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• The inclusion of hydrogen in the existing energy mix for global Net-Zero target is discussed. • Prospects and feasibility of underground hydrogen storage (UHS) in India are presented. • The connection between the human development index, hydrogen energy, and carbon footprint is emphasized. • Indian geological basins and formations suitable for UHS are highlighted. • Technical data, regulations, and policies to explore the opportunity of UHS are described. The world is combating climate change by adopting a low-carbon energy mix and pursuing a net-zero target. Being the third-largest CO 2 emitter, net-zero target for India is crucial for global decarbonization. Hydrogen is a promising fuel for the future to aid in this transition, but its storage is a significant challenge. Surface-based storage options are inadequate and expensive, making underground hydrogen storage (UHS) in geological formations a more practical solution for its ample storage capacity and economic feasibility. Herein, this review critically investigates (i) the possibility of surplus renewable energy storage in India, (ii) UHS's applicability to store this surplus energy, and (iii) scientific aspects, feasibility, and challenges with suitable geological storage locations for storage of hydrogen in India. This review starts with briefing 'net-zero' at both the global and national (India) levels. The urgency for green energy is highlighted by the disparity between the Human Development Index and ecological footprint. The Indian hydrogen market and the production of green hydrogen from renewable resources are also discussed. The review discusses UHS storage types, reservoir and fluid parameters, and flow behavior, including physiochemical, geochemical, and microbial interactions. Based on the geological basins and formation, availability and capacity of renewable energy sources, the potential of UHS in India is analysed. This review also provides vital information including technical data, regulations, initiatives, and policies needed to explore the opportunity of UHS and its development. Finally, the article presents challenges and opportunities for UHS development and emphasizes the need for continued research and development. [ABSTRACT FROM AUTHOR]

Abstract The tetrapod footprint record of Permian eolian environments has long been underestimated because of overall poor preservation and its apparent monospecifity. The best known and most abundant Cisuralian record of tetrapod footprints is from the Coconino and De Chelly formations of Arizona, which, however, thus far encompassed only the ichnogenera Chelichnus and Dromopus. We revised the locomotion and taphonomy of these footprints and propose a new model, basing it on: 1) trackways changing direction, 2) trackways heading in different directions on the same surface, 3) trackways in situ, and 4) laboratory experiments with common wall lizards, Podarcis muralis. In all cases, the Chelichnus -like appearance of footprints is due to digit tip sliding on inclined depositional surfaces, masking the original footprint shape and orientation. Also, the trackway pattern and body position are largely influenced by the angle of inclination (dip) of the substrate being walked on. Based on an anatomy-consistent ichnotaxonomy, Chelichnus and Laoporus are here considered nomina dubia, and the footprints from the Coconino and De Chelly formations are revised and assigned to: parareptiles/captorhinomorph eureptiles (Erpetopus, Varanopus curvidactylus), bolosaurid parareptiles/ diapsid eureptiles (cf. Dromopus), varanopid synapsid (cf. Tambachichnium) and reptiliomorph amphibians (Amphisauropus, Ichniotherium sphaerodactylum). The ichnoassociation is dominated by parareptile/captorhinomorph tracks, similarly to all the late Cisuralian marginal marine, floodplain, alluvial fan and ephemeral lacustrine tetrapod ichnoassociations of North America, Europe and North Africa. A review of all the available data including the new results suggests a facies-crossing transition between an early-Cisuralian amphibian- and synapsid-dominated ichnofauna (Dromopus track biochron) and a late Cisuralian parareptile/captorhinomorph-dominated ichnofauna (Erpetopus track biochron) at low latitudes of Pangea. [ABSTRACT FROM AUTHOR]

Abstract This study investigated the geochemical characteristics of source rocks in each sequence and studied on the oil-source correlation under a sequence stratigraphic framework of the Upper Cretaceous Yogou Formation. The Yogou Formation as the most primary and productive source bed in the Termit Basin (Niger), was further divided into three third-order sequences (i.e. YSQ1, YSQ2 and YSQ3, from bottom to top) in the study. On the basis of the geochemical analysis of a total of 35 rock samples from YSQ3, YSQ2 and YSQ1, the variation of the organic matter abundances, types and maturity of source rocks within different sequences were compared. The TOC content of YSQ2 and YSQ1 source rocks ranges from 0.76% to 1.69% with mature to high mature thermal evaluation stage (R o : 0.63%–1.29%), while the TOC content of YSQ3 source rocks is much higher (1.11%–6.63%) with a relatively lower maturation levels (R o : 0.59%–0.91%). All source rocks are dominated by Type II 2 –III kerogen. Moreover, in order to determine the oil-source correlation under a sequence stratigraphic framework, the composition of biomarkers in 35 source rocks and 25 crude oils were analyzed systematically. Various non-biomarker and biomarker parameters were applied involving source-related, maturity-related and environment-related parameters. The YSQ3 source rocks are typically characterized by relatively higher Pr/Ph ratio, higher concentration of dibenzofurans (DBFs) and lower gammacerane index indicating a suboxic transitional environment, and the relatively higher values of oleanane index and C 24 TeT/(C 24 TeT + C 26 TT) ratio reflecting a significant terrigenous organic matter input. Conversely, the YSQ2, YSQ1 source rocks and studied oils have relatively lower Pr/Ph ratios, higher content of gammacerane and dibenzothiophenes (DBTs) suggesting suboxic to anoxic conditions under shallow marine depositional environment, and low values of oleanane index and C 24 TeT/(C 24 TeT + C 26 TT) ratios indicating predominant algal aquatic organisms input. In addition, other molecular parameters in crude oils, including OEP, CPI, TAR, n C 21 -/ n C 22 +, (n C 21 + n C 22)/(n C 28 + n C 29), Pr/ n C 17 , Ph/ n C 18 ratios and the distribution patterns and relevant parameters of tricyclic terpane series, also have the obviously similar characteristics to YSQ2 and YSQ1 source rocks, rather than YSQ3 source rocks. The result of oil-source correlation shows that the discovered oils are dominantly derived from YSQ2 and YSQ1 source rocks, with minor contribution of YSQ3 source rocks. Therefore, the oil-source correlation within a sequence stratigraphic framework can provide a practical model for source rock assessment and petroleum system study. Highlights • The source rock was evaluated under a sequence stratigraphic framework. • The compositions of molecular markers for 3 sequences were discussed. • Oil-source correlation shows that the YSQ1 and YSQ2 are the main source beds. • Geochemical sequence stratigraphy has practical implication in petroleum system. [ABSTRACT FROM AUTHOR]

Abstract For tight sandstone reservoirs, the pore structure is sensitive to the stress variations during the production process. The alternation of pore structure influences the petrophysical properties and therefore the dynamic flow of tight sandstone reservoirs. In this research, Nuclear Magnetic Resonance (NMR) technique is employed to quantitatively study the pore volume variation under different confining pressures. Not only during the compression process, this research also investigated the sandstone pore sensitivity to stress during the recovery process of rock samples, which is equally important and happens during the production process when the formation pressure is building up due to the temporary well shut-in or water injection. The core samples selected from Ordos basin are first tested routinely for basic parameters like porosity, permeability and detrital components, and then through both compression and recovery processes where the pressure decreases and increases, respectively. It is found that the core samples with high permeability show stronger stress sensitivity of pores than those with low permeability due to the higher content of large pores. Besides permeability, detrital components and interstitial material can also affect the stress sensitivity of pores. During the recovery process, the low recovery degree of pore volume is related to the recoverable pore size (mainly smaller size pores) and the narrow distribution range of recovered pores. Furthermore, the power function relationship and the exponential relationship are suggested to evaluate the variation of pore volume in compression and recovery process, respectively. The results demonstrated that the compressibility of larger pores is bigger than that of smaller pores. During the compression process, the higher permeability rocks, containing a higher content of larger pores, are easier to be compressed, and has a greater decrease in permeability. Besides, the recoverability of the total pore volume after compression is mainly controlled by smaller pores, which results in the lower recoverability of the total pore volume and the permeability for the higher permeability core samples. Highlights • Using Nuclear Magnetic Resonance (NMR) technique to quantitatively study stress sensitivity. • High permeability cores show stronger stress sensitivity than that in low permeability. • Detrital component and interstitial material can affect stress sensitivity of pores. • Power function relationship can be used to evaluate the variation of pore volume. • Exponential relationship can be used to estimate the recovery degree of pore volume. [ABSTRACT FROM AUTHOR]

Abstract The bioturbated horizons identified from Member Two of the Paleocene Funing Formation (E 1 f 2) in the Jinhu sag, have not been previously assessed to ascertain their influence on reservoir quality. Owing to that, conventional thin section, pulse decay permeametry, pressure decay porosimetry, Field Emission Scanning Electron Microscope (FESEM) and Energy Dispersive X-ray Spectroscopy (EDS) methods were used to analyze the significance of bioturbation in controlling porosity and permeability on selected cored intervals from one well. Intensely bioturbated core slab samples A and B selected from Bian-5 Well are dominantly controlled by Ophiomorpha and Macaronichnus , and Macaronichnus , respectively. Results showed that Ophiomorpha and Macaronichnus in sample A exhibited sediment cleaning activity with better sorted grains in burrow fillings. Mud and clay were removed from the burrow filling and directed towards burrow lining and/or host sediment by burrowing organism resulting in well sorted burrow fill matter; the net effect, enhanced isotropy and increased porosity in burrows fills. Macaronichnus ichnofabrics in sample B also exhibited sediment cleaning activity causing reduction of local porosity in host sediment and an outcome of permeability improvement in burrows. Reservoir quality of bioturbated sandstone of Bian-5 well in the Jinhu sag is, therefore, dependent on activity of burrowing organism, burrow structure, bioturbation extent as well as the existence or nonexistence of burrow fill or lining. Highlights • Intensely bioturbated samples embody Ophiomorpha – Macaronichnus and Macaronichnus ichnofabrics. • Sediment cleaning activity resulted in better isotropy and increased porosity in burrow fills. • Permeability enhancement resultant from well sorted and clean burrow fill by burrowing organism. [ABSTRACT FROM AUTHOR]

Highlights • Laterally Confined Volcanic Successions record rift-jumps during SDR emplacement. • 2 km of subsidence can be generated during SDR emplacement without faulting. • Rift-jumps may occur to establish a laterally continuous magmatic spreading centre. Abstract Seaward Dipping Reflectors (SDRs) are a characteristic feature of magma-rich margins, and represent the generation of large volumes of flood basalts at the point of continental breakup. A number of recent studies provide new insights into the emplacement and tilting of SDRs and conclude that the majority of SDRs are contained within new magmatic crust that has a close affinity to oceanic crust. However, the process by which these initial magmatic systems evolve into a fully established spreading centre remains poorly understood. Several characteristic features of magma-rich margins may be explained by the occurrence of rift-jumps during SDR emplacement, yet the cause and prevalence of such rift-jumps remain unknown. Here we constrain the 3D geometry of the continent–ocean transition in the Orange Basin, offshore South Africa. This allows us to test if, where and why such rift jumps occur. Our results demonstrate an order of along-strike segmentation previously unobserved in these settings. We demonstrate that the SDR belt is disrupted by the occurrence of a volcanic-stratigraphic package, defined as the Laterally Confined Volcanic Succession (LCVS), not previously identified on a rifted margin. We interpret this as a magmatic spreading centre that was abandoned by a subsequent rift-jump. Identification of LCVSs is important for two reasons. First, we argue that the LCVS formed via the same process as SDRs, and hence provides a unique example of SDR geometry prior to their separation onto conjugate plates. Second, as we can map out the 3D geometry of the LCVS and SDRs, we propose that rift-jumps during magma-rich margin formation may be fundamental to the establishment of a laterally continuous incipient spreading centre. [ABSTRACT FROM AUTHOR]

Highlights • Redox-stratified water column in a Mesoarchean epicontinental sea, Pongola basin. • Mesoarchean aerobic iron and manganese cycling in the Pongola epicontinental sea. • Aerobic biological catalysis enhanced manganese oxidation in a Mesoarchean sea. • Evidence for oxygenated surface seawater ca. 3 billion years ago. Abstract Redox conditions in the marine realm prior to the Great Oxidation Event (GOE; ∼2.46–2.32 Ga ago), during which the atmospheric oxygen level rose dramatically for the first time, are still debated. Here, we present C, O, Fe, and Mo stable isotope systematics of Fe-, Mn-, and carbonate-rich shales, deposited at different water depths in association with iron formations (IFs) of the Mesoarchean Mozaan Group, Pongola Supergroup, South Africa. δ 13 C values between −22.3 and −13.5‰ VPDB, and δ 18 O values between −21.1 and −8.6‰ VPDB for Fe–Mn-rich carbonate minerals indicate their precipitation out of equilibrium with seawater. Instead, early diagenetic reduction of Fe–Mn-oxyhydroxide precursor minerals, along with microbially induced oxidation of organic matter (OM), formed these carbonates. δ 56 Fe IRMM - 014 values between −1.27 and 0.14‰ and δ 98 Mo NIST 3134 + 0.25 values between −0.46 and 0.56‰ co-vary with Mn concentrations and inferred water depth of deposition. This suggests that, despite the diagenetic origin of the Fe–Mn carbonates, the primary light Fe and Mo isotopic signature of Fe–Mn-oxyhydroxides that originally precipitated from seawater is still preserved. While isotopically light Mo implies that Mn(II) was oxidized to Mn(IV) due to the availability of free, photosynthetically produced O 2 , Mn enrichment suggests that the water column was redox stratified with a Mn-redoxcline situated at a depth below the storm wave base. A trend to highly negative δ 56 Fe values with increasing Mn/Fe ratios and decreasing depositional depth suggests progressive oxidation of Fe(II) as deep-waters upwelled across a redoxcline towards shallow, locally oxygenated waters where Mn(IV) oxyhydroxides precipitated. Combined δ 56 Fe and δ 98 Mo data indicate pervasive oxygenation of seawater with the O 2 content in the photic zone likely reaching levels higher than the maximum value of 10 μM proposed for Archean oxygen oases. Since abiotic Mn(II) oxidation is kinetically very slow in marine environments, it is likely that Mn-oxidizing microorganisms catalyzed Mn-oxidation in the oxygenated Pongola surface waters during deposition of IFs. This implies that aerobic metabolism had evolved before the GOE in shallow, aquatic habitats, where it exerted a first-order control on the deposition of shallow-marine, Mn-rich iron formations. [ABSTRACT FROM AUTHOR]

Reconstructing the evolution of paleofluid pressure in carbonate reservoirs is a challenging problem, particularly when no oil-bearing fluid inclusions are available to provide barometric constraints on the fluid system. To recover the paleofluid pressure in the Sinian Dengying Formation (Z 2 dn) and the Cambrian Longwangmiao Formation (Є 1 l) of the central paleo-uplift of the Sichuan Basin, we conducted a comprehensive analysis combining the inclusion pressure-volume-temperature simulation, inclusions in situ Raman shifts, and basin modeling. The results showed that the evolution of the pore pressure could be divided into four stages. Prior to the end of the late Triassic, the whole central paleo-uplift was in a normal pressure state. During the Jurassic, a moderate overpressure prevailed in the dolomitic reservoirs of the Gaoshiti-Moxi area and decreased in the adjacent units. At the end of the early Cretaceous, the overpressure reached a maximum in the whole region and was strongest in the Longnüsi area, decreasing to the west. From the late Cretaceous to the present day, the central area of the basin has uplifted rapidly with the overpressure significantly decreasing. Z 2 dn has been restored to normal pressure during the Neogene, while Є 1 l remains moderately overpressurized. Based on an analysis of the structural background and preservation conditions, the types of sedimentary facies, the preservation condition of the traps, and the differential weathering process are the primary reasons for the different pressure states in Z 2 dn and Є 1 l during the last stages. [ABSTRACT FROM AUTHOR]

The Lower Permian Collyhurst Sandstone Formation is an important gas reservoir both offshore and onshore in the East Irish Sea Basin of north-western UK, and is considered to be a tight gas reservoir. Diagenetic processes have significantly influenced the reservoir quality of the sandstone. This study employs petrographic, mineralogical, and stable isotopic analyses on two different onshore core successions to investigate the composition and texture, the diagenetic history, and the impact of lithology and diagenesis on reservoir quality. The Collyhurst Sandstone Formation consists of fine- to very coarse-grained, poorly to well-sorted, sub-arkosic to lithic arenite sands. The reservoir properties of the sandstone are relatively poor, with average porosity and permeability of 6.6% and 0.387 mD respectively. On the basis of petrographic analyses, five sandstone lithofacies were identified: fine-grained, conglomeratic, carbonate-cemented, quartz-cemented and matrix-free. Contrasting characteristics between the reservoir units suggest the development of divergent diagenetic pathways. Features observed during early diagenesis include compaction, growth of pore-filling early carbonate cements (calcite I, dolomite and siderite I) and vermicular kaolinite. Late diagenetic features include: further carbonate cementations (ferroan dolomite/ankerite, calcite II and siderite II-III), clay mineral cementation (illite and chlorite) with both grain-coating and pore-filling textures. Also common in late diagenesis are syntaxial quartz overgrowths. Albite, barite and anhydrite are minor late diagenetic cement phases. Compaction processes rather than cementation accounts for the major destruction of the initial sandstone porosity. But cementation still accounts for up to 32% of porosity loss. Carbonate cements mostly fill intergranular pores, thus block pore throat and prevent migration of fluid. Clay minerals bridge and occlude pores, resulting in further destruction of the reservoir quality. The study provides insights into how lithofacies (primary composition, texture and sorting) and diagenesis can influence reservoir qualities of tight gas sandstones. [ABSTRACT FROM AUTHOR]

We present a case-study in Palaeolithic archaeology focusing on formation processes at cave- and rock-shelter-sites in a Mediterranean context and on the correlation between site deposits, Quaternary surface sediments and the morphology of the surrounding land. We study three sites located in the basin of River Mula (Murcia, Spain): the cave-site of Cueva Antón and the rock-shelters of Finca Doña Martina and Abrigo de la Boja, in the Rambla Perea valley. They are examined through an integrated geoarchaeological approach that takes into account geomorphological, stratigraphic and soil micromorphological data. The three sites are found within a short distance and cover similar time spans within the Upper Pleistocene but have different formations and show distinct degrees of preservation of the archaeological record. Cueva Antón features a densely-stratified, mostly alluvial succession, with excellent preservation of stratigraphic layout and sedimentary facies. The succession represents a well-preserved record of past activity of the River Mula and can be correlated to its open-air alluvial system. The Rambla Perea sites, despite their immediate vicinity, underwent distinct formation dynamics, with a complex interaction among the site deposits and the hillslope along which they are found, and exhibit different degrees of preservation of archaeological layers and features. We summarise the evidence from these sites and then discuss its implications for site formation, for the preservation of the archaeological record and for the correlation between the ‘inside’ (the archaeological successions preserved within caves and in rock-shelters) and the ‘outside’ (the geomorphological setting of the land and the evolution of Quaternary surface dynamics through time). [ABSTRACT FROM AUTHOR]

The thermal and diagenetic evolution of shale units has received renewed focus because of their emergence as unconventional hydrocarbon reservoirs. The Serra Alta Formation (SAF) is a Permian shale unit of the Paraná Basin, which is the largest South American cratonic basin. The SAF stands out as a pathway for aqueous fluids and hydrocarbon migration from the Irati organic-rich shales to the Pirambóia fluvial-eolian sandstone reservoirs. Vertical NNW and NNE opening fractures would be the main pathways for the migration of buried pore waters and aqueous fluids, besides the input of meteoric water. These fractures would be associated to the reactivation of basement discontinuities such as the Jacutinga (NE) and Guapiara (NW) faults. Thus, vertical NNE and NNW associated fractures would represent the main pathways for fluid migration in the studied area. The vertical calcite filled opening fractures from SAF record moderately low salinity (0–4.5 wt % of NaCl eq.) aqueous fluids, suggesting the input of meteoric water in the buried fracture system. Eutectic melting temperatures at −52±5 °C indicate an H 2 O + NaCl + CaCl 2 system with CaCl 2 or MgCl 2 in solution. Homogenization temperatures recorded in fluid inclusion assemblages (FIAs) of calcite filled opening fractures indicate that the SAF in the studied area reached temperatures above 200 °C, suitable for generation of gaseous hydrocarbons. The recorded paleotemperatures point to a thermal peak associated with Serra Geral volcanic event during the Early Cretaceous, with the thermal effect of volcanic rock cap possibly overcoming the effect of intrusive igneous bodies. The detection of methane in SAF shale pores indicates conditions for hydrocarbon generation. However, additionally studies are necessary to confirm the thermogenic and/or biogenic origin of the methane within the SAF. [ABSTRACT FROM AUTHOR]

The Pico do Itapeva Formation is a key metasedimentary unit to the understanding of the extensional events that occurred between the late stage of the southern Brasília Orogen collision and the main collision in the central Ribeira Orogen. The formation crops out in a 20 km long NE-trending narrow belt in the Mantiqueira mountain range in eastern São Paulo State, Brazil. It is located in the interference zone of the southern Brasília and the central Ribeira orogens and records deformation and greenschist facies metamorphism (biotite zone) related to the Brasiliano orogeny. The Pico do Itapeva Formation rests unconformably on a metaigneous substratum of the Socorro-Guaxupé Nappe/Embu Terrane and, on the southern side, is truncated by a steep SE-dipping dextral reverse shear zone. It consists of a coarsening- and thickening-upward succession, with minimum thickness of 800 m, composed of lutite, arkose and conglomerate. These rocks constitute three distinct lithofacies associations: LAI- arkose, arkose-lutite composite beds, lutite and fine conglomerate beds; LAII- arkose, pebbly arkose and scarce lutite and; LAIII- conglomerate and pebbly arkose. Most of the beds are massive; graded beds, dish and convolute structures occur locally. Bed thickness varies from thin to very thick and amalgamated bodies constitute up to 30m thick strata. Three mappable units at scale 1:20,000 were recognized based on different proportions of the three lithofacies associations. The deposits are interpreted as the record of mass flows and associated processes in a fan delta setting developed in an intermontane rift basin. U-Pb LA-ICP-MS detrital zircon ages suggest the maximum depositional age at ca. 611 Ma and the basin evolution is interpreted in the range between 611 and 580 Ma during an inter-orogenic stage between the Brasília and Ribeira orogenies. [ABSTRACT FROM AUTHOR]

Shales in the middle Permian Lucaogou Formation in the intracontinental Santanghu rift basin have been considered as “typical” organic-rich profundal shales for decades. Our study of well cores using petrographic microscope and scanning electron microscopy suggests an otherwise complex hydrovolcanic and hydrothermal origin. This paper describes characteristics of a particular type of the shales, composed of fine-grained detrital minerals and lithic grains. Some of them are orthopyroxene, calcite, peralkaline feldspars, and analcime that are interpreted as derived from peralkaline-alkaline carbonatite, pyroxenite, analcime phonolite, and andesite, whereas others are quartz, dolomite, ankerite, serpentine, and calcite that were precipitated from syndepositional or penecontemporary hydrothermal fluids. Grain size ranges from 0.001 to 2 mm, mostly 0.01–0.1 mm. Well-developed laminae are mostly 0.5–3 mm thick and alternate with tuffaceous dolomicrite. The rocks are interpreted as sublacustrine hydrovolcanic deposits, which had been altered by syndepositional hydrothermal fluids. The interpretation is substantiated by abundant cone-shaped stratigraphic buildups on seismic sections in the basin. This study shows an ancient example of volcanic-hydrothermal deposits in a rift basin. [ABSTRACT FROM AUTHOR]

Measurements of TOC, Rock-Eval pyrolysis, carbon isotope composition, solvent extract, extract fractionation, and the gas chromatography-mass spectrum (GC-MS) of the saturated fraction were conducted on the Lower Permian Fengcheng Formation carbonate-bearing shales and argillaceous dolomites of Well FN7 in the northwestern Junggar Basin to understand the geochemistry and depositional environmental evolution of the carbonate-bearing lacustrine source rocks. The results indicate that the Fengcheng Formation carbonate-bearing shales are fair to good source rocks with Type II kerogen for generating oil and minor gas, and the argillaceous dolomites are poor source rocks. The thermal mature shales contain more residually generated hydrocarbon, whereas the argillaceous dolomites contain more migrated hydrocarbon. This property is favorable for shale oil or tight oil accumulation in the Fengcheng Formation. Biomarker analyses indicate that the Fengcheng Formation source rocks were formed in the anoxic reducing lacustrine depositional environment with a stratified water column and high salinity. The source rock organic matter has major aquatic phytoplankton and algae input and less terrestrial organic matter input. During the Fengcheng Formation period, oxidation had a weak increasing tendency and salinity had a weak decreasing tendency from the early to late periods under the background of a reducing environment. Except for poor source rocks in upper and lower parts, the good source rocks are primarily in the middle part of the Fengcheng Formation. [ABSTRACT FROM AUTHOR]

A geometrically quantitative plate-kinematic model, based on paleomagnetism, for the initial assembly of Laurentia has taken form in the past few decades. Within this framework, there remains but one problematic interval of data predominantly from the Slave craton, which is the 1.96–1.87 Ga Coronation apparent polar wander path (APWP). The Coronation APWP shows large (∼110°) back-and-forth oscillations that are difficult to explain in terms of plate motion. Nonetheless, poles from the Coronation APWP have been incorporated in various paleogeographic reconstructions of Laurentia and the supercontinent Nuna, pointing to the importance of testing its veracity. In this study, we conducted a detailed paleomagnetic and rock magnetic study of the ca. 1.87 Ga Pearson Formation, East Arm of Great Slave Lake, Northwest Territories, Canada. Our results show that Pearson Formation yields a characteristic remanent magnetization carried by single-domain or small pseudo-single-domain magnetite. The age of the magnetization is constrained to be older than Paleoproterozoic deformation and is interpreted as primary. Paleomagnetic declinations reveal a one-to-one correlation with local structural attitudes, indicating that some small blocks in the fold belt likely experienced significant (∼60°) vertical-axis rotations, presumably related to large dextral displacements along the McDonald Fault system. Alternative explanations, such as true polar wander or a non-dipole magnetic field, are considered less parsimonious for the data presented here. It is suspected that some existing Christie Bay Group poles (the Stark and Tochatwi Formations), which were sampled in areas with anomalous structural attitudes and differ from time-equivalent poles obtained from areas of the Slave craton far from major transcurrent faults, may similarly suffer from vertical-axis rotation. We suggest further study before using possibly rotated Christie Bay Group poles for paleogeographic reconstructions. [ABSTRACT FROM AUTHOR]

The paleolake that once existed in the Hetao Basin, China, represents an important stage of the Yellow River drainage system evolution in the region, but it has not been studied in detail to date. In particular, the formation and driving forces of the paleolake evolution since the late Pleistocene is still poorly understood. In this study, optically stimulated luminescence and radiocarbon dating were combined with multiproxy analysis of lacustrine sediments in the central Hetao Basin to reconstruct the paleolake evolution. Proxies included carbonate 18 O and 13 C isotopes, grain size, magnetic susceptibility, and color. In combination with stratigraphic records, the proxies indicated the existence of a lake environment in the Hetao Basin between ∼150 and ∼18 ka BP. The paleolake between 150 and 130 ka BP was an open system. Between 130 and 54 ka BP, the paleolake basin closed, with high lake levels at the beginning of the period and then a gradual decrease. After 54 ka BP, the paleolake began to outflow and gradually shrank in size, and after 18 ka BP an alluvial fan covered the region, and the paleolake disappeared. We propose a combination of tectonic uplift and climate changes as the main drivers of paleolake formation and evolution since the late Pleistocene. [ABSTRACT FROM AUTHOR]

Source rock screening analysis was performed on four stratigraphic units from the Muglad basin namely; Abu Gabra, Zarqa, Ghazal, and Baraka formations using pyrolysis and Vitrinite Reflectance (Ro). Results, integrated with the chromatographic and isotopic data from these rocks extracts and a Tertiary oil from El Mahafir-1 wild cat, were used to determine the origin of the oil. A good organic source within the Middle Abu Gabra Formation is observed in wells El Toor-6 and Neem Deep-1 (TOC, 1.0–2.0% & S2 5.0–10.0 mg C/g rock), with mixed kerogens I, II, & III, and thermally mature (% Ro = 0.74–0.94). The Campanian-Early Maastrichtian sequence, i.e. Zarqa and Ghazal formations are generally poor (TOC, <0.5% & S2 <2.5 mg C/g rock), dominated by type III kerogens, and immature at the studied locations. The Baraka shale nevertheless, is good at El Mahafir-1 well (avg. TOC 1.8% & S2 5.0–10.0 mg C/g rock) and fair at Timsah-1 well (Avg. TOC 0.69% & S2 2.5–5.0 mg C/g rock) with a Kerogen that is predominantly Sapropellic at the former, and an exclusively Humic at the later. The formation is mature at Timsah (% Ro = 0.77–1.16) and early mature at El Mahafir-1 (% Ro = 0.64–0.81). Consistent with the pyrolysis, chromatographic data of the rock extracts confirms the mixed source nature of the Abu Gabra Formation which consists of both algal [prominent LMW n-alkanes & elevated C 27 steranes (36–47%)], as well as terrigenous material [higher diasterane/regular sterane ratios (0.50–0.56), abundant rearranged hopanes, & relatively high CPIs (1.22–1.9)], accumulated in an oxic to sub-oxic environment (Pr/Ph, 1.3–3.0). Abu Gabra further shows low C 29 /C 30 hopanes (0.45–0.54), low C 28 steranes (21–26%) with high Gammacerane index (20.3). In contrast, the environment during the Late Cretaceous was strongly reducing (Pr/Ph, 0.37–1.0), associated with a wide organic diversity, both in space and time and is characterized by dominant algal input at some areas and or stratigraphic intervals [Elevated tricyclics, higher C 29 /C 30 hopanes (0.5–1.14), and relatively low Gammacerane indices (4.6–14.4)], while mixed with abundant terrigenous material at others. A direct correlation between El Mahafir oil and the Abu Gabra extracts is thus inferred based on: its mixed organic source nature, oxic to sub-oxic depositional environment (Pr/Ph 1.22), relatively low C 29 /C 30 hopanes (0.54), low C 28 steranes (29%), and a high gammacerane index (20.5). This is largely supported by the maturity modeling results which suggest generation is only from the Abu Gabra at this location. [ABSTRACT FROM AUTHOR]

Facies analysis of the Cretaceous Bima Formation in the Gongola Sub –basin of the Northern Benue Trough northeastern Nigeria indicated that the Lower Bima Member is composed of alluvial fan and braided river facies associations. The alluvial fan depositional environment dominantly consists of debris flow facies that commonly occur as matrix supported conglomerate. This facies is locally associated with grain supported conglomerate and mudstone facies, representing sieve channel and mud flow deposits respectively, and these deposits may account for the proximal alluvial fan region of the Lower Bima Member. The distal fan facies were represented by gravel-bed braided river system of probably Scot – type model. This grade into sandy braided river systems with well developed floodplains facies, forming probably at the lowermost portion of the alluvial fan depositional gradient, where it inter-fingers with basinal facies. In the Middle Bima Member, the facies architecture is dominantly suggestive of deep perennial sand-bed braided river system with thickly developed amalgamated trough crossbedded sandstone facies fining to mudstone. Couplets of shallow channels are also locally common, attesting to the varying topography of the basin. The Upper Bima Member is characterized by shallow perennial sand-bed braided river system composed of successive succession of planar and trough crossbedded sandstone facies associations, and shallower channels of the flashy ephemeral sheetflood sand – bed river systems defined by interbedded succession of small scale trough crossbedded sandstone facies and parallel laminated sandstone facies. The overall stacking pattern of the facies succession of the Bima Formation in the Gongola Sub – basin is generally thinning and fining upwards cycles, indicating scarp retreat and deposition in a relatively passive margin setting. Dominance of kaolinite in the clay mineral fraction of the Bima Formation points to predominance of humid sub – tropical to tropical climatic conditions. This favors pedogenic activities which are manifested in the several occurrences of paleosols. Pronounced periods of arid climatic conditions are also notable from the subordinate smectite mineralization. Chlorite mineralization at some localities is indicative of elevation of the provenance area, and this is synonymous with deposition of the Bima Formation, because of its syn – depositional tectonics. The absences of lacustrine shales in the syn – rift stratigraphic architecture of the Bima Formation indicates that the lower Cretaceous petroleum system that are common in the West and Central African Rift basins are generally barren in the Gongola Sub – basin of the Northern Benue Trough. [ABSTRACT FROM AUTHOR]

Large quantities of natural gases have recently been discovered in the marine carbonate formations in the Sichuan Basin, southern China. The origins and isotopic compositions of the gases are poorly understood and highly controversial. We examined the chemical and isotopic compositions of the carbonate reservoir gases in marine formations from the Sinian to Lower Triassic as well as shale gas from the Silurian and Cambrian Formation in the Sichuan Basin. It is found that the carbon isotopes of the carbonate reservoir gases had been evolved through three stages corresponding to pre-rollover, rollover and post-rollover in the plot of δ 13 C 2 versus wetness, which is consistent with that found in shale gas by Tilley and Muehlenbachs (2013). The unique carbon isotopic compositions indicate that both kerogen and oil-cracking gas have contributed to the accumulation of the carbonate reservoir gases in the Sichuan Basin. A mixing of two thermogenic gases with different maturities can cause the δ 13 C reversal between methane and ethane, which is commonly observed in the carbonate reservoir gases in the Carboniferous, Permian and Triassic formations, and the shale gas in the Silurian and Cambrian formations in the Sichuan Basin. We established two mixing models for quantitatively determining the contribution of each type gas for the natural gases in different formations. Kinetic extrapolation based on thermal history of the Anyue gas field demonstrates that ethane cracking indeed occurred in the deep reservoir formations, which may explain the disappearance of the 13 C reversal for methane and ethane in the carbonate reservoir gases in the post-rollover zone. It is proposed that organic-inorganic interactions may be responsible for the presence of D isotope rollover for methane in carbonate reservoir gases with ultra-high maturity. The organic-inorganic interactions include thermal reactions between water and residual organic matters or carbons and hydrogen isotope exchange between water and methane, and their mechanism needs further investigations in future. [ABSTRACT FROM AUTHOR]

A time series of produced shale gas samples from Lower Silurian Longmaxi Formation (Fm.) were collected from six production wells during 3.5 years production period from the Weiyuan and Changning shale gas pilots in Sichuan basin, China. The molecular compositions and carbon isotopes of CH 4 , C 2 H 6 , C 3 H 8 and CO 2 , and the hydrogen isotope of methane were analyzed for these samples by an integrated method of gas chromatography combined with mass spectrometry. The results showed that the produced shale gases are dominated by CH 4 (90.1–99.3%) with minor C 2 H 6 (0.3–0.8%) and non-hydrocarbon gases, and have low wetness (0.3–0.8%). The non-hydrocarbon gases are mainly CO 2 and N 2 , and trace amount of He. The Longmaxi shale gases display completed reversal pattern of carbon isotopic compositions with carbon number (i.e., δ 13 C 1 > δ 13 C 2 > δ 13 C 3 ) except for Wells W201 and W201-H1 with normal patterns between C 2 H 6 and C 3 H 8 (i.e., δ 13 C 2 < δ 13 C 3 ), and it is a mixture of oil cracking gas and kerogen cracking gas at high-over level of thermal maturity. Some CO 2 (δ 13 C CO2 ranging from −2.5 to −6.0‰) is mainly derived from the decomposition of carbonate minerals. CH 4 concentration and the carbon isotopes of methane and ethane during 3.5 years shale gas production from year 2012 to 2016 displayed small fluctuations, which possibly suggested that gas supply from shale matrix to fracture network after hydraulic fracturing is abundant. The Weiyuan shale gases show larger decrease of CH 4 content than the Changning shale gases, indicated more adequate supply in the Changning area. The difference in δ 13 C 1 values between horizontal and vertical wells on the same drilling site in the Weiyuan pilot indicated that the vertical heterogeneity of Lower Silurian Longmaxi shale plays an important role to shale gas production. Interestingly, the shale gases in the Weiyuan pilot display much lighter δ 13 C 1 (−34.46 to −36.82‰) and δ 13 C 2 (−37.59 to −41.89‰) values than those (δ 13 C 1 = −27.18–−27.27‰, δ 13 C 2 = −33.70–−34.14‰) in the Changning pilot. The difference in carbon isotopic compositions in Weiyuan and Changning pilots mainly attributes to mixing between varied proportion of oil cracking gas and kerogen cracking gas because of the regional heterogeneity of the Longmaxi marine shale, in particular amount of residual oil retained in organic matter rich shale. The isotopic difference between Weiyuan and Changning shale gases doesn't disappear in the period about 3.5 years of shale gas development, which further suggests the regional heterogeneity of retained residual oil exists in the Longmaxi Fm. marine shale. [ABSTRACT FROM AUTHOR]

The concentration and distribution of organic matter, phosphate nodules, and framboidal pyrite preserved in shales are controlled by organic matter decomposition and the cycles of phosphorus, sulfur, and iron, which, in turn, are controlled by bottom-water redox conditions. The organic-rich shale in the 7th member of the Upper Triassic Yanchang Formation (Ch7) contains high organic carbon concentrations and abundant phosphate nodules and pyrite framboids, providing an opportunity to better understand the redox conditions at the time of deposition. Framboidal pyrite in the Ch7 organic-rich shales are composed of octahedral, pyritohedral, and spherulitic microcrystals with uniform crystal size. SEM photographs and statistical data suggest that pyrite framboids have large mean diameters (10.0 to 18.1 μm) and standard deviations (2.8 to 5.9 μm). This may indicate that the organic-rich shale was deposited under oxic-suboxic bottom-water conditions. The C org :P ratios of the organic-rich shale exhibit considerable variation, ranging from 7.2 to 216, but the majority of them (83.2%) belong to the lower-intermediate range (< 130), suggesting that the redox conditions were predominantly oxic-suboxic and intermittently anoxic. The C org :P ratios also show periodic vertical variation, which suggests that redox conditions fluctuated between oxic-suboxic and anoxic. Additional support for the trends based on C org :P ratios is provided by DOP T values, which show similar vertical variation and redox patterns. All evidence shows that the redox conditions during deposition of the Ch7 organic-rich shale was predominantly oxic-suboxic accompanied by intermittent anoxic environments. The result of this study will be helpful in better understanding the formation mechanism of the Ch7 organic-rich shale. [ABSTRACT FROM AUTHOR]

The giant sediment-hosted Jinding zinc-lead deposit is located in the Lanping Basin, northwestern Yunnan Province, China. The genesis of the deposit has long been debated and the sources of the ore-forming fluids and metals are controversial. This study presents rare earth element (REE) and noble gas isotope data that constrain the origins of the ore fluids and the heat source driving the hydrothermal circulation. The early-stage sulfides are enriched in light REEs and have high ∑REE values (30.8–94.8 ppm) and weakly negative Eu (δEu 0.85–0.89) and Ce anomalies (δCe 0.84–0.95), suggesting that the fluids were likely derived from dissolution of Upper Triassic marine carbonates with input of REEs from aluminosilicate rocks in the basin. In contrast, the late-stage sulfides have irregular REE patterns, generally low ∑REE values (0.24–10.8 ppm) and positive Eu (δEu 1.22–10.9) and weakly negative Ce anomalies (δCe 0.53–0.90), which suggest that the ore-forming fluids interacted with evaporite minerals. The 3 He/ 4 He (0.01–0.04 R a ) and 40 Ar/ 36 Ar values (301–340) of the ore-forming fluids indicate crustal and atmospheric origins for these noble gases. These findings are in agreement with the published fluid inclusion microthermometry data and the results of H, O, C, S, Pb and Sr isotope studies. Our data, in combination with published results, support a two-stage hydrothermal mineralization model, involving early-stage basinal brines and late-stage meteoric water that acquired metals and heat from crustal sources. [ABSTRACT FROM AUTHOR]

To gain a better understanding of the oceanic environment of the transitional Lower-Middle Cambrian boundary and provide additional data for the global ROECE correlation, we studied the carbon isotope records of shallow-water platform carbonates of the Cambrian Stage 4 Longwangmiao Formation in the Bandenggou section in Chongqing, South China, and correlated with previously reported contemporaneous δ 13 C carb records across a range of depositional environments on the Yangtze Platform and other basins. The δ 13 C carb curve of the Longwangmiao Formation fluctuates frequently approximately 0‰ with multiple excursions at its lower sequence, exhibits a conspicuous negative δ 13 C excursion with a minimum peak value of − 4.76‰ at the middle sequence, and then possesses a stable positive distribution upward. The correlation across the platform-to-basin transect on the Yangtze Platform shows that the Toyonian sections, from slope to basin, have similar shapes of δ 13 C curves, with a depleted trend upward and consistent timings of the largest-amplitude negative δ 13 C excursion, termed ROECE, while contemporaneous shallow-water platform sections have different δ 13 C profile shapes and timings of their largest-amplitude negative δ 13 C excursions, as do sections at a range of depositional environments from a multitude of continents. We infer that the carbon isotopic compositions from the Bandenggou section in the shallow-water environment layer may retain a regional signal. The negative δ 13 C distribution of the lower sequence is primarily due to a local, long-term riverine influx and low relative sea level at the early stage, which amplified organic matter decay and diminished primary productivity. Eventually, this elevated the oxidation and eutrophication of the shallow water, while the stable positive δ 13 C distribution upward is attributed to the integrated influence of a long-term scant riverine flux and high relative sea level. Additionally, the location of ROECE in the shallow-water sections still needs substantiation at the global scale. [ABSTRACT FROM AUTHOR]

A combined investigation of bulk geochemistry and molecular geochemistry was carried out on 31 source rock samples from the Lower Permian Lucaogou Formation, in the Jimsar Sag of the Junggar Basin, northwestern China. Geochemical characterization reveals the depositional environments and other factors favorable for the generation and accumulation of β-carotane, an important hydrocarbon precursor. Many core samples have a total organic carbon content > 1%, and Rock-Eval S 2 values > 2 mg HC/g rock, suggesting relatively high hydrocarbon generation potential. Biomarkers in the Lucaogou source rock extracts are characterized by short- to long-chain n-alkanes, a wide range of Pr/Ph ratios (0.74–1.42) and β-carotane index values (β-carotane/C max , 0.16–1.92), high concentrations of C 29 sterane, relatively low tricyclic terpane content, and moderate gammacerane index values (0.14–0.29). This is consistent with a weakly oxidizing to weakly reducing depositional environment, with fresh to brackish water conditions, rapid input of terrigenous organic matter, and a relatively small contribution of aquatic algal-bacterial organic matter. Cross-plots of biomarker parameters and the β-carotane index show that β-carotane in the Lucaogou source rocks is mainly derived from terrigenous organic matter, and is found predominantly in sediments deposited in weakly reducing, brackish environments. It does not appear to have been affected by thermal maturation. [ABSTRACT FROM AUTHOR]

The Permian-Triassic sedimentary succession of the southern Sydney Basin in southeastern Australia contains a wealth of well-preserved trace fossils that are important for systematic ichnological, palaeoecological, and palaeoenvironmental interpretations. In this study, a new ichnofossil assemblage comprising Macaronichnus , Palaeophycus , Psammichnites , Protovirgularia , Rosselia , and Teichichnus is documented from the tide-influenced shoreface deposit of the Middle Permian Jamberoo Sandstone Member of the Broughton Formation. Two distinct ichnofabrics are recognized: i) the Psammichnites ichnofabric characteristic of the upper–middle shoreface deposit; and ii) the crowded Rosselia ichnofabric (CRI) representing the lower shoreface. The Psammichnites -dominated ichnofabric is interpreted to have resulted from opportunistic behaviours of unknown trace makers, whereas the CRI is interpreted to represent possible strategic behaviours of stress-tolerant polychaetes (e.g., terebellid, spionid, and cirratulid polychaetes) in a distal shoreface with high sedimentation rate. Thus, the Psammichnites –crowded Rosselia ichnofabric succession can be considered to represent a response to an environmental shift from proximal to distal shoreface. As such, this study highlights the utility of these two ichnofabrics as stressed environmental indicators. [ABSTRACT FROM AUTHOR]

During the Early Cretaceous, a large area in Brazil and other South American countries was covered by an extensive paleoerg field, the Botucatu Desert. The Araraquara County (São Paulo State, Brazil) contains some of the most diverse ichnological assemblages in the Botucatu Formation. Mammaliform trace fossils from the Botucatu Formation are of great interest, as they may represent the only record of Lower Cretaceous Mammaliamorpha from Brazil. These trace fossils can be grouped into two distinct classes, based on the dimensions of the footprints. The larger morphotype is described and classified as a new ichnotaxon, Aracoaraichnium leonardii igen. nov. isp. nov., with a discussion of preservational features and paleoecological aspects. This morphotype can be differentiated from Brasilichnium Leonardi, 1981 by digit morphology, with digits III and IV being larger and sub-equal in size. The tracks are mesaxonic, unlike Brasilichnium , which is ectaxonic, and the stride is proportionally smaller, suggesting a trackmaker with shorter limbs in relation to body length. Another characteristic that reinforces the new ichnotaxon is the larger size of the footprints of Aracoaraichnium igen. nov. Footprints representing intermediate ontogenetic stages between Brasilichnium and the larger morphotype are not observed, suggesting that the two track types were made by different taxa. Compared with others tetrapod ichnotaxa, the new ichnotaxon is characterized by differences in the number and morphology of the digits, track morphology, the degree of heteropody, and locomotor parameters. Tracks with good preservation quality were likely originally produced in the humid subsurface, below the surficial dry sand layer. Tracks with smaller dimensions, and trackways with the presence or absence of the manus, may be due to variable degrees of impression into subsurface layers, and substrate heterogeneity. The Aracoaraichnium igen. nov. trackmakers probably inhabited areas near the desert, making occasional incursions into desert areas, while Brasilichnium trackmakers may have been mostly nocturnal residents of the desert environment. [ABSTRACT FROM AUTHOR]

Sediment gravity flow deposits are common, particularly in sandy formations, but their origin has been a matter of debate and there is no consensus about the classification of such deposits. However, sediment gravity flow sandstones are economically important and have the potential to meet a growing demand in oil and gas exploration, so there is a drive to better understand them. This study focuses on sediment gravity flow deposits identified from well cores in Palaeogene deposits from the Liaodong Bay Depression in Bohai Bay Basin, China. We classify the sediment gravity flow deposits into eight lithofacies using lithological characteristics, grain size, and sedimentary structures, and interpret the associated depositional processes. Based on the scale, spatial distribution, and contact relationships of sediment gravity flow deposits, we defined six types of lithofacies associations (LAs) that reflect transformation processes and depositional morphology: LA1 (unconfined proximal breccia deposits), LA2 (confined channel deposits), LA3 (braided-channel lobe deposits), LA4 (unconfined lobe deposits), LA5 (distal sheet deposits), and LA6 (non-channelized sheet deposits). Finally, we established three depositional models that reflect the sedimentological characteristics and depositional processes of sediment gravity flow deposits: (1) slope-apron gravel-rich depositional model, which involves cohesive debris flows deposited as LA1 and dilute turbidity currents deposited as LA5; (2) non-channelized surge-like turbidity current depositional model, which mainly comprises sandy slumping, suspended load dominated turbidity currents, and dilute turbidity currents deposited as LA5 and LA6; and (3) channelized subaqueous-fan depositional model, which consists of non-cohesive bedload dominated turbidity currents, suspended load dominated turbidity currents, and dilute turbidity currents deposited as LA2–LA5, originating from sustained extrabasinal turbidity currents (hyperpycnal flow). The depositional models may be applicable to oil and gas exploration and production from sediment gravity flow systems in similar lacustrine depositional environments elsewhere. [ABSTRACT FROM AUTHOR]

The ∼1500 km diameter Caloris basin is the largest, most well-preserved impact structure on Mercury. The interior of Caloris contains an enigmatic geological unit at depth subsequently excavated by smaller impacts – low-reflectance material (LRM). LRM is interpreted to be the original basin floor material and be composed of melted and re-crystallized mantle material. In this work, a comprehensive numerical modeling study of Caloris basin formation is undertaken to provide insight into basin formation on Mercury (which may differ to that on the Moon), and to address the origin of the LRM. Analysis of the modeled basin attributes demonstrates that the Caloris impact would have penetrated to depths approaching the core/mantle boundary, excavating mantle material to the surface and producing melt volumes on the order of 10 7 km 3 . Such a melt volume could differentiate, forming a layer (buried beneath later volcanism) that could be the LRM, in agreement with previous spectroscopic analyses. Furthermore, LRM deposits at other smaller basins on Mercury may also represent impact melted mantle material. A comparison to lunar basin counterparts demonstrates that basin formation on Mercury is similar to that on the Moon. This observation suggests that the observed difference in the number of large basins between the two bodies (fewer on Mercury) is due to processes that modify basins (i.e., greater impact melt volume and more extensive volcanism on Mercury), rather than first-order impact process differences. The greater average impact velocity and gravity exert a slight effect on some basin properties, including an increased excavation depth-to-transient crater diameter ratio on Mercury. [ABSTRACT FROM AUTHOR]

Mantle convection could have a significant effect on basin evolution; however, research quantifying this relationship is controversial. To understand the formation mechanism and evolution of the Cenozoic rift basins in eastern China, we applied the back-stripping technique and strain rate inversion modeling to 119 wells from Sangjiang Basin, Bohai Bay Basin, North Yellow Sea Basin, South Yellow Sea Basin, East China Sea Shelf Basin, and four basins within the northern South China Sea margin. The modeled results can be used to reconstruct the tectonic subsidence history and further assess the potential subsidence mechanisms of eastern China. Residual subsidence is defined as the difference between the theoretical and observed tectonic subsidences. Our results show that the residual subsidence since 20 Ma in eastern China generally increases from ∼100 to 300 m in terrestrial areas to ∼1.2–1.8 km on the continental shelf. Our observed residual subsidence is generally consistent with the present-day dynamic topography that is predicted from mantle flow models, and it is associated with a stagnant slab within the mantle that has been observed by seismic tomography. The migration pathway of the residual subsidence since 20 Ma is consistent with the movement direction of the Pacific and Philippine plates. Therefore, we suggest that the residual subsidence might be a dynamic subsidence induced by a negative buoyancy of the sinking slab in the deep mantle beneath eastern China. [ABSTRACT FROM AUTHOR]

The origin of anhydrite caprocks found in many evaporite basins is currently under debate. Existing models in the literature for anhydrite formation include rotated stratigraphic layering, rotated crestal caprock and diagenetic anhydrite. In this work, we interpret high-quality 2D seismic reflection data to understand the processes associated with anhydrite caprock formation. Our study focuses on anhydrite caprock formation in the Tromsø Basin, which represents one of several underexplored salt basins in the Arctic. We use well log data and seismic stratigraphic as well as seismic attribute analyses to characterize the ‘anhydrite’ layers associated with salt diapirs. Our results reveal that the anhydrite layers are high-amplitude reflections at the crests, which are often found close to the seabed and lateral caprocks on the flanks of salt diapirs. A third class of anhydrite layers is folded, faulted and boudinaged intrasalt stringers encased by the salt diapirs. Lateral caprocks in the Tromsø Basin are formed in response to rotated crestal caprock failure. The crestal caprocks are secondary in origin, occur at shallow depths, and are formed as insoluble residues from the dissolution of halite by the ingress of basinal fluids into the tops of diapirs. Anhydrite stringers, or floaters, originate from tectonic-related multi-dimensional salt flow. Salt diapirs in the Tromsø Basin are associated with varied anhydrite caprocks; their formation cannot be fully explained using a single model. Our work demonstrates from seismic reflection data the occurrence of caprock events that formed progressively on the flanks of salt diapirs due to local folding. [ABSTRACT FROM AUTHOR]

The image logs played a key role in interpreting the lithology and sedimentary structures of uncored units. An integrated analysis of the borehole images and open-hole logs, which are calibrated with core observations, is conducted to reveal the detailed sedimentary characteristics of the Upper Triassic Xujiahe Formation in the Central Sichuan Basin. To establish borehole image interpretation guidelines, the interpretations of sedimentary textures and lithology from cores and outcrops are calibrated with borehole images and conventional logs. Lithologies in the Xujiahe Formation include conglomerates, fine to medium-grained sandstones, siltstones, mudstones and coals. Sedimentary structures in terms of bedding, scour surface and stacking style of beds are interpreted from the image and open-hole logs. Depositional microfacies are defined on the basis of grain size and sediment texture, sedimentary structures, thickness, basal and upper contacts derived from core observations, openhole logs and image logs. The Xujiahe Formation sandstones in Central Sichuan Basin are deposited in a braided delta front, of which the depositional microfacies consist of underwater distributary channel, river mouth bar, distal bar or sand sheet and underwater interdistributary bay. The depositional microfacies is predicted in a single well using a combination of openhole conventional logs, borehole images, and cores. The distributary channels, which show box-shaped or bell-shaped GR log character, are generally infilled with fine to medium-grained sandstone with massive, tabular, wedge-shaped or trough-cross-laminated bedding. Repeated fining-upwards cycles, as well as the stacked nature of individual bedsets and the presence of scour surfaces represent multistory, amalgamated, channel-fill deposits. The underwater distributary channel deposits can also be overlain by interdistributary bay mudstones, known as channel abandonment. Mouth-bar deposits, which are characterized by coarsening- up successions, infill the channels as they migrate laterally, and sometimes could be overlain by a channel deposit. The methodology has potential application to other braided delta front reservoirs with appropriate calibration and scaling. [ABSTRACT FROM AUTHOR]

A reservoir in the upper Miocene Huangliu formation from the Dongfang (DF) area of Yinggehai Basin is characterized by overpressure and hydrothermal fluids with CO 2 accumulations. The shallow-marine gravity-flow deposits of the Huangliu formation provide an opportunity to study the impact factors on reservoir quality under overpressure and a CO 2 -rich environment. The current study examined the impact of sandbody types, diagenetic alteration, high-overpressure and CO 2 accumulations on reservoir quality by comparing between the DF-A (CO 2 -rich) and DF-B (CO 2 -poor) areas. The channel sandbody, sheet sandbody and neritic sandbar are recognized from the Huangliu formation in the study area. The sandbody type is the most important factor for controlling reservoir quality. The bottom current reworked channel (BCRC) and reworked sheet sandstones of gas layer #I in the DF-B area have the best reservoir properties. This is due to winnowing away the original (very) fine component (such as mud) by bottom current, resulting in a slight increase of grain size and marked increase of pore-throat radius and permeability. Mechanical compaction and cementation are the main diagenetic factors for reducing primary porosity. High-overpressure can protect the primary porosity to some extent by resisting the compaction effect. The intrusion of late CO 2 -rich hydrothermal fluids affects mesodiagenesis in two ways. On the one hand, it can cause intense dissolution of detrital minerals and early calcite cements, resulting in a higher proportion of secondary porosity in the CO 2 -rich DF-A area than in the CO 2 -poor DF-B area. On the other hand, it can result in precipitation of cements, such as siderite and ankerite, through CO 2 -water-rock reactions. Overall, CO 2 accumulations has an insignificant contribution on the increase of porosity. Intense dissolution by CO 2 accumulations can increase the connectivity of pores and permeability, whereas precipitation of minerals leads to decreased permeability. [ABSTRACT FROM AUTHOR]

Sturtian cap carbonate deposits, previously unknown in South China, are documented in this study from the base of the Datangpo Formation in the Nanhua Basin (Guizhou Province). The age of these deposits is constrained by an LA-ICP-MS U-Pb zircon age of 662.7 ± 6.2 Ma from the overlying Mn-shale layer, which is approximately coeval with the termination of the Sturtian glaciation globally. The newly discovered cap carbonates are composed of massive, laminated, pisolitic, and sandy dolomites, and formed subtidally on the summits of horsts within the Nanhua Basin. Two δ 13 C carb profiles (from the JJS-1 and ZK01 sections) show positive shifts of 4–5‰ from the base to the top of the cap carbonates with a ∼2‰ offset between the profiles. These features suggest both a large global perturbation as well as local influences on carbonate carbon isotopes during the Sturtian deglaciation, which is consistent with aspects of both the Snowball Earth and gas hydrate destabilization hypotheses for the origin of cap carbonates. The newly discovered cap carbonates are inferred to be correlative with rhodochrosite (Mn-carbonate)-bearing deposits of the basal Datangpo Formation in adjacent grabens of the Nanhua Basin. This correlation is based on (1) the equivalent stratigraphic positions of the cap carbonates and Mn-deposits, both of which are found at the base of the Datangpo Formation, and (2) consistent U-Pb dates of 667–663 Ma for tuff layers interbedded with these deposits at two locales. The Mn-deposits show significantly higher TOC content (1–3% vs <0.7%) and more negative δ 13 C carb values (−10.5 to −5.5‰ vs −4.0 to +2.5‰) than the cap carbonates. The upsection trend toward higher δ 13 C carb in the cap carbonates may reflect increases in marine productivity and the burial rate of organic carbon following the Sturtian glaciation. Despite coeval formation, the dolomitic cap carbonates deposited on horsts and the Mn-carbonate sediments deposited in grabens are heterogeneous facies with different formation mechanisms. [ABSTRACT FROM AUTHOR]

The Mesoproterozoic Ikpiarjuk Formation (Borden Basin, Nunavut) consists of a series of very large (>200 m thick; kms diameter) dolostone mounds that accumulated on the floor of a restricted basin. The mounds are isolated from contemporaneous shallow-water carbonate rocks, accumulated at the same time as black shale in deep-water regions of the basin, and are geographically limited to the vicinity of basin-scale fault zones. The mounds contain extensive deposits of benthic thrombolites, a microbial texture that is not common in Mesoproterozoic carbonate rocks. Field relationships of individual mounds indicate that mound growth was initiated during regional deposition of organic-rich black shale, mounds developed significant relief above the basin floor, and some mound tops were eventually exposed and karsted during differential uplift. The mounds accumulated in a deep-water, sub-photic-zone setting as a result of fluid venting along basin-scale faults. Interpreted basin depths indicate that the thrombolitic mound texture is probably not related to cyanobacteria. Chemosynthetic microbes exploited the chemical gradient in the vicinity of the vents and built a mound framework of thrombolites. Framework voids contain both carbonate mud and isopachous linings of fibrous cement, indicating that the mixing of vent fluid with basin water favoured production of abiogenic carbonate minerals in addition to providing a chemical gradient for microbes to exploit. Proterozoic deep-water carbonates and thrombolites are rare, and the Ikpiarjuk Formation mounds represent microbial buildups that are fundamentally different from other examples in the Precambrian rock record. [ABSTRACT FROM AUTHOR]

To better understand reservoir quality and to produce accurate petrophysical interpretations, it is necessary to understand complex and heterogeneous pore throat structures in tight sands and to develop a technique to reveal the full range of pore and throat distributions. In this study, in order to characterize the features and evolutions of pore throat structures, nine samples from the Lower Cretaceous tight gas sandstone in the Songliao Basin of China are measured by nuclear magnetic resonance (NMR), rate-controlled porosimetry (RCP) and scanning electron microscopy (SEM). Throats with bifractal structures are found in these tight sands and can be divided into a backbone formation (BF) region and a percolation region using RCP data. Because (i) throats in the percolation region record treelike pore structures and are predominant in small pore spaces and (ii) a good correlation exists between NMR-derived T 2 relaxation times and the RCP-derived radius of throats, the throat distribution obtained via RCP can be used to calibrate the NMR PSD and then to partition the PSD into distributions of pore bodies and throats. These data indicate that (i) throats are more common than pore bodies in pore spaces of tight sands with lower permeability, (ii) pore bodies are connected to throats from both the BF and percolation regions, in which the fluid mobility in pore bodies is mainly controlled by the pore to throat ratio (PTR), which is related to the intersection throat of these two regions, and (iii) compaction, dissolution, clay cementation and sediment properties (e.g., rock compositions) have different impacts on the evolutions of pore bodies and throats, in which larger PTR values in tight sands are mainly produced by compaction and pore-bridging clay cementation, and lower contents of pore bodies are commonly related to abundant plastic compositions and pore-bridging clay cementation. [ABSTRACT FROM AUTHOR]

The occurrence of hydrocarbons in self-sourced reservoirs strongly depends on the concentration and maturity of organic matter in sediments. Therefore, understanding the distribution of organic heterogeneity at the time of deposition is key to reduce the risk in exploration and development of unconventional resources. This study focuses on the Lower and Middle Triassic Montney and Doig Formations (Alberta and British Columbia). Samples from outcrops, cores and cuttings were analyzed for organic content with a Rock-Eval VI and for major and trace element concentration using ICP-MS and ICP-AES techniques. The interpretation of the analysis results in relation with the stratigraphic architecture provides a mean to better understand the distribution of the organic heterogeneities and the variations of primary productivity, sedimentation rates and anoxia that control the development of source rocks. The key findings of this analysis are: - The basin-scale distribution of the organic matter suggests that the two major source-rock intervals of the Lower and Middle Triassic correspond to the sequence 3 (Spathian Montney unit) and to the transgressive systems tract of sequence 4 (Doig phosphate zone). - The dominant controls on organic matter accumulations vary through time. The Montney source-rock interval is interpreted to be associated with a major basin restriction triggering anoxia during a second-order falling stage of relative sea level. The organic accumulation of the Doig phosphate zone is interpreted as being controlled by a sharp decrease of the sedimentation rate, combined with an increase of the primary productivity. - The spatial and temporal variations of anoxia, primary productivity and dilution reflect the geodynamic evolution of the basin that ultimately controls the basin physiography as well as the sources of nutrient and sediments. [ABSTRACT FROM AUTHOR]

The ongoing exploration activities at the Maiganga coal mine uncover some missing gaps in the stratigraphy of the Late Cretaceous Gombe Formation. Therefore, there is a need to further investigate this sequence, including the organic facies variations and distributions of the coals, mudstones and sandy-shaly sediments in the Gombe Sandstone (Formation), Gongola Sub-basin, Northern Benue Trough of Northeastern Nigeria. Geochemical and petrographic methods were used to assess the organic matter source input, paleodepositional conditions, thermal maturity and hydrocarbon generation potential of the analysed Maiganga and Yaya-Ngari samples. Field observations showed four coal seams interbedded with mudstones and shales deposited in coarsening upward sequence of a deltaic environment. Evaluation based on the source rock analyses (SRA) reveals that Maiganga sedimentary facies is dominated by terrestrial source input of Type III kerogen. This is supported by the biomarker study suggesting land plant origin for the organic matter with minor lacustrine and marine influences, in Maiganga, that grades into predominantly mixed organic matter input of a transitional (terrestrial to marine) environment in the Yaya-Ngari area. This shows a progressive movement towards marine source input. Paleodepositional conditions of organic matters were sub-oxic to oxic in Maiganga, while the mixed organic matters in Yaya-Ngari were deposited in sub-oxic conditions. Based on organic petrology and bitumen extraction data, the Maiganga coal was distinctly observed to be immature of pre-oil generation window, while the Yaya-Ngari is in early oil generation window. These analysed sediments are mainly gas prone based on the dominance of Type III kerogen, although the presence of Type II/III kerogen supported by relatively higher HI may indicate gas/oil potential for the Maiganga coal. On the other hand, Type IV kerogen dominated the Yaya-Ngari samples. This suggests that these shales are gas prone or do not possess any hydrocarbon generation potential as indicated by the very low HI values. [ABSTRACT FROM AUTHOR]

The Yanchang Formation in the Ordos Basin is the most important petroleum play not only for conventional oil and gas accumulations, but also for newly emerging shale oil and tight gas resources. The molecular characterization of the basinwide source rocks predicts three groups of generative petroleum types: Paraffinic High Wax Oil, mixed base (Paraffinic-Naphthenic-Aromatic) Low Wax Oil, and Gas and Condensate. Supplementary to previous work, 68 samples including the crude oils, source bitumens and reservoir extracts from the Yanchang petroleum play are analyzed. The distribution of two terpane classes (eight tricyclic terpanes and eight pentacyclic terpanes) are determined with subsequent simultaneous RQ-mode factor analysis for a composite data set of these samples alongside 216 published crude oils worldwide with known facies descriptions. Thermal maturity has been evaluated as a consistent distribution at first using a combined method of a maturity-related biomarker [Ts / (Ts + Tm)] and aromatic parameters (Methyldibenzothiophene Ratios) to alleviate the maturity differences effect when discussing geochemical characterization. The R-mode factor analysis consists of the first two factors that are describing 45 present of the cumulative total variance in the data set, and presents a sample grouping pattern in Q-mode factor analysis which is determined by different contributions of terpane associations, i.e., the tricyclic C 21 coupled with pentacyclic C 26 , C 27 , C 28 and C 30 , in the same factor space. Three terpane associations, the C 26 and C 28 terpanes, the C 21 and C 30 terpanes and the C 27 pentacyclic terpenes, are respectively responsible for discriminating crude oil, reservoir extracts and source bitumens in RQ-mode factor analysis. Molecular compositions further address more detailed interrelationships among three sample groups that crude oils and reservoir extracts are sharing close genetic relationships both in depositional environment typing and C 27 -C 28 -C 29 sterane distribution. Samples from source rocks vary much significantly. A mixing process which occurs after oils has been expelled from host source rocks into carrier units during accumulation. In addition, the migration-contamination of C 29 sterols when oils are cross through the Chang 7–2 unit along migration pathways might also explain this lack of correlation between source rocks and oil-reservoir system. [ABSTRACT FROM AUTHOR]

The opening of the Gulf of Mexico during the Mesozoic led to the formation of the Sabinas Basin. Large carbonate platforms were developed throughout the Lower and Middle Cretaceous. The basin provided ideal conditions for the formation of a suite of carbonate-hosted, stratabound deposits such as barite, celestine, fluorite, and lead–zinc of Barremian–Aptian age. These deposits resemble Mississippi Valley-type (MVT) and associated deposits. The mining district of Sierra de Santa Rosa is located approximately ~ 7 km SE from Melchor Múzquiz in the state of Coahuila, Mexico. Barite is the economic mineral and the shape of the ore bodies is considered “mantos”, the gangue minerals are calcite, local gypsum, traces of celestine, silica, and iron (oxy) hydroxides. The barite deposits show relict textures such as rhythmic, alternating black and white bands due to the presence of organic matter, and globular clusters similar to the “chicken-wire” anhydrite, typical of evaporites. A fluid inclusion and stable isotope analysis (S from barite, C and O from carbonates) were conducted. The lower manto yielded a melting ice temperature between − 26 °C and − 5 °C (salinities of 7.9 to 27 wt.% NaCl equiv.) and a homogenization temperature ranged between 59 °C to 155 °C. The eutectic temperature was − 51 °C ± 2 °C denoting a primary calcic brine. The upper manto yielded a melting ice temperature between − 22 °C and − 15 °C (salinities of 18.6 and 24 wt.% NaCl equiv.) and a homogenization temperature was ranging from 60 °C to 126 °C. Isotopic analysis of barite showed δ 34 S VCDT ranges from + 14.9‰ to + 19.5‰ (average of 16.9‰). Sulfur isotope data for barite from the Sierra de Santa Rosa is consistent with a sulfur source formed during the Lower Cretaceous, which coincides with the age of the Cupido Formation. The carbon isotope analysis of the host limestone yielded a δ 13 C VPDB range from − 0.01‰ to + 0.11‰. The δ 13 C values for clear and gray calcites ranged from − 0.15‰ to − 1.5‰, and − 1.41‰ to − 2.3‰, respectively. The oxygen isotope analysis showed a range between δ 18 O VSMOW − 4.55‰ and − 10.04‰. Fluid inclusion microthermometry and isotopic measurements lead us to conclude that brines from the Sabinas Basin led to the replacement of the evaporite strata (gypsum) by barite in the Cupido Formation and thus classify these deposits within the category of MVT and associated deposits. [ABSTRACT FROM AUTHOR]

Widmerpool Gulf is one of the basins in the carboniferous Bowland Hodder shale in Central Britain. The area of interest in the Widmerpool Gulf is the Bowland Shale Formation within the Upper Bowland. This paper focuses on evaluating the total organic carbon (TOC) from Rempstone 1 and Old Dalby 1 both penetrating the Bowland Shale Formation using Passey's delta Log R method. TOC is an essential property needed to have a productive shale gas play – accurate characterisation of TOC is critical in evaluating the potential of shale gas reservoirs because it closely relates to the amount of kerogen and therefore total gas content. Passey's method involves the use of well logs to obtain the organic richness i.e. TOC. It relies on the response of well logs to organic matter using an overlay of porosity logs and resistivity logs. Which causes a separation i.e. a delta Log R separation. The reasons for the delta Log R separation in an organic rich reservoir/non reservoir rocks is due to the presence of low density, high transit time kerogen and the presence of formation fluid i.e. oil, gas or water. Immature source rocks also observe a separation but only from the porosity log whilst non source rocks observe no separation. Rempstone 1 and Old Dalby 1 well logs were exported on Interactive Petrophysics to analyze the lithology of the intervals before been exported to Microsoft Excel to evaluate the parameters of Passey's equation to enable TOC determination. TOC estimates for the Upper Bowland Shale section in the Widmerpool Gulf and was found comparable to estimates obtained from British Geological Survey (BGS) RockEval core analysis report. This method proved to be a suitable way of estimating the total organic carbon using simple well logs. [ABSTRACT FROM AUTHOR]

Continental rifting and ocean basin formation is occurring today in the Red Sea, providing a possible modern analogue for the creation of mid-ocean ridges. Yet many of the seafloor features observed along the axis of the Red Sea appear anomalous compared to ancient and modern examples of mid-ocean ridges in other parts of the world, making it unclear, until recently, whether the Red Sea is truly analogous. Recent work suggests that the main morphological differences between the Red Sea Rift (RSR) and other mid-ocean ridges are due to the presence and movement of giant, submarine salt flows, which blanket large portions of the rift valley and thereby the oceanic crust. Using ship-based, high-resolution multibeam bathymetry of the central RSR between 16.5°N and 23°N we focus here on the RSR volcanic terrains not covered by salt and sediments and compare their morphologies to those observed along slow and ultra-slow spreading ridges elsewhere. Regional variations in style and intensity of volcanism can be related to variations in volcanic activity and mantle heat flow. The Red Sea oceanic seafloor shows typical features of mature (ultra)slow-spreading mid-ocean ridges, such as 2nd order discontinuities (overlapping spreading centres) and magma focussing in the segment centres (forming spreading-perpendicular volcanic ridges of thick oceanic crust). The occurrence of melt-salt interaction at locations where salt glaciers blanket the neovolcanic zone, and the absence of large detachment faults are unique features of the central RSR. These features can be related to the young character of the Red Sea and may be applicable to all young oceanic rifts, associated with plumes and/or evaporites. Thus, the RSR falls in line with (ultra)slow-spreading mid-ocean ridges globally, which makes the Red Sea a unique but highly important type example for initiation of slow rifting and seafloor spreading and one of the most interesting targets for future ocean research. [ABSTRACT FROM AUTHOR]

Lithofacies types of the marine shale of the Lower Cambrian Qiongzhusi Formation in the southwestern Sichuan Basin and the continental shale of the fifth member of Upper Triassic Xujiahe Formation in the western Sichuan Basin were classified based on a modified three-end diagram concerning the contents of siliceous minerals, carbonate minerals and clay minerals. Various experiments including X-ray diffraction, low pressure nitrogen adsorption, high pressure methane adsorption, and gas content measurement were designed for comparative analyses among different lithofacies in terms of core, pore structure, methane adsorption and gas-bearing characteristics. Then, the discrepancies between marine and continental shales lithofacies were analyzed, and the effects of lithofacies on gas adsorption and storage capacities were investigated. It is demonstrated that there are mainly five types of lithofacies developed in the marine shale of the study area, namely siliceous shale lithofacies (S), mixed siliceous shale lithofacies (S-2), clay-rich siliceous shale lithofacies (S-3), silica-rich argillaceous shale lithofacies (CM-1), and argillaceous/siliceous mixed shale lithofacies (M-2), while there are also mainly five types of lithofacies developed in the continental shale of the study area, which are carbonate-rich siliceous shale lithofacies (S-1), mixed siliceous shale lithofacies (S-2), clay-rich siliceous shale lithofacies (S-3), silica-rich argillaceous shale lithofacies (CM-1), and argillaceous/siliceous mixed shale lithofacies (M-2). Geological characteristics significantly vary among different lithofacies in terms of core, pore structure, methane adsorption and gas-bearing characteristics. Compared to the continental shale lithofacies, the marine shale lithofacies was characterized by a higher proportion of siliceous shale lithofacies group and a lower proportion of argillaceous shale lithofacies group, which could be attributed to different sedimentary environments and sediment provenances. On the condition that the organic matter content keeps constant, the silica-rich argillaceous shale lithofacies (CM-1) is favorable for adsorbed gas storage due to its strong methane adsorption capacity resulting from the highest clay minerals content, while the siliceous shale lithofacies group is favorable for gas storage due to its well-preserved primary and organic pores resulting from the highest siliceous mineral content. [ABSTRACT FROM AUTHOR]

Although extensive studies have been conducted on the shale reservoir in recent years, little work has been focused on argillaceous dolomite. In this study, investigation of microscopic pore structure in the argillaceous dolomite reservoir in the Xingouzui Formation from the Jianghan Basin has been undertaken to further the understanding of the oil and gas storage mechanism in these rocks. Based on the understanding of pore characteristics by scanning electron microscope (SEM), areal porosity method (APM) and material balance principle (MBP) were respectively used to calculate the inorganic and organic porosities. Moreover, effective porosity and connectedness of the pores were indicated by mercury intrusion porosimetry (MIP) and focused ion beam-scanning electron microscope (FIB-SEM) respectively. Inorganic pores dominate in the argillaceous dolomite reservoir, and there also exist a small amount of organic pores. The calculated inorganic porosities range from 6.37% to 18.58%, close to the measured porosities. The pore radii are mainly within 2.0 μm. With the increase of porosity, pores with large pore radii make much more contribution to porosity. Organic porosities are much lower, with an average of 0.15%, which is meaningless to the porosity of the shale reservoir in the Jianghan Basin. Compared with the mudstones, the argillaceous dolomites have relatively higher porosity values. Pores in the range of 25.5–253.0 nm contribute a larger fraction of the porosity of the argillaceous dolomite, while pores with smaller throat radii (below 22.4 nm) mainly contribute to porosity in the mudstones. Pores in the argillaceous dolomite have good connectivity, although there are some isolated and dispersed pores. [ABSTRACT FROM AUTHOR]

The basal unit of the succession in the Illela borehole belongs to the Dange Formation comprising thick calcareous and variably coloured dark-greyish shale of 36.30 m thick which is overlain by a 31.44 m thick limestone of Kalambaina Formation with 1.7 m thick shaly-limestone inclusive. The uppermost part of the section belongs to the Gwandu Formation which has intercalation of silty-clay, muddy siltstones with well lithified ironstone capping the borehole section. The limestone/carbonate microfacie as deduced from their salient lithologic, sedimentologic and paleontologic features are comparable to standard microfacie (SMF) types 9 and 10, i.e. bioclastic wackestone/bioclastic micrite and packstone-wackestone respectively. Diagenetically, syndepositional and early diagenesis have taken place particularly cementation and replacement in the carbonate rocks and these have greatly affected the reservoir potential negatively. The matrix/grain relationships indicate a shallow marine environment of deposition. The borehole section is delineated into upper foraminifera and lower ostracod biostratigraphic units as no formal biostratigraphic zonation could be attempted due to low diversity of both benthic foraminifera, marine ostracods and the absence of planktonic foraminifera. The similarity of the ostracod assemblages between this study area, Illela borehole, West Africa, North Africa (Libya), Mali and Niger Republic) and South-Western Nigeria (West Africa) suggests that a marine connection exists between the Gulf of Guinea and the Sokoto Basin via the area occupied by the River Niger during the Paleocene. [ABSTRACT FROM AUTHOR]

The Yudongliang Outcrop, located in the northern part of the Sichuan Basin, nicely exposes oolitic shoal complexes of the Lower Triassic Feixianguan Formation. The outcrop is an analogue for the subsurface Longgang and Yuanba gas fields and other Permo-Triassic reservoirs with dolomitized and nondolomitized oolite-dominated grainstone bodies as the main producing zones. Based on five measured sedimentological sections, and petrographic and petrophysical analysis of 200 plugs, the architecture of the reservoir bodies has been reconstructed in detail and placed within a high-frequency sequence-stratigraphic framework. The studied interval belongs to the highstand system tract (HST) of a single composite sequence and comprises three shallowing-upward high-frequency sequences, which consist from base to top of oncoid/peloid/ooid grain-dominated packstone, cross-bedded ooid grainstone, dolograinstone, fenestra-laminated dolomite, and crystalline dolomite. The internal architecture of the oolitic shoal is the result of complex tidal-dominated hydrodynamics. Facies and diagenetic partitioning with each high-frequency sequence are the primary controls on pore types and reservoir properties. Original lithofacies texture and subsequent cementation resulted in interbedded thin effective reservoirs and permeability barriers that created a complex internal architecture of reservoir bodies. The shapes of reservoir bodies generally follow those of the lithofacies, but with the addition of intercalated thin flow barriers. [ABSTRACT FROM AUTHOR]

Pore-throat size is a very crucial factor controlling the reservoir quality and oiliness of tight sandstones, which primarily affects rock-properties such as permeability and drainage capillary pressure. However, the wide range of size makes it difficult to understand their distribution characteristics as well as the specific controls on reservoir quality and oiliness. In order to better understand about pore-throat size distribution, petrographic, scanning electron microscopy (SEM), pressure-controlled mercury injection (PMI), rate-controlled mercury injection (RMI), quantitative grain fluorescence (QGF) and environmental scanning electron microscopy (ESEM) investigations under laboratory pressure conditions were performed on a suite of tight reservoir from the fourth member of the Lower Cretaceous Quantou Formation (K 1 q 4 ) in the southern Songliao Basin, China. The sandstones in this study showed different types of pore structures: intergranular pores, dissolution pores, pores within clay aggregates and even some pores related to micro fractures. The pore-throat sizes vary from nano- to micro-scale. The PMI technique views the pore-throat size ranging from 0.001 μm to 63 μm and revealed that the pore-throats with radius larger than 1.0 μm are rare and the pore-throat size distribution curves show evident fluctuations. RMI measurements indicated that the pore size distribution characteristics of the samples with different porosity and permeability values look similar. The throat size and pore throat radius ratio distribution curves had however significant differences. The overall pore-throat size distribution of the K 1 q 4 tight sandstones was obtained with the combination of the PMI and RMI methods. The permeability is mainly contributed by a small part of larger pore-throats (less than 30%) and the ratio of the smaller pore-throats in the samples increases with decreasing permeability. Although smaller pore-throats have negligible contribution on reservoir flow potential, they are very significant for the reservoir storage capacity. The pore-throats with average radius larger than 1.0 μm mainly exist in reservoirs with permeability higher than 0.1mD. When the permeability is lower than 0.1mD, the sandstones are mainly dominated by pore-throats with average radius from 0.1 μm to 1.0 μm. The ratio of different sized pore-throats controls the permeability of the tight sandstone reservoirs in different ways. We suggest that splitting or organizing key parameters defining permeability systematically into different classes or functions can enhance the ability of formulating predictive models about permeability in tight sandstone reservoirs. The PMI combined with QGF analyses indicate that oil emplacement mainly occurred in the pore-throats with radius larger than about 0.25–0.3 μm. This result is supported by the remnant oil micro-occurrence evidence observed by SEM and ESEM. [ABSTRACT FROM AUTHOR]

Organic-rich black shale of the Upper Yangtze Basin from the Late Ordovician and Early Silurian is considered an excellent source rock in South China. The formation and preservation conditions of this resource are revealed by its geochemical characteristics in this study. Geochemical indices, including redox indices (V/(V + Ni), V/Cr, V/Sc, and Ni/Co) and primary productivity indices (P/Ti and Ba/Al), and paleoclimate, clastic flux and sedimentary rate analyses are presented to investigate the accumulation mechanism of organic matter. Redox indices suggest that a stagnant, anoxic environment predominated in the Upper Yangtze Basin during accumulation of Wufeng and Longmaxi formations. In contrast, ventilated and oxygenated marine conditions pervaded the Upper Yangtze Basin during deposition of Linxiang and Guanyinqiao formations. The concentrations of V and U demonstrate that accumulation of organic matter was mainly controlled by redox conditions. Besides, such factors as clastic fluxes, fresh water inflows or a mixed deposition with a rapid sedimentary rate cannot be ignored due to their influences on organic matter enrichment and preservation. However, weak co-variance relationship of TOC content and productivity proxies, including P/Ti and Ba/Al, demonstrates that the accumulation of organic matter was not controlled by primary productivity. Results of the present study suggest a depositional model that stresses the importance of tectonic movements and glacial events on the accumulation and preservation of organic matter. The model shows that the Upper Yangtze Basin was a semi-restricted basin system influenced by the isolation of Xuefeng, but also it implies that oxygen-depleted bottom water of the basin favored the accumulation and preservation of sedimentary organic matter, resulting in the formation of organic-rich black shale. [ABSTRACT FROM AUTHOR]

Source rock formation influenced by river-delta system, especially in continental margin basins, is still poorly understood. This article aimed to reveal the effect of river-delta system on the formation of the source rock by taking the Baiyun Sag of the Pearl River Mouth Basin for example. Paleo-Pearl River began to develop since the Enping Formation, providing abundant organic matter beneficial for the formation of the source rocks in the Baiyun Sag. The main controlling factor of source rock formation in the Baiyun Sag is terrestrial organic matter supply rather than the paleoproductivity or redox conditions. Low Al/Ti and P/Ti ratios suggest low marine productivity, which may be associated with a large number of terrigenous detritus input, occupying about 43.04%–94.91%. There is a positive correlation between the oleanane/C 30 hopane ratio and the TOC value, showing that terrigenous organic matter controls the source rock formation. The size of the delta below Pearl River estuary determines the extent of terrestrial organic matter supply. Source rocks with high organic matter abundance mainly formed in delta environment, and those in neritic environment in Enping and Zhuhai Formations also have high TOC values as a result of adequate terrestrial organic matter supply. [ABSTRACT FROM AUTHOR]

Field emission scanning electron microscope (FESEM) is a widely used technique to examine pore structures in shale reservoirs. Prior to a FESEM scan, one must decide upon the location of sample to be viewed as well as the magnification ratio to be used, which is, however, prone to human errors. One common practice in FESEM study is that one tends to use a fixed magnification ratio for observing the pore structure on a given location of a sample. Also, the pore structure observed based on a given magnification factor cannot entirely represent the true pore structure characteristics of the shale sample, primarily due to the multi-scale heterogeneity of the shale rocks. To improve the FESEM characterization of pore structure in shale samples, we propose a methodology to reveal the multi-scale characteristics of shale samples. In this work, we propose an improved method for taking multiple FESEM images for performing more accurate characterization of the pore structures in shale samples. With the improved method, we can determine the appropriate number of FESEM images to be taken as well as the appropriate magnification ratio to be used in FESEM scan. A given magnification ratio is used throughout all the FESEM scans. This method can be used to retrieve important multiscale pore-structure characteristics of shale samples. We demonstrate the efficacy of the improved FESEM characterization technique by applying it to a shale gas formation in Sichuan basin in China. [ABSTRACT FROM AUTHOR]