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During the Middle to Late Ordovician, the Sino-Korean Plate changed its movement from diverging to converging along its southern and western margins. This resulted in shifts of the sedimentary facies in the Ordos Basin: the environment changed from carbonate platforms in an epeiric sea to deep-sea slopes and a deep-sea basin. This had more than regional consequences: the area is not only the key for understanding the evolution of the tectonic activity and sedimentary environment of the Sino-Korean Plate, but also for a better insight into the structural history of the Yangtze Plate, and the Qinling Orogene and the Qilian Orogene. In addition to autochthonous sediments, slumps, debrites, turbidites and contourites are present, as found both in the field and in drilling cores. The lithological successions allow, in combination with well-logging data, facies correlations of the Ordovician rocks. It appears that the sedimentary facies along the southern and western margins of the Sino-Korean Plate show distinct shifts during the Middle-Late Ordovician. These facies shifts were a consequence of the change in the tectonic regime of the basin from divergence to convergence. A 3-D structural and depositional model of the passive plate margin and carbonate platform within its finally compressive setting is compared with the model of the ocean-trench/island arcs/back-arc rift basin/slope/platform in its initial tensional setting. These models illustrate the change of the structural regime in combination with the resulting shifts in sedimentary facies. The present contribution thus provides a link between geodynamics and sedimentary basin filling.

Tight sandstone gas reservoirs constitute an important type of unconventional gas resources and may be of crucial significance as environment-friendly source of energy in the world. However, the dynamics of densification and evolution of the low-porosity and low-permeability sandstone reservoirs are intensively debated in literature. Recently, the question of sequence of reservoir densification and hydrocarbon accumulation has arisen. A typical tight sandstone reservoir (the first member of the Shanxi Formation and the eighth member of Shihezi Formation) in the Permian Sulige gas field in Central China provides an excellent case study to probe these questions. By using a variety of different methodologies such as scanning electron microscopy (SEM), X-ray diffraction (XRD), cathodoluminescence, fluid inclusion analysis and confocal Raman spectrometer analysis, we identified different authigenic minerals such as illite, kaolinite, dickite, chlorite and smectite illitization coexisting with siliceous and calcitic cements. These authigenic minerals precipitated at different diagenetic stages, related to the changing conditions during compaction and lithification, and therefore, changed the porosities of the host rocks during each of these stages. The types of pores in the Sulige gas field are dominated by illite and kaolinite intercrystalline pores, matrix solution pores, and primary intergranular pores. The results of this study suggest that compaction and cementation are the reasons for the most distinct loss in porosity. On the other hand, kaolinite and illite preserve their intercrystalline pores, which could be identified as the main reserve space of the tight gas reservoirs. Therefore, the secondary porosity of the sandstone reservoirs forms the most important contributor to the gas reservoir potential in the Sulige gas field. In addition, it could be inferred that the tight gas accumulation took place during the densification of the reservoirs. The implications of the presented results may also help to understand the formation of other tight reservoirs world-wide.

A large, typical, marine, organic-rich shale is developed in the Ordovician Wufeng Formation-Silurian Longmaxi Formation in the Sichuan Basin, China, which has rich shale gas resources. This paper uses this type of shale as an example to analyze the influence of the depositional environment on the enrichment mechanisms and sedimentary processes of its organic matter (OM). Based on thin sections and field emission scanning electron microscope observations from well core samples, along with X-ray diffraction and geochemical analysis, the mineralogy, organic and inorganic geochemistry characteristics, as well as the lithofacies, are analyzed. The results show that the shale is mainly composed of quartz and clay minerals but also contains calcite, feldspar, pyrite, etc. The OM includes solid bitumen, algae, graptolite, spongy spicule, radiolarians, etc. The total organic carbon (TOC) content ranges between 0.5% and 6.17%, with an average value of 2.5%. Based on the mineral compositions, the TOC content and sedimentary structure, five kinds of lithofacies are identified. According to the primary productivity, redox conditions, paleosalinity and sedimentation rate characteristics, the formation's sedimentary environment is divided into five intervals. During the OM enrichment processes, the primary productivity, sedimentation rate and redox conditions affect the generation, accumulation and preservation of OM. There is a good positive correlation between primary productivity and TOC content, as well as the redox condition. Sedimentation rate is inversely proportional to TOC content. The change and evolution of the sedimentary environment over these five intervals form different OM occurrence characteristics and associations with lithofacies. Based on the characteristics of OM, lithofacies and the sedimentary environment, as well as the geological background of the study area, the mode of OM occurrence and enrichment was proposed.

In the purview of groundwater pollution, a rational exploitation, utilization, protection, and risk assessment of groundwater has become an important means of groundwater management and pollution prevention. Xiashan Economic Zone of Weifang City which is located in the central region of the Shandong Peninsula, China, has been chosen to investigate the risk assessment of pollution by heavy metals (Pb, Cr, Cd, and Hg) permeating into groundwater periodically through the soil layers or the vadose zone. This study deals with the risk assessment and forewarning model of groundwater pollution by the use of comprehensive risk index method along with GIS overlay analysis and groundwater pollution forewarning system. Soil samples (n = 988) and water samples (n = 327) from groundwater resources of Xiashan Economic Zone and eight major factors, i.e., runoff factor, overburden strata factor, atmospheric precipitation factor, pollutant source type, pollutant generation quantity, pollutant release possibility, water yield from aquifers, and groundwater conservation district, have been thoroughly studied. Based on data analysis and study, groundwater pollution risk assessment zonation map and groundwater forewarning distribution map were established. The results show that the distribution area accounting for 3.07% of the total area is of medium warning with high risk of pollution. Mainly spread on both banks of the river, the serious warning zone is covering 26.80% of the total area for groundwater, and the groundwater is distressed with a risk of moderate pollution. While the tremendous warning area for groundwater pollution is distributed in over 70.13% of the total area, thus, putting in a state of high-risk pollution. The warning degree of groundwater pollution in typical water source areas was identified, and the possible alarming warning instance can be forecast. Based on the warning degree, the governance protection area and key protection area were divided. The corresponding prevention and control measures were put forward after formulating the prevention and control plan of groundwater pollution based on this kind of study.

Interpretation of the continental or marine character of sediments on the basis of the paleosalinity is increasingly based on the concentration of some trace elements. The paleosalinity can be affected, however, also in deep-marine environments, by discharge peaks of major sediment-laden rivers. These induce hyperpycnal flows that run down the basin slope. The huge volume of fresh water gradually mixes with the ambient saline seawater, diminishing the paleosalinity. The temporary lowered paleosalinity may be preserved in the sedimentary record in the form of ‘diverging’ concentrations of trace elements and trace-element ratios. This is exemplified for the Early Cretaceous Lingshandao Formation on Lingshan Island (western Yellow Sea). It was found during field work and geochemical analysis that hyperpycnal currents may not only lower the salinity of a deep-marine environmental setting, but may also carry along continental plant fragments and other biological remains, which may obscure the marine character of the sediments. It is deduced that detailed facies analysis in the field is consequently required to reveal the true character of a marine environment.

The ongoing exploration for shale oil and gas has focused sedimentological research on the transport and deposition mechanisms of fine-grained sediments, and more specifically on fine-grained mass-flow deposits. It appears, however, that no easily applicable classification scheme for gravity-flow deposits exists, and that such classifications almost exclusively deal with sandy and coarser sediments. Since the lack of a good classification system for fine-grained gravity flow deposits hampers scientific communication and understanding, we propose a classification scheme on the basis of the mud content in combination with the presumed transport mechanism. This results in twelve types of gravity-flow deposits. In order to show the practical applicability of this classification system, we apply it to the Triassic lacustrine Yanchang Formation in the southern Ordos Basin (China), which contains numerous slumps, debris–flows deposits, turbidites and hyperpycnites. The slumps and debrites occur mostly close to a delta front, and the turbidites and hyperpycnites extend over large areas from the delta slopes into the basin plain. The case study shows that (1) mud cannot only be transported but also deposited under active hydrodynamic conditions; (2) fine-grained gravity-flow constitute a significant part of the lacustrine mudstones and shales; (3) muddy gravity flows are important for the transport and deposition of clastic particles, clay minerals and organic matter, and thus are important mechanisms involved in the generation of hydrocarbons, also largely determining the reservoir capability for unconventional petroleum.

The Anambra Basin in the west of the lower Benue Trough is one of Nigeria's most petroliferous provinces. The subsurface structure has been extensively studied using various geophysical methods except for the mapping of structural lineaments . For the present study, the satellite-derived World Gravity Map 2012 (WGM 2012) global gravity model was utilized to study structural features located at different depths. Upward continuation at the height of 30 km was used to extract the regional anomaly from the Bouguer anomaly . The residual anomaly was determined after the deduction of the regional gravity anomaly from the Bouguer anomaly. The total horizontal derivative (THD) of the complete Bouguer, regional, and residual anomalies was used to map the lineaments. The Bouguer gravity anomaly ranges from −58 to more than +28 mGal and presents two generic trends in the E-W and NNE-SSW directions. The regional Bouguer anomaly reveals a Moho depth of 38.0 km from the CRUST1.0 model. The amplitude pattern of the residual gravity anomaly was used to distinguish seven high residual anomalies (HR1-HR7) and four low residual anomalies (HL1-HL4). The residual anomalies of HR1-HR7 are inferred to be hypabyssal igneous rocks. The low residual anomalies of HL1-HL4 are interpreted to be Paleogene and Cretaceous deposits of the Niger Delta complex, and the Bende-Ameki, Imo, Nsukka, Ajali, Mamu, and Nkporo formations. Based on THD analysis, the major structural trends are found to be in NE-SW and NW-SE directions. In addition, 2.5-dimensional forward modelling using the residual Bouguer anomaly along profile CD was performed, providing a variation of the basement depth between 3.5 km and 6.5 km. Therefore, the sedimentary thickness is adequate for the recommended minimum depth for source rock formation and petroleum accumulation with respect to the other petroleum system elements.

Fine-grained sediments from the Late Triassic Yanchang Fm. in the Ordos Basin (central China) were studied by core analysis and geophysical logging. Part of the mudstones in this formation are stratified, part of them are unstratified; the various mudstones can be subdivided into eight types on the basis of their structures and textures. They represent a variety of environments, ranging from delta fronts and subaqueous fans to deep-water environments. Part of the sediments were reworked and became eventually deposited from subaqueous gravity flows, such as mud flows, turbidity currents and hyperpycnal flows that easily developed on the clay-rich deltaic slopes. The sediments deposited by such gravity flows show abundant soft-sediment deformation structures. Understanding of such structures and recognition of fine-grained sediments as gravity-flow deposits is significant for the exploration of potential hydrocarbon occurrences. Because fine-grained deposits become increasingly important for hydrocarbon exploration, and because the sediments in the lacustrine Yanchang Formation were deposited by exactly the same processes that play a role in the accumulation of deltaic and prodeltaic fine-grained sediments, the sedimentological analysis provided here is not only important for the understanding of deep lacustrine sediments like the Yanchang Formation, but also for a better insight into the accumulation of fine-grained prodeltaic deep-marine sediments and their potential as hydrocarbon source rocks and reservoir rocks.

Pebbles up to 10 cm in diameter, as the main clastic component of gas reservoirs in the Permian Sulige Gas Field, deposited more than 150 km away from their northward provenance (Yinshan Mountains) in the northern Ordos Basin. The transportation and depositional mechanisms of these pebbles have been unclear for decades. Based on core examinations, facies analysis, and mathematical modeling (stress field calculation of pebbles), it is thought that (1) these coarse-grained deposits were transported and deposited in braided rivers; (2) the size of coarse-grained sediments and pebbles in the Sulige Gas Field is in accordance with the size of flash flood; (3) flash flood is regarded as a crucial important transporting mechanism of pebbles; and (4) the “flash flood genesis” braided river-braided delta sedimentary model provides a reasonable understanding for prediction of coarse-grained reservoirs for gas exploration not only in the Permian Sulige Gas Field but also in similar coarse-grained sediments elsewhere.

During the Upper Paleozoic large-scale tight sandstone oil and gas reservoirs developed in the southern Ordos Basin of China. Due to the heterogeneity of sedimentary facies and diagenesis, the reservoir quality of these deposits is extremely uneven. Based on field profile observations, conventional thin section, cast thin section and scanning electron microscope (SEM) analysis, combined with general physical analysis, the sedimentary characteristics, physical properties and main controlling factors of the reservoirs of the Upper Paleozoic Shanxi Formation of the southern Ordos Basin were determined. The results of this contribution show that the Shanxi Formation is dominated by delta facies, with small-scale lacustrine facies. The delta facies can be further divided into distributary channel, natural levee, interdistributary swamp, underwater distributary channel, underwater distributary bay, mouth bar and sand sheet microfacies. Diagenesis within the study area has generally entered the mesodiagenetic B phase. Compaction and cementation were the main destructive stages of diagenesis. While cementation blocked the reservoir pores and reduced the physical properties, dissolution effectively improved the physical properties of the reservoir. The physical reservoir properties of the studied sandstone reservoirs are controlled by different sedimentary microfacies that are mainly, represented by distributary channel, underwater distributary channel and mouth bar microfacies that also show the best properties. These microfacies are followed by sand sheets. The lowest properties can be observed in the natural levees.

Labuan Island represents the emergent part of a north-eastward plunging anticline. The stratigraphic succession at Labuan is made up of four formations: the Crocker Formation, the Temburong Formation, the Setap shale Formation and the Belait Formation. The sedimentological aspects of the Labuan stratigraphic succession have been studied by several authors but the stratigraphic relationships between the formations is still a matter of conjecture. In this paper we have undertaken an outcrop based (1) sedimentary facies analysis of the Middle Miocene Belait Formation and (2) inferred the mode of stratigraphic sequence building of the Oligocene-Miocene successions. In significant contrast to the tide-storm interactive shallow-marine Belait Formation of Brunei and Sarawak, the Belait Formation of the Labuan island is made up of a lower fluvial and an upper shallow marginal marine to shelf facies association. The sedimentary succession from the Crocker, Temburong, Setap Shale and Belait formations represents a marine regression either due to a relative sea-level fall (forced regression) or to sediment supply exceeding the rate of sea-level rise (normal regression). The ~17 million years unconformity separating the fluvial facies association of the Miocene Belait Formation from the underlying shelfal sedimentary units (Oligocene Temburang and/or Setap shale formations) is a subaerial unconformity that is a consequence of rapid uplift and associated forced regression, and thus indicates the influence of regional tectonics on the mode of stratigraphic sequence building The change in paleocurrent pattern recorded by earlier researchers across the Oligocene-Miocene boundary also indicates tectonic tilt, and a change in sediment dispersal direction because of upliftment. The upper marginal marine facies association of the Belait Formation indicates transgression and the thick shelfal mudstone unit that overlies the marginal marine deposit indicates subsequent maximum flooding. Our sedimentological and stratigraphic analyses reveal that the shift in depositional environment during the Oligocene-Miocene transition is linked to regional tectonics and consequent sea level change and unrelated to paleoclimatic shift.

The occurrence and enrichment of organic matter (OM) are influenced by the deposition of minerals in shale reservoirs. However, inadequate research has been published on the origin, spatial relationships, and significance of organic matter–mineral associations (OMMA) for predicting “sweet spots” in shale gas reservoirs. We describe OMMA in the Wufeng Formation and the Longmaxi Formation in the Sichuan Basin, which is the largest commercial shale gas accumulation outside of North America. We characterize the presence of solid bitumen (SB) and graptolite fragments (the most important kinds of OM in these reservoirs) with four important minerals: i.e., quartz, clay minerals, carbonates, and pyrite. The occurrence of OMMA is influenced by the physical (e.g., morphology, texture, and density) and chemical properties of the associated minerals, which influences the accumulation of OM and the development of porosity. By carefully studying the type and relative amount of OM, we have determined how the surface porosity, pore diameters, and amount of porosity in each kind of OMMA influence the quality of shale gas reservoirs: e.g., diagenetic quartz or clay minerals encased in SB is the most favorable kind of OMMA. The quality of OM–carbonate associations is lower because they contain a relatively low amount of OM. Similarly, the relatively low abundance and dispersed distribution of OM–pyrite associations do not improve the quality of shale gas reservoirs. We identified four important lithofacies using the color, composition, and sedimentary structures of samples. The heterogeneous distribution of OMMA has important implications for shale gas reservoirs by influencing hydrocarbon generation and gas storage conditions in each lithofacies: i.e., siliceous shale > argillaceous siliceous shale > calcareous shale > silty shale.

The Triassic Yanchang Formation contains the main oil-bearing strata in the Ordos Basin, central China. But the sedimentology of the Upper Triassic is still under debate, and flood-generated, hyperpycnal-flow deposits and their implications for unconventional petroleum development have long been overlooked. Our study indicates that hyperpycnites are well developed in the seventh oil member of the Yanchang Formation. They are characterized by couplets of upward-coarsening intervals and upward-fining intervals, separated by microscale erosion surfaces. The origination of hyperpycnal flows was controlled mainly by episodic tectonic movements and the humid climate. The deposits extend from distributary estuaries into the deep lake, have intercalations of dark shales and tuffs, and coexist with debrites and turbidites as a result of the progradation of subaqueous fans. The hyperpycnites have implications for unconventional petroleum reservoirs, because the flows supplied not only large amounts of coarse grains and organic material to the deep-water, fine-grained central lake sediments but also affected the ecosystems, resulting in a higher total organic carbon content in the sediments.

Some sections on Lingshan Island, in the Yellow Sea, close to the city of Qingdao, show the Early Cretaceous Lingshandao Formation. The exposed succession consists mainly of turbidites, some of which show uncommon soft-sediment deformation structures, and contains well-developed, large-scale slumps that consist of sets of turbidites. The sediments were deposited as part of lobes of turbidites and hyperpycnites (deposits of flood-generated sediment-laden bottom currents derived from an estuary) in a deep-sea environment that formed part of an inter-arc basin with a tensional tectonic regime. Some of the slumps have dimensions of the order of metres to tens of metres; they consist of the turbidites that build most of the succession. The soft-sediment deformation structures in the turbidites represent a wide variety of types, most of which are common in subaqueous environments with a high accumulation rate. The most interesting are, however, structures that indicate active tectonic tension. These structures are boudins, which are present with two subtypes. The most common are pinch-and-swell structures, sometimes leading to isolated parts of the affected sandy layer, enveloped by shales of the under- and overlying shale layers. The second type consists of domino boudins, which have, as far as we are aware of, not earlier been described from sediments that were still unlithified during their formation. The presence of the boudins and associated structures, which indicate synsedimentary stretching related to an extensional tectonic setting, form an additional argument in the long-lasting debate about the question of whether a compressional or an extensional tectonic regime dominated in the area during the Early Cretaceous.

The fine-grained autochthonous sedimentation in the deep part of a Late Triassic lake was frequently interrupted by gravity-induced mass flows. Some of these mass flows were so rich in water that they must have represented slurries. This can be deduced from the soft-sediment deformation structures that abound in cores from these lacustrine deposits which constitute the Yanchang Fm., which is present in the Ordos Basin (central China). The flows and the resulting SSDS were probably triggered by earthquakes, volcanic eruptions, shear stress of gravity flows, and/or the sudden release of overburden-induced excess pore-fluid pressure. The tectonically active setting, the depositional slope and the high sedimentation rate facilitated the development of soft-sediment deformations, which consist mainly of load casts and associated structures such as pseudonodules and flame structures. Sediments with such deformations were occasionally eroded by slurries and became embedded in their deposits.