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Palaeo-exposure surfaces are common geologic features in the Cretaceous sedimentary record of the Middle East. Two important disconformity surfaces have influenced the Cenomanian-Turonian intervals of the Sarvak Formation in the Abadan Plain. These surfaces had substantial controls on reservoir quality of their underlying succession. In this study obtained data from macroscopic core descriptions, microscopic petrography, geochemical analysis, spectral gamma ray log signatures, and clay minerals frequency in three hydrocarbon fields are combined and used for interpretations. Dissolution, cementation and recrystallization are the main diagenetic features related to the disconformity surfaces. Intensity and development of these diagenetic alterations are highly dependent on depositional facies characteristics of the underlying sequences. In mud-dominated facies, the main diagenetic process is recrystallization, while the grain-dominated facies were highly subjected by dissolution and cementation. Based on the geochemical analysis, there is an increase in Al 2 O 3 , SiO 2 , Fe 2 O 3 , Al, Fe, Mn, and Rb content beneath the disconformities. In contrast, the Sr concentration displays a decreasing trend towards the disconformities. On spectral gamma ray logs, depletion of the uranium and high values of CGR/SGR and TH/U ratios are attributed to the meteoric diagenesis. In addition, obtained data from SEM and XRD analysis indicate that the montmorillonite is the most abundant clay mineral in uppermost part of the Sarvak Formation. It shows the highest concentration at the disconformity surfaces which can be quoted to the impact of meteoric diagenesis and development of palaeo-soil horizons. These results are used for reconstruction of diagenetic sequence of the upper Sarvak Formation in subsurface sections of the Abadan Plain. [ABSTRACT FROM AUTHOR]

This study aims to establish the diagenetic evolution of carbonate rocks of the Barremian-Aptian Tirgan Formation (Kopet-Dagh Basin, NE Iran), with special emphasis in their impact on reservoir quality to be used as analog for their equivalent carbonate reservoirs. To achieve this target, basic petrography complemented by elemental and stable isotopic (δ 18 O and δ 13 C) analyses of the studied carbonate rocks was used. In addition, the visible porosity was detected. The recognized carbonate diagenetic processes include micritization, cementation, dolomitization, compaction, dissolution, stylolitization, fracturing, silicification and neomorphism. These diagenetic processes took place in four diagenetic environments: marine-phreatic, meteoric-phreatic, meteoric-vadose and burial. Early dolomitization of lime muds, micritization of skeletal allochems and cementation by isopachous equant calcite represent marine-phreatic diagenesis. The meteoric-phreatic diagenesis was accompanied with the development of granular non-ferroan calcite cement, neomorphosed carbonate matrix and recrystallized bioclasts. The meteoric-vadose diagenesis led to prevalence of dissolution process and sparry calcite infilling the fractures. Late dolomitization, ferroan blocky and poikilotopic calcite cement, stylolitization and silicification processes were developed during burial diagenesis. An integration of the petrographic study with the geochemical analysis confirms that the studied carbonate rocks were originally composed of aragonite, which was eventually recrystallized to calcite and replaced by dolomite during diagenesis. In terms of reservoir quality, the recognized diagenetic alterations and visible porosities show that the diagenetic processes have a direct effect on modification of both inter- and intra-granular/crystalline porosity. In this manner, it is found that the calcite cementation, micritization and compaction decline the pore spaces, whereas the dolomitization, dissolution, formation of moldic pores and fracturing increase the pore volumes. Consequently, prevalence of dolomitization, dissolution, occurrence of moldic pores and fracturing in the Tirgan carbonate rocks gives the Tirgan Formation and its equivalent rocks a good opportunity to be an excellent reservoirs. [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]

New detailed lithological, sedimentological, chemostratigraphic data were obtained from exploration drilling samples on the C5 carbonate-dominated formation of the Neoproterozoic Lukala Subgroup (former Schisto-Calcaire Subgroup) from the West Congo Belt (WCB) in the Democratic Republic of Congo. This formation records the last post-Marinoan sea-level events that occurred in the whole basin, followed by the development of the Araçuaï-West Congo Orogen between 630 and 560 Ma. The C5 Formation consists of back-reef lagoonal and peritidal/sabkha cycles of ∼2.0 m in thickness, that record a short-time marine regression, rapidly flooded by a marine transgression with deposition of organic-rich argillaceous carbonates or shales under dysoxia and anoxia conditions. These dysoxic/anoxic waters were rapidly followed by a regional-scale marine transgression, favouring mixing with well-oxygenated waters, and the development of benthic Tonian to Cambro-Ordovician Obruchevella parva -type ‘seagrasses’ in the nearshore zones of the lagoons. New δ 13 C and 87 Sr/ 86 Sr isotopic data in the C5 Formation of the Lukala Subgroup are used in the frame of a correlation with the Sete Lagoas Formation in Brazil. Relatively comparable negative to positive δ 13 C excursions point to marine flooding of the whole basin and allow extension of the debatable Late Ediacaran age of the uppermost Sete Lagoas and C5 formations. Sr isotope “blind dating” failed due to low Sr concentration related to a dolomitization event close 540 Ma. Several tentative datings of the C5 Formation converge to a Late Ediacaran age ranging between 575 and 540 Ma. As the overlying Mpioka folded Subgroup, the C5 series suffered the Pan African deformation, dated at 566 ± 42 Ma. Unlike the previously generally accepted interpretation, our data suggests that the Mpioka Subgroup was deposited in the Early Cambrian. [ABSTRACT FROM AUTHOR]

The hinterland of Campo is marked by Archean, Paleoproterozoic, and Cretaceous geological formations. Petrographic, mineralogical and geochemical analyzes of beach sediment samples were performed to determine the origin, alteration, and paleoclimate of the various coastal sediment layers and to reconstruct the structural context of the source region. The beach sediments are fine to medium-grained in size. Morphoscopic examination showed a predominance of very angular and shiny quartz grains. These data reflect a relatively close source with transport in a fluvial environment. The Chemical Index of Alteration (CIA: average = 79.94%), Plagioclase Index of Alteration (PIA: average = 85.31%), as well as the A–CN–K and (A-K)-C-N triangular diagram, show that the weathering affected the source area rocks. The ratios of SiO 2 /Al 2 O 3 , K 2 O/Na 2 O, and the chemical composition variation index (ICV ∼ 1.32) show that the sediment samples are immature to mature in terms of texture and chemical composition. Al 2 O 3 /TiO 2 ratio values ranged from 3.04 to 33.44, indicating an intermediate to felsic rock signature, confirmed by Zr vs TiO 2 , La/Sc, and Th/Co binary diagrams. Geochemical results reveal a passive marginal continental environment. The passive margin emerged during the opening of the South Atlantic Ocean, which is compatible with the overall geology and structural setting of the studied area. • Campo beach sediment textural vary from clays to silt. • Beach sediments derived from moderate to intense source area weathering. • The source rocks vary from felsic to mafic rocks. • Campo beach sediments are compositionally immature to mature. • The source rock is controlled by sedimentary processes and diagenetic impacts. [ABSTRACT FROM AUTHOR]

The Triassic-Jurassic boundary marks a global faunal turnover event that is generally considered as the third largest of five major biological crises in the Phanerozoic geological record of Earth. Determining the controlling factors of this event and their relative contributions to the biotic turnover associated with it is on-going globally. The Upper Triassic and Lower Jurassic rock record of southern Africa presents a unique opportunity for better constraining how and why the biosphere was affected at this time not only because the succession is richly fossiliferous, but also because it contains important palaeoenvironmental clues. Using mainly sedimentary geochemical proxies (i.e., major, trace and rare earth elements), our study is the first quantitative assessment of the palaeoclimatic conditions during the deposition of the Elliot Formation, a continental red bed succession that straddles the Triassic-Jurassic boundary in southern Africa. Employing clay mineralogy as well as the indices of chemical alteration and compositional variability, our results confirm earlier qualitative sedimentological studies and indicate that the deposition of the Upper Triassic and Lower Jurassic Elliot Formation occurred under increasingly dry environmental conditions that inhibited chemical weathering in this southern part of Pangea. Moreover, the study questions the universal validity of those studies that suggest a sudden increase in humidity for the Lower Jurassic record and supports predictions of long-term global warming after continental flood basalt emplacement. [ABSTRACT FROM AUTHOR]

The shallow marine shales of the Cretaceous formations namely Yolde, Dukul, Jessu, Sekuliye and Numanha ranging in age from Cenomanian to Coniacian within the Yola Sub-basin in the Northern Benue Trough, northeastern Nigeria were analysed to provide an overview on their hydrocarbon generation potential. This study is based on pyrolysis analysis, total organic carbon content (TOC), extractable organic matter (EOM), biomarker distributions and measured vitrinite reflectance. The present-day TOC contents range between 0.24 and 0.71 wt. % and Hydrogen Index (HI) values between 8.7 and 113 mg HC/g TOC with Type III/IV kerogens. Based on the present-day kerogen typing, the shale sediments are expected to generate mainly gas. Biomarker compositions indicates deposition in a marine environment under suboxic conditions with prevalent contribution of aquatic organic matter and a significant amount of terrigenous organic matter input. Organic matter that is dominated by marine components contains kerogens of Type II and Type II-III. This study shows that the organic matter has been affected by volcanic intrusion and consequently, have reached post-mature stage of oil generation. These higher thermal maturities levels are consistent with the vitrinite reflectance ranging from 0.85 to 2.35 R o % and high T max ( 440–508 °C ) values as supported by biomarker maturity ratios. Based on this study, a high prospect for major gas and minor oil generation potential is anticipated from the shallow marine Cretaceous formations from Yola Sub-basin. [ABSTRACT FROM AUTHOR]

This study discusses the mechanism of dolomitization of carbonate rocks of the Upper Turonian Wata Formation in west Sinai, Egypt. It has been done in terms of study of textural, mineralogical, and geochemical characteristics of the dolostones and dolomitic limestones of the Wata Formation. The Wata Formation is composed mainly of dolostone and dolomitic limestone intercalated with few shale and sandstone beds. Beside the studied dolostone facies, five associated limestone microfacies have been identified. The limestone microfacies also provide an evidence of sparse dolomitization. Dolomites occur as replacive dolomites with minor dolomite cements. Four textural types of dolomite are distinguished: (1) fine-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite; (2) medium-to coarse-crystalline, planar-e (euhedral, idiotopic) replacive dolomite; (3) medium-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite, and (4) fine-crystalline, planar-e (euhedral, idiotopic) void-filling dolomite cements. The recorded dolomite is nearly non-stoichiometric (CaCO 3 ranges from 51 to 56 mol % with an average of 53.5 mol %) and has similar geochemical features. It has δ 18 O VPDB values range from −7.16‰ to +0.26‰ and δ 13 C VPDB values vary between −0.52‰ and +5.2‰. Its strontium content lies between 11 and 53 ppm. Petrographic investigations and geochemical data indicated that the dolomitization of the studied carbonates probably took place in a meteoric water-sea water mixing zone at shallow burial depth during the early stage of diagenesis. In this situation, the dolomitization was developed through an increase of Mg/Ca ratio of the pore water, with no salinity increase. The increase of Mg/Ca ratio took place by two possible ways: (1) through leaching of Mg from high Mg-calcite grains (micrite) and aragonitic shells in the studied limestone (2) through leaching of Mg adsorbed on clays of the adjacent mudrock horizons. Such leaching brought about by freshwater-dominated fluid during meteoric water diagenesis. [ABSTRACT FROM AUTHOR]

Wastes of the former Pb–Zn mining district of Touiref (Pb/Zn) can be eroded during strong rainfall. The presence of a downstream lake offers the opportunity to perform geochemical characterisation of lacustrine sediments and analyze the temporal evolution of metal contamination. For this reason, a sedimentary core (C1) has been collected at the outlet of the Oued Sidi Bou Said which runs along the North side of the dike flotation tailings and a second core (C2) was sampled in front of the river flowing outside the mine site and reaching the plain on the East side of the endoreic study area. The scope of the present paper is characterize the temporal variability of major and minor elements in both sedimentary cores. The study of the historical contamination in both Touiref cores shows a clear impact of mining on the total endorheic basin, especially for the period going from 1914 until 1955 as well as the influence of urban development. [ABSTRACT FROM AUTHOR]

The study area is situated in the imbrication zone (northwest of Adıyaman–Gölbaşı, Helete–Savran) of the Southeast Anatolian orogen, part of the Mediterranean–Himalayan orogenic belt. This area is characterized by NEE/SWW-trending, northward-dipping imbricated thrust slices located low in the nappe zone that was finally emplaced onto the Arabian platform during the Mid-Miocene. From top to bottom the main units in the study area are: (1) the Malatya Metamorphics- (2) the Harami Formation- (3) the Göksun Ophiolite–Esence Granitoid- (4) the Helete volcanics and (5) the Savran–Alacık Formation, Hompur–Lice Formation and Midyat Formation. This study contains the Upper Cretaceous–Eocene tectono-magmatic succession (ophiolitic rocks and related felsic intrusions and volcanic rocks), which are located in the Helete area. The unmetamorphosed ophiolitic rocks display an irregular ophiolitic sequence. These rocks are represented by tectonites, cumulated gabbros, isotropic gabbros and sheeted dyke complex and were intruded locally by felsic intrusions (granite, granodiorite, tonalite, dacite, and aplite). These ophiolites locally contain tectonically disrupted thin amphibolitic rocks. The mineral chemistry and geochemical features of these ophiolites indicate that these rocks exhibit sub-alkaline characteristics, and they are similar to those of island arc tholeiites formed in a supra-subduction zone. These ophiolitic rocks, located in the imbrication zone, are interpreted as being the equivalent of the Göksun Ophiolites. The felsic intrusions, which cut these ophiolitic rocks, correspond to the calc-alkaline Esence Granitoids which formed in an arc setting. Based on the 40Ar/39Ar dating method, a plateau age of 80.92 ± 0.29 Ma was obtained from a granite sample that cuts the gabbroic rocks. The Helete volcanics, which have a tectonic relationship with the Göksun Ophiolite and the Esence Granitoid, are represented by andesite-basalt (locally pillow lavas), and are overlain by nummulitic carbonate sediments. Geochemical data indicated that, the Helete volcanics evolved from a tholeiitic parental magma and formed in a subduction-related setting. [ABSTRACT FROM AUTHOR]

The following study was undertaken on inorganic geochemical data acquired for 832 core and cuttings samples taken from Cretaceous sediments encountered in 11 wells in the North Central Rub’ al-Khali Basin, Saudi Arabia. The study sections extend from Khafji Member at the base of the Wasia Formation to the basal part of the Aruma Formation. The principal objective of the study was to produce a chemostratigraphic scheme for these wells, though a comparison was also made with a previous lithostratigraphic scheme. Although ICP–OES (Inductively Coupled Plasma–Optical Emission Spectrometry) and ICP–MS (Inductively Coupled Plasma–Mass Spectrometry) were used to acquire data for 50 elements, the chemostratigraphic scheme is based on changes in the following ‘key’ elements and ratios: Ca, Al, Zr, P/Y, Zr/P, Zr/Y, Zr/Nb, Mo and Zr. The scheme comprises a hierarchical order of five zones, 18 subzones and four divisions, with the zones labelled C1, C2, C3, C4 and C5 in ascending stratigraphic order. The definition of these zones are mainly based on changes in Ca and Al reflecting variations in bulk lithology as a response to changes in base level. Zone C1 is dominated by elevated Al and low Ca concentrations and represents calcareous mudrocks deposited in a lagoonal–prodelta environment. An increase in base level led to the formation of highly calcareous rudistic limestones deposited in a shallow marine environment, with a further increase in base level resulting in the deposition of calcareous mudrocks with high Al values in zone C3. Zone C4 comprises rudistic limestones (with elevated Ca and low Al) deposited in shallow marine conditions, while C5 is dominated by calcareous mudrocks and argillaceous limestones deposited in a prodeltaic paleoenvironment. Although the definition of zones mainly relates to changes in base level/bulk lithology, more subtle changes in provenance are modelled using the P/Y, Zr/P, Zr/Y and Zr/Nb ratios which are used to define most subzones and divisions. Variations in this ratio are almost entirely dependent on changes in source/provenance. Although all of the zones and most of the subzones are correlative between the study wells it is noted that different criteria are used to define the zone C2 and C4 subzones in the eastern and western wells of the study area. This suggests that separate sediment sources were responsible for the delivery of clastic material to these carbonate ‘reefs’ during the deposition of these subzones. One important conclusion from this study is that there are close links between the chemostratigraphic and lithostratigraphic correlations but the former offers a much higher level of resolution. [ABSTRACT FROM AUTHOR]

The Cretaceous coals of Lamja Formation located in Yola Sub-basin of the Northern Benue Trough, northeastern Nigeria, were analyzed based on a combined investigation of organic and inorganic geochemistry to define the paleodepositional environment condition, organic matter source inputs and their relation to paleoclimate and tectonic setting. The total organic carbon and sulfur contents of Lamja Formation coals ranges from 48.2%–67.8% wt.% and 0.42%–0.76% wt.%, respectively, pointing their deposition in freshwater environment with inferred marine influence during burial. Biomarkers and chemical compositions provide evidence for a major contribution of land-derived organic matter, with minor aquatic organic matter input. Minerals such as quartz, pyrite, kaolinite, illite, montmorillonite and calcite were present in the coals, suggesting that these minerals were sourced from terrigenous origin with slightly marine influence, considered as post-depositional. This is consistent with a significant amount of the oxides of major elements such as SiO 2 , Fe 2 O 3 , Al 2 O 3 , TiO 2 , CaO, and MgO. The investigated biomarkers are characterized by dominant odd carbon numbered n -alkanes ( n -C 23 to n -C 33 ), moderately high Pr/Ph ratios (1.72–3.75), very high Tm/Ts ratios (18–29), and high concentrations of regular sterane C 29 , indicating oxic to relatively suboxic conditions, delta plain marine environment of deposition with prevalent contribution of land plants and minor aquatic organic matter input. Concentrations of trace elements such as Ba, Sr, Cr, Ni, V, Co and their standard ratios also suggested that the organic matter was deposited under oxic to relatively suboxic conditions, which is in parts deposited under marine influenced. Some standard binary plots of SiO 2 versus (Al 2 O 3 + K 2 O + Na 2 O) indicate a semi-arid paleoclimatic condition whereas log SiO 2 versus (K 2 O/Na 2 O) also revealed passive continental margin setting. The inferred tectonic setting is in agreement with the tectonic events witnessed in the West and Central Africa during the Cretaceous period. [ABSTRACT FROM AUTHOR]

Petrographic, major and trace element compositions of sandstones from the Pliocene Gabir Formation, Central Eastern Desert, Egypt have been investigated to determine their provenance, intensity of paleo-weathering of the source rocks and their depositional tectonic setting. Gabir Formation is composed mainly of sandstones alternating with limestone and shale beds. The Gabir sandstone is yellowish gray to yellowish brown color, calcareous and fossiliferous. The composition of this formation refers to shallow warm agitated marine conditions. Texturally, Gabir sandstones are immature, poorly sorted and grain supported. Abundance of feldspars indicates rapid deposition of sediments from a nearby source rocks. Their average modal composition (Q 71.35 F 16.6 L 12.05 ), classifies them as sublitharenite and arkose with subordinate litharenite and subarkose, which is also supported by geochemical study. Chemical analyses revealed that sandstones have high SiO 2 , K 2 O > Na 2 O, and low Fe 2 O 3 values, which are consistent with the modal data. Also, sandstone samples are enriched in most trace elements such as Ba, Sr, Ni, Cr and Zr and depleted in U and Th. The petrography and geochemistry suggest that Gabir sandstones were deposited in an active continental margin basin. They were mainly derived from granitic and low grade metamorphic sources. The CIA values (41.69–74.84) of the Gabir sandstones indicate low to moderate degree of chemical weathering, which may reflect cold and/or arid climate conditions in the source area. The source rocks are probably identified to be Proterozoic granites, metagabbros and metavolcanics, which must have been exposed during rifting, initiated during Oligocene and continued till post Miocene. [ABSTRACT FROM AUTHOR]