Depositional Environment Conditions Research Articles

Geochemical fingerprints of Dongpu Depression crude oils in Bohai Bay Basin, northern China: Insights from biomarkers and isotopic compositions of selected alkylnaphthalenes and alkylphenanthrenes

The Dongpu Depression is a rift lake in the Bohai Bay Basin of eastern China, where abundant oil and gas resources were discovered. Previous geochemical investigations of Dongpu Depression oils have revealed a notable lack of data regarding the isotopic compositions of individual polycyclic aromatic hydrocarbons (PAHs) from the discovered Cenozoic crude oils. Using gas chromatography-mass spectrometry (GC–MS) and gas chromatography-isotope ratio mass spectrometry (GC-IRMS), a novel approach combining molecular parameters and stable carbon isotope compositions of individual PAH was applied to study the depositional environment conditions, the different source inputs, the maturation stage and to identify the potential family of five crude oils selected from Dongpu Depression.A decrease of Ga/C30H together with low Pr/Ph ratios and changes in isotopic compositions indicate that the five Dongpu Depression oils were derived from OM deposited in an anoxic saline environment with a stratified water column where mixing of saline water and freshwater occurred. The low isoprenoid, tricyclic and tetracyclic terpane ratios in combination with the δ13C values of 1,6-DMN, 1,2,5-TMN, 1,3,6,7-TeMN, 9-MP and 1-MP attest that their sources have a mixed origin with high aquatic organism (planktonic) input and a lower land plant (C3 plant) contribution. The δ13C values of phenanthrene between −30.0 ‰ and −20.0 ‰ indicate that the studied Dongpu Depression oils belong to a single family generated from the peak to the late oil generation stage (Rc-TNR-2 between 0.85 and 1.15 %). A correlation with various basins indicates that oil samples featured by δ13C values of Phen between −30.0 ‰ and −20.0 ‰ correspond to crude oils derived from mixing of aquatic and high plant contributions with different degrees of mixtures, which have not yet reached their cracking stage (Rc < 2.0 %). Crude oils from the Carboniferous and Mesozoic-Cenozoic Tarim rocks, Australian petroleum systems, Termit Basin, Bongor Basin, Fushan Depression and the studied Dongpu Depression oils belong to that group of oils. Crude oils characterized by lighter phenanthrene isotopic compositions (δ13C values of Phen < −30.0 ‰) are mainly derived from marine/aquatic input and have already entered the cracking window (Rc > 2.0 %). This group of oils is represented in this study by the Cambrian-Ordovician Tarim Basin oil samples. The research shows the importance of the aromatic isotopic compositions in petroleum system characterization and could be used as a reference for a practical exploration campaign of petroleum.

Sedimentary age and paleoenvironment restoration of paleogene and cretaceous oil and gas reservoirs in the Termit Basin, Niger

The Termit Basin is a typical rift basin of the Mesozoic-Cenozoic era and is considered the most promising area for hydrocarbon exploration in the West African Rift System. This region's Paleozoic and Upper Cretaceous Formations have relatively complete source-reservoir-cap assemblages. This study aims to provide an in-depth understanding of the excellent environmental conditions and background that are conducive to the preservation of hydrocarbons in the Paleocene and Upper Cretaceous Formations. The study aims to explore various aspects of the sedimentary age, depositional environment, source direction, and hydrodynamic conditions in the area.In this study, zircon dating, geochemical characterization, and particle size analysis were performed on samples collected from two high-quality exploration areas, the Paleocene and Upper Cretaceous Formations. The Sokor-2 Formation of the Paleocene and the Madama and Yogou Formations of the Upper Cretaceous were the principal strata under investigation. The zircon dating results show that the primary rocks of detrital zircons experienced the Garidonian movement, with primary peaks in the Cambrian period. Geochemical characterization reveals that the sedimentary environment in the area was oxygen-rich, low-salinity, and warm. Particle size analysis helps identify the direction of material sources and hydrodynamic conditions in the palaeoenvironment.The experimental results indicate that the Paleogene and Upper Cretaceous Formations are high-quality exploration layers for oil and gas. These findings provide valuable data support for future hydrocarbon discoveries in the region.

Carbonate Stable Isotope Data Suggest Freshwater Environment for the McMurray Formation (Aptian), Alberta, Canada

Stable isotope, trace element, and cathodoluminescence analyses were performed on gastropods and matrix from rock cores of the Middle to Upper McMurray Formation (Lower Cretaceous/Aptian, Northwestern Alberta, Canada) to characterize the extent to which pristine material remained from which depositional environmental conditions could be estimated. Aragonitic gastropod shells retain their original depositional chemistry, and along with other closely associated authigenic carbonate components, indicate that the Middle to Upper McMurray Formation was deposited in a freshwater environment that was part of a continent-scale river system.

The organic geochemistry of crude oil in the Saltpond Basin (Ghana): Organic source input, depositional environment, and thermal maturity

The Saltpond Basin, situated within the South Atlantic margin of Ghana, is a significant area for petroleum exploration but has received relatively limited research attention. Previous studies have examined source rock composition, but data on crude oil organic chemistry are lacking, hindering understanding of the basin’s petroleum system and evolution. To address this gap, we analyzed biomarkers and stable carbon-isotope ratios in Saltpond Basin crude oil using gas chromatography–mass spectrometry and gas chromatography–isotope ratio mass spectrometry to elucidate organic matter source, depositional environment, and thermal maturity. Findings were compared with oils from the West African segment of the South Atlantic margin, namely the Tano Basin and the Niger Delta Basin, to identify potential correlations and gain insights into regional variations. Molecular and isotopic results unveiled a significant prevalence of organic matter derived from lower marine organisms. Patterns of organic matter deposition and preservation in Saltpond oil samples suggested a suboxic marine transitional environment, contradicting conventional understanding of terrestrial dominance in such settings. Moreover, the potential for degradation processes to obscure differentiation between terrestrial and marine organic matter origins underscores the complex nature of organic matter dynamics in transitional marine environments. Analysis of molecular thermal maturity indices suggested Saltpond oils were expelled from source rocks exhibiting thermal maturity at the early maturity stage. Correlation analysis unveiled genetic disparities among crude oils sourced from the Saltpond Basin and those from the Tano and Niger Delta Basin, primarily due to variations in source input and depositional environment conditions. Saltpond oil exhibits lower terrestrial organic input than Tano Basin’s crude oils, which also have less terrestrial input than Niger Delta Basin crude oils. Additionally, its paleodepositional environment notably differs from oils in the Tano Basin (anoxic transitional marine-lacustrine settings) and the Niger Delta Basin (suboxic–oxic terrigenous deltaic or marine or lacustrine environments). Thermal maturity range of Saltpond oil is comparable to oils in the Tano Basin but lower than oils in the Niger Delta Basin. These findings provide valuable insights into petroleum generation history and unique organic geochemical characteristics within the Saltpond Basin, essential for exploration, production, and environmental management efforts in the region. Furthermore, correlation studies provide evidence that distinct biological, geological, and paleoenvironmental conditions shaped various oil types in the West African segment of the South Atlantic margin.

The lower Cambrian Cranbrook Lagerstätte of British Columbia

Discovered over a century ago, the lower Cambrian (Series 2, Stage 4) Cranbrook Lagerstätte of southeastern British Columbia's Eager Formation is one of the oldest Burgess Shale-type deposits in North America. This Konservat-Lagerstätte is rich in olenelloid trilobites, but also yields a very low-diversity soft-bodied fossil assemblage including Tuzoia and Anomalocaris , and a low-diversity ichnofauna. Its scientific study, however, remains limited. A 2015 field-based investigation by the Royal Ontario Museum has revealed new information about the site's biota, depositional environment and taphonomic conditions. Not only is the Cranbrook Lagerstätte significant for early Cambrian biostratigraphy and comparisons with other Burgess Shale-type deposits, it also reveals some of the little-known diversity of life in a distal outer shelf environment during the Cambrian period. Supplementary material: Three supplementary figures (historical map, Tuzoia outlines, Anomalocaris specimens), supplementary material and methods, and a supplementary data file (generic presence/absence matrix) are available at https://doi.org/10.6084/m9.figshare.c.6850810

Petrographic and geochemical analysis of Barmer Basin Paleogene lignite deposits: Insights into depositional environment and paleo-climate

The current study attempts to characterize the samples from Kapurdi, Giral and Sonari lignite mines of Barmer Basin within Rajasthan Tertiary Lignite deposits using petrographic and geochemical techniques. The data generated has been studied in order to better understand the geochemical properties, hydrocarbon potential, depositional environment and paleo-climatic condition of the paleo-mires of these lignites. The proximate analysis of the samples reveals that the average ash content of the Barmer Basin is 11.93 %, the average moisture content is 14.08 %, average volatile matter yield is 42.03 % and the average fixed carbon is 31.95 %, which suggesting that the physical properties of lignite are dominated by volatile matter on dry basis. The ultimate investigation reveals the chemical properties of the lignite with the average carbon content found to be 58.79 wt%, hydrogen value 3.83 wt%, nitrogen percentage 1.60 wt% and sulphur content 2.75 wt%. This clearly indicates high sulphur concentration in Barmer lignite deposits, based on dry basis. The lignite samples subjected to FTIR studies identified aliphatic CH group, aromatic CO group, hydroxyl OH and Aldehyde HCO group. The Rock-Eval pyrolysis studies have been carried out to know the coalification profile of the deposits and hydrocarbon source rock potential. The varying behaviour between brown coal lithotype are primarily reflected by changes in oxygen index values, whereas hydrogen index values demonstrate a more pronounced correlation with coal type rather than the level of maturation. The results show the average total organic carbon (TOC) 47.29 %, S1 peak 2.18 mg HC/g, S2 peak 88.70 mg HC/g, S3 peak 19.22 % mg CO2/g, Tmax 410.53 °C, hydrogen index 219.2 mg HC/g TOC and oxygen index 46.33 mg CO2/g TOC. According to petrographic investigations, the huminite macerals dominate followed by liptinite and inertinite maceral groups in mineral matter-free basis. In all the three different lignite mines, the huminite group dominates with preserved sub-maceral ulminite indicating less transformation of structured plant matter. Reconstruction of depositional environment and paleo-climatic condition of the paleo-mires has been attempted by using organic petrographic indices. The high gelification index (GI) and low tissue preservation index (TPI) values suggest a prolonged wet environment during the decomposition of organic material, accompanied by a gradual subsidence process. The ground water influence index (GWI) and vegetation index (VI) denote ombrotrophic to mesotrophic hydrological conditions of basin at the time of deposition.

Determination and petroleum geochemical significance of short-chain alkylbenzenes in lacustrine source rocks

The determination of short-chain alkylbenzenes (C0-4) in source rocks has always been a challenge due to potential losses during pretreatment processes such as solvent extraction of the soluble organic matter. This work proposes a method for analyzing short-chain alkylbenzenes in solid powder samples from source rocks by combining headspace (HS)-solid phase microextraction (SPME)-Arrow and gas chromatography-mass spectrometry (GC–MS). The recommended optimal conditions of HS-SPME-Arrow are: use of a polydimethylsiloxane (PDMS) phase with a 250 μm thickness coated fibre, an extraction temperature of 90 °C, exposure time of 13 min, and desorption time of 8 min. The biomarkers and short-chain alkylbenzenes of 37 lacustrine source rocks with varying degrees of thermal maturation from different sedimentary basins in China were analyzed using this method. Three parameters based on short-chain alkylbenzenes isomers were then proposed, including the dimethyl-4-ethylbenzene ratio (DM4EBR), the 1,2-dimethylethylbenzene ratio (1,2-DMEBR), and the trimethylbenzene ratio (TMBR), which showed significant linear correlation with vitrinite reflectance of the source rocks. These can be used to evaluate the maturity or hydrocarbon generation stage of such source rocks, ranging from immature-low mature to postmature stages. An additional three parameters, including the dimethylethylbenzene ratios (DMEBR-1 and DMEBR-2) and the 1-methylisopropylbenzene ratio (1-MiPBR), were also proposed; these related to the organic matter inputs and depositional environments of the source rocks and can be used to identify the organic matter input and depositional environment conditions of source rock samples prior to the late stage of oil generation (Ro < 1.2 %). The usefulness of short-chain alkylbenzene parameters in indicating organic matter inputs, depositional environment conditions, and thermal maturity levels makes them valuable tools for geochemical studies of highly matured light oils and condensates.

Taphonomic analysis of shell concentrations in deltaic Miocene deposits of the South Caspian Basin, eastern Paratethys

ABSTRACT The depositional environment and palaeoecological conditions of a Miocene delta in the Paratethys Basin have been reconstructed based on an integrated sedimentological, ichnological and taphonomic analysis of shell concentrations. Four facies associations, i.e. delta plain, distributary channel, delta front and prodelta, are recognised. The shell concentrations, which are a characteristic feature of the Miocene successions, display different taphonomic signatures in the various parts of the delta system. Delta plain shell concentrations are characterised by unabraded shells with abundant micro-borings and thick micrite envelops formed during colonisation windows and optimum ecological conditions such as good illumination, high nutrient availability and low sedimentation rate. Distributary channel shell concentrations consist mainly of highly abraded, highly fragmented, disarticulated shells that point to long-term high-energy hydrodynamic conditions during their formation. Sporadic shell concentrations in the delta front are composed of chaotic and highly fragmented skeletal elements and co-occur with predominantly suspension-feeding trace fossils such as Arenicolites, Diplocraterion and Skolithos. They reflect a high sedimentation rate and suspension organic particles in a high-energy depositional setting. Prodelta shell concentrations are characterised by moderately fragmented, parallel-oriented shells, preserved within thin-bedded sandstones, intercalated between thick mudstone packages. These concentrations were produced by storm-induced, offshore-directed high-density currents.

Provenance, Depositional Environment, and Paleoclimatic Conditions of a Near-Source Fan Delta: A Case Study of the Permian Jiamuhe Formation in the Shawan Sag, Junggar Basin

The science of the provenance, sedimentary system, and distribution of sand bodies is unclear, limiting oil and gas exploration. Here, we combined heavy mineral, rare earth element, petrographic, and outcrop data to shed new light on the provenance, depositional environment, and paleoclimatic conditions of the Permian Jiamuhe Formation. The provenance is characterized by “a main provenance system, and four provenance zones,” and this result could be interpreted from analyses of its seismic reflection, clastic composition, REES, and heavy minerals. A detailed sedimentological study performed in the excellent outcrops, a comprehensive analysis of logging, and the legalistic cores of this formation allowed for the identification of ten lithofacies and three lithofacies associations. Four distributary/underwater channels were observed. Furthermore, the redox and paleoclimatic conditions based on trace elements (Th/U, V/(V + Ni), V/Cr, Cu/Zn, Sr/Ba, and Sr/Cu) suggested a weak reduction in the environment, as well as semi-humid and semi-dry conditions of the Jiamuhe Formation. These conditions are also supported by the mudstone color and plant fossils. The tectonic setting belongs to the acid island arc area based on the trace element discrimination diagram of La-Th-SC and the values of the La, Ce, ΣREE, L/H, La/Yb, and (La/Yb)N criteria. The research results further confirm that there are differences in the mineral compositions in the same provenance area, and they provide a geological basis for the fine sedimentary facies characterization and a favorable zone prediction in this area.

Sabkha and salina dolomite preserves the biogeochemical conditions of its depositional paleoenvironment

Precipitation of Mg-carbonate minerals, ranging from Mg-calcite to Ca-rich dolomite, is typically associated with microbial activity, and commonly takes place in saline environments along with other evaporitic minerals. The geochemistry of such evaporitic minerals can contribute to the interpretation of the environmental conditions and those (bio)geochemical processes occurring at the time of deposition. We use a multiproxy approach that combines stratigraphic, petrographic, mineralogical and geochemical data, such as Fe and Sr isotope values, to describe the environmental and biogeochemical conditions under which dolomite precipitated in a sabkha/salina environment using the Miocene Vilobí Gypsum Unit (Penedès Basin, NE Spain) as a case study.The lithofacies of Vilobí Gypsum Unit are interpreted to indicate sedimentary environments alternating between sabkha and coastal salina. The δ13C and δ18O values of Vilobí dolomites (from −6.3 to −0.4‰ and from 0.2 to 2.3‰, respectively) compare well with microbially influenced dolomite from modern sabkha and coastal environments. The dolomite 87Sr/86Sr ratios (0.70816 to 0.70874) show significant variations and indicate that the coastal sabkha/salina was strongly influenced by seawater. Microbial activity is evidenced by petrography (microbially induced sedimentary structures interpreted as cyanobacterial mats, and spheroidal dolomite with empty bacterial voids), and geochemical data (elemental concentrations enrichment in Fe2+ and Mn2+, up to 1388 and 605 ppm, respectively; supported by δ56Fe values of between −0.16 and 0.44‰). Collectively, these data suggest that the studied dolomite is a primary precipitate that records the geochemistry of the saline depositional environment influenced by microbial activity. The Vilobí Gypsum Unit is here dated to 16.05 Ma and 16.30 Ma (Burdigalian) by correlating the 87Sr/86Sr values of dolomite and shell of the oysters (shells between 0.70870 and 0.70872) with the global Sr curve. Moreover, dolomite records marine (porewater) values (C-O-Fe-Sr) that can be used to infer the depositional environment and geochemical conditions at the time of deposition. Results of non-traditional isotopes and trace metal concentrations support using dolomite formed in evaporitic environments as an archive of paleoenvironmental conditions.

Sequence stratigraphy and petrographic characteristics of Oligocene sediments in the West trough of the Cuu Long Basin, Vietnam

The Cuu Long Basin is the major source of Vietnam's oil and gas production. In clastic sediments, the Oligocene and Lower Miocene formations are the main objects of oil and gas exploration and exploitation activities. Oligocene sediments are both source and reservoir rocks; therefore, studying the petroleum system of these formations is of great interest. This paper discusses detailed stratigraphy and lithological properties to clarify the Oligocene sedimentary characteristics and their significance in the petroleum system. Study results show that Oligocene sediments are divided into three sequences, including eight parasequences and are dominated by the terrestrial sediments. Sequence E, mainly coarse-grained, contains low palynomorphs and rich organic matter with mainly palynomaceral 1 and is deposited in freshwater fluvial settings under high-energy conditions, showing sediments not so far from the original areas. The high level of detrital matrix materials is the main cause of poor or negligible porosity of the rock. The parasequences of Sequence D contain abundant palynomorph assemblages, characterized by swamp and freshwater alga taxa. They are deposited mainly in the shallow lake with a decreasing energy level pointing upward. The volume of organic matter in sediments is very rich, with sapropel organic matter (SOM) rates reaching up to 80%, indicating that the hydrocarbon generation potential is mainly oil-prone and some oil-gas mixtures. The porosity is poor and reservoir quality is affected by detrital matrix and authigenic minerals. The parasequences of Sequence C are characterized by the extreme abundance of freshwater algae, deposited mainly in the shallow-to-deep lake under moderate energy conditions, and rich sapropel organic matter, suggesting mainly oil-prone in hydrocarbon generation potentials. The porosity ranges from fair to good and the reservoir quality is estimated as the best in Oligocene sediments. A new finding of marker assemblages provides evidence to pinpoint the stratigraphic boundary between the Lower Miocene and Upper Oligocene. This finding helps to complete the stratigraphic framework of the area and to correlate the stratigraphy to adjacent sedimentary basins. This study provides information on depositional environmental conditions, organic matter content, hydrocarbon generation potential, and reservoir quality that relate to source rock, reservoir, and seal; these are the elements of a petroleum system. A complete petroleum system assessment requires a holistic view of all the petroleum system elements.

Geochemical approach for decoding the paleoenvironmental and depositional evolution of a coastal lacustrine Konservat-Lagerstätte (Early Cretaceous, south-Central Pyrenees)

The lithographic limestone of La Pedrera de Meià (LPM) in south-Central Pyrenees (NE Spain) is considered one of the best preserved lacustrine-coastal successions of the Early Cretaceous in Europe, hosting a taxonomically diverse record of Barremian biota. While this Konservat-Lagerstätte has been extensively surveyed for paleontological purposes, little is known about the paleoenvironmental and depositional conditions prevailing during the formation of this fossil lagerstätte deposit. LPM limestone is made up of a homogeneous, dark gray to pale brown, faintly laminated 50 m-thick succession of micrite-rich intervals. To shed light on the environmental conditions of deposition, 303 rock samples were studied using a multiproxy approach based on different techniques including petrography, SEM-EDX, XRD, ICP-MS, XRF, Rare Earth Element distributions and C and O stable isotope geochemistry. The combination of sedimentological and geochemical data reveals a vertical shift of facies from littoral to profundal environments and then returning to littoral conditions. Profundal facies are made up of filament-rich mudstones originated from in situ precipitation of micrite either through biotically mediated or physico-chemical processes in undisturbed low energy settings, under anoxic conditions, characterized by the presence of transported terrestrial and lacustrine organisms that are exquisitely well-preserved. Littoral areas are characterized by the presence of in situ charophytes, smooth ostracods, and miliolid foraminifera that accumulated in mud-rich facies indicating low/moderate-energy, shallow-water, and relatively oxygenated conditions. Stable isotopic data and elemental geochemistry suggest a hydrologically closed to semi-closed lacustrine system with a restricted water circulation encouraging dysoxic to anoxic conditions in profundal sediments. Paleoproductivity proxies suggest a good correlation between catchment-derived siliciclastic discharge to the lake and primary productivity peaks. Furthermore, a decline in terrigenous input coupled with a reduction of meteoric water entrance to the lake is interpreted based on elemental proxies derived from paleosalinity and paleoredox analysis. Collectively, the strontium/barium (Sr/Ba) ratios, the sparse presence of miliolid foraminifera, and the isotopic signatures indicate an increase in salinity toward the younger intervals of the lake infilling, although their ultimate causes are unclear (i.e., increased evaporation, or a combination of reduced meteoric input and sporadic marine inputs might explain our data). The present work shows the importance of performing detailed multiproxy geochemical-sedimentological studies to better constrain the unique paleoenvironmental and paleohydrological conditions associated with the exceptional preservation of fossil biotas in Konservat-Lagerstätte deposits in analogous depositional settings.

Synergistic impacts of climate change and human activities on spatiotemporal organic nitrogen burial variation in a plateau lake in southwest China

ABSTRACT The concentration and sources of organic nitrogen (ON) in lake sediments affect lake nitrogen cycles. However, the factors influencing ON accumulation rate (ONAR) are unclear. We collected 3 sediment cores from northern, eastern, and southern Dianchi Lake (DC-N, DC-E, and DC-S, respectively) in July 2014, to study the effects of autochthonous and allochthonous sources of ON. The ON and ONAR increased 2.4–5.1 and 2.6–4.8 times, respectively, from 1900 to 2000, especially since the 1980s, when algal blooms have occurred more frequently. The ON decreased in the order DC-S > DC-N > DC-E, whereas the ONAR decreased in the order DC-N > DC-S > DC-E, suggesting that ONAR was influenced by ON content as well as the depositional environment. The total concentrations of n-alkanes (n-C12 to n-C34) ranged from 4719 to 61 960 ng g−1 in the 3 sediment cores, with proportions varying with vertical depth. The sources of ON were mainly allochthonous (soil erosion and terrestrial plants) and autochthonous (algal and aquatic plants) in DC-S and DC-N, respectively, and primarily mixed planktonic and terrestrial in DC-E. The stochastic impacts by regression on population, affluence, and technology (STIRPAT) model revealed that a 1% increase in air temperature and nitrogen fertilizer corresponded to an increase in ONAR by 23–33% and 20–79% in the Dianchi Lake basin, especially in DC-S and DC-E. However, a 1% increase in urban land area reduced ONAR by 2–11%, especially in DC-N. Our study suggests that the spatial and temporal ONAR in Dianchi Lake may increase in response to a warmer and wetter climate combined with increasing chemical nitrogen fertilizer application.

Preliminary study of crude oils from Shabwah depression in the Central Sab'atayn Basin (Yemen): Revealing oil-oil correlation with organic matter input and maturity based on geochemical properties of maltene and asphaltene fractions

This study was investigated eleven crude oil samples from representing Jurassic reservoir sandstones from the Al–Uqlah Oilfield in the Shabwah depression, using physical and geochemical aspects for saturated, aromatic, and asphaltene fractions to gain information about their source rock characteristics, particularly the conditions of depositional environment, thermal maturity and organic matter (OM) character. The overall results show that the examined oils have API gravity values between 38° and 43°, indicating that most crude oils are light and not biodegradable; nevertheless, oil samples, sweet oils with a low sulfur content, with S of <1 wt%. The molecular structure of the asphaltene in these oils derived from Py–GC suggests mainly paraffinic oil exhibiting high to low wax content derived from Types I and II kerogens. This claim agrees with the high saturated HC of more than 38% and significant levels of aromatic HC and polar components. The studied oil were generated from clay-rich source rocks, containing a mixed OM origin of marine and terrestrial organic matter and deposited under sub-oxic depositional environmental conditions as indicated by their biomarker of the saturated and aromatic fractions. The Py–GC structure of the asphaltene together with the environment-related biomarker indicators reveals the existence of two oil families. The first oil family represented in the majority of the studied oils and characterised as waxy paraffinic oil and generated from a mixture of organic matter, with significant terrestrial land plant input and deposited in sub-oxic environmental conditions. Other oil samples belonged the second oil family; these samples are predominantly P–N-A oils with low wax content and were derived from marine-source rock, containing significant contributions from aquatic OM and little input from terrestrial OM and deposited under more reducing environmental conditions. The biomarker and non-biomarker maturity ratios indicate that thermally mature source rocks generated the investigated crude oils of both oil families, corresponding to the peak oil window.

Organic matter accumulation in lacustrine shale of the Permian Jimsar Sag, Junggar Basin, NW China

The lacustrine organic-rich shale in the Permian Lucaogou (LCG) Formation of the Jimsar Sag, Junggar Basin, is one of the main shale oil plays in China. In this paper, geological and geochemical research techniques were employed to evaluate the geochemical variability of the lacustrine shale and the production of organic matter and its preservation conditions. The LCG Formation is characterized by its complex mineral compositions and a wide range of organic matter richness and quality. The presence of high proportions of β-carotane and C29 steranes, indicates that the organic matter mainly originated from phytoplankton and aquatic algal-bacterial organisms, especially cyanobacteria. This study found that the productivity of the Lower LCG Member (P2l1) was highest, and the Middle LCG Member (P2l2) was the lowest. During the deposition of the Lower LCG Member, the lake's bottom water was predominantly a reducing environment, and the degradation of organic matter was largely a result of bacterial sulfate reduction. During the deposition of the Middle and Upper LCG members, the lake's bottom water was mainly oxidizing, and the degradation of organic matter was likely to be caused by aerobic processes. Based on a comprehensive analysis of the origin and production of organic matter, as well as its depositional environment and preservation conditions, two organic matter accumulation models were proposed to explain the distribution of the organic-rich shale. In model A, the high influx of volcanic ash released nutrients and brought abundant sulfate into the water, the accumulation of organic matter was mainly controlled by the preservation of organic matter, which was mainly controlled by BSR. In the model B, the influx of volcanic ash was small, organic matter was mainly degraded by oxygen and the accumulation of organic matter is mainly determined by the production of organic matter.

Grain size distribution and fluvial environmental implications of Sebou estuary (Morocco)

The hydrodynamics and grain size distributions of sediment in estuarine and coastal environments provide important information on the material source, environmental events, and so forth. According to our knowledge, there is no research on the sediment characteristics of the entire Sebou estuary (Morocco), particularly, on the correlation between hydrodynamics and sediment properties. Grain-size analysis of estuary sediments along Sebou estuary (Morocco) has been carried out to reveal the depositional environment condition, energy and hydrodynamic conditions, as well as the mode of transportation along the study area. Linear discriminate analysis technique and bivariate plotting of grain size parameters are used to explain and understand the environment of sediment deposition. Hydrodynamic processes working during the deposition of sediments have been identified by CM plotting. As the mean grain size increased, sorting improved, and skewness became more positive and leptokurtic to very leptokurtic nature was observed in both periods. The spatial distribution indicates majority of sample fall in silt and clay category in both periods (i.e., spring and neap). Based on the CM pattern it was inferred that the sediment fall in rolling and suspension field. The sediments may be derived from offshore and catchment area deposits of the clay and silt, and evidence was found to suggest that these deposits are being eroded by tidal currents.

Sedimentological and geochemical signatures of the Ebolowa Municipal Lake sediments (South-Cameroon): implication for provenance, weathering, and depositional environment conditions

Sedimentological and geochemical signatures of the Ebolowa Municipal Lake sediments (South-Cameroon): implication for provenance, weathering, and depositional environment conditions

Tectonic and stratigraphic evolution of the Santos Basin rift phase: New insights from seismic interpretation on Tupi oil field area

Most recent published work aims to understand pre-salt carbonates interval development and depositional environment conditions. Previous contributions which debate about rift extension usually consist of regional analysis regarding the South Atlantic Ocean opening and keep the debate about syn-rift and sag stage limits. This work uses 3D seismic volume analysis, structural contour maps and isopach maps analysis in order to understand and establish rift extension development in Santos Basin pre-salt interval. Our analysis concludes that the pre-salt interval is inserted into the syn-rift context, divided into 3 phases: Lower Rift, Upper Rift and Later Rift. Fault analysis shows that the 3 main fault directions act as a structural control for the rift phases. Lower and Upper Rifts are controlled by SW-NE and N–S trend faults, while Later Rift is controlled by SE-NW trend faults. Seismic interpretation and well log analysis indicates that, at Santos Basin, Lower Rift includes Camboriú and Piçarras Formation, Upper Rift comprehend Itapema and Lower Barra Velha Formation, while Later Rift contains Upper Barra Velha Formation, as sag-type basin.

Organic matter characteristics and conventional oil potentials of shales from the Early Jurassic Datta Formation in the Upper Indus Basin, Northern Pakistan

This study deals with organic matter in the Early Jurassic Datta shale in the Upper Indus Basin, Pakistan. Organic geochemical and microscopic techniques were used to investigate organic matter input, environmental conditions of deposition as well as petroleum generation potential. The geochemical results suggest that the shales display mainly fair to good source rock generation potential, with poor petroleum generation potential in some samples. The HI varies between 32 and 519 mg HC/g TOC, with many of the samples appearing to be mainly Type II and mixed Type II-III kerogen, and thereby exhibiting the ability to generate both oil and gas. The dominance of such Type II kerogen in some samples is consistent with the high HI values (>300 mg HC/ g TOC) and high fluorescence alginite macerals. A section of the shale within the Datta Formation appears to be gas-prone, with HI values of less than 200 mg HC/g TOC. Biomarkers also indicate the presence of mixed organic matter of planktonic/bacterial and land plant input, which were deposited in a marine environment under relatively oxic conditions. The biomarker characteristics of the extracted shales correlate well with those of the oil from the Chanda Oilfield, indicating that the Datta shales are likely to be the effective source rock. This is supported by the thermal maturity, which ranges from moderate to peak-maturity, as indicated by vitrinite reflectance values of 0.69–0.82%, and the biomarker maturity indicators.

Sedimentological and mineralogical-petrographic characteristics of Miocene evaporitic deposits (SW Erzincan)

This study aimed to find the depositional environment and formation conditions of the gypsum-dominated Miocene succession alternated and intercalated with clastics and carbonates in the southwestern part of the Erzincan Basin. As a result of sedimentological, mineralogical-petrographic investigations, it was determined that the gypsum in this sequence was formed as primary and secondary. Primary lithofacies; nodular anhydrite, selenitic, discoidal, gypsum arenitic secondary lithofacies; It is divided into massive, laminated, banded, nodular, brecciated, satin-spar. The facies repetitions and folds, fractures, cracks and faulting in the sequence showed the effect of tectonism with the water level fluctuations in the basin. In addition, these investigations revealed sedimentary structures such as cross-bedding, ripple undulations, chicken-wire, enterolithic and bacterial-algal structures, parallel laminations in gypsum lithofacies. In addition to these, it was understood that gypsum lithofacies were exposed to diagenetic processes under the influence of a hot-humid and semi-arid climate, organic matter activity, meteoric/underground-water and hydrothermal solution factors, and were subjected to high temperature, pressure, salinity and pH conditions.