Paleo-water Depth Research Articles

Sedimentary Environment and Organic Matter Accumulation of Continental Shales in Xiahuayuan Formation in Xuanlong Depression, Yanshan Area

The shale sedimentary environment is crucial for evaluating shale gas reservoirs and sweet spot zones. The Xiahuayuan Formation in the Xuanlong Depression of the Yanshan area is an important exploration and development region for shale gas due to its multi-layer dark shale. The paleosedimentary environment and organic matter accumulation mechanism of organic-rich shale were discussed through geochemical methods such as total organic carbon (TOC) content and elemental analysis. The results indicate that the shale exhibits a high TOC content. The Mo content and the P/Al and P/Ti ratios indicate that the primary productivity of the ancient lake is high. The Ceanom, V/(V + Ni) ratio and MoEF-UEF covariation model reveal that the sedimentary environment of shale is characterized by anoxic conditions. The ratios of K/Al and Ti/Al suggest significant variations in the input of fine-grained clay clastics and terrigenous clastics. The Ca/(Fe + Ca) and Sr/Ba ratios suggest that the paleowater was a freshwater environment. The paleoclimatic conditions, as indicated by CIA, Sr/Cu, and C-value, suggest a range from semi-humid to humid. The ratios of Rb/K and Mn/Ti reflect that the water primarily existed in a shore–shallow lake environment. The correlation analysis between organic matter accumulation and sedimentary environment parameters indicates that the primary factors influencing the organic matter accumulation in the Xiahuayuan Formation shale are redox conditions, terrigenous clastic input, paleoclimate conditions, and paleowater depth. The organic matter accumulation is characterized by a “preservation condition” pattern. This study provides theoretical support for the accumulation mechanism, potential evaluation of resources, and optimal selection of favorable regions for Jurassic shale gas in the Xuanlong Depression.

Analysis of lake level fluctuations in the Early Cretaceous Songliao Basin supports aquifer eustacy

Clear geochronology and precise interpretation of sequence stratigraphy enhance our understanding of continental lake-level evolution. The Songliao Basin features well-preserved Cretaceous continental strata. Nevertheless, the correlation between lake-level fluctuations and global sea-level changes remains ambiguous. High-resolution gamma ray logging data were used to analyze the cyclical stratigraphy of the lower member of the Early Cretaceous Shahezi Formation in the Songliao Basin. X-ray fluorescence experiments characterized the sedimentary environment of this formation. Lake-level variations in the lower part of the Early Cretaceous Shahezi Formation were reconstructed using sedimentary noise modeling based on finely tuned gamma ray logging data. Time series analysis using the tuned gamma ray data established an astronomical timescale of approximately 2.4 Myr within the lower section of the early Cretaceous Shahezi Formation in the Songliao Basin. A volcanic ash layer dating (118.20 ± 1.5) Ma from the base of the Shahezi Formation served as an anchor point, providing an absolute astronomical timescale ranging from 115.80 to 118.20 Ma for the study region. The sedimentary model indicates that variations in paleowater depth within the lower section of the Shahezi Formation closely match fluctuations observed in the Fe/Mn index, which reflects paleowater depth changes. This introduces a novel approach to assess changes in continental lake levels. The sedimentary noise model revealed a notable obliquity cycle of about 1.2 million years, strongly associated with fluctuations in lake levels. This indicates that prolonged obliquity periods affect lake level variations. Intriguingly, when lake levels rise, global sea levels concurrently decline, highlighting an inverse relationship between these phenomena. This observation offers insights into how long-term obliquity-driven climate changes regulate sea and lake levels.

Organic matter enrichment model of fine-grained rocks in volcanic rift lacustrine basin: A case study of lower submember of second member of Lower Cretaceous Shahezi Formation in Lishu rift depression of Songliao Basin, NE China

Based on sedimentary characteristics of the fine-grained rocks of the lower submember of second member of the Lower Cretaceous Shahezi Formation (K1sh2L) in the Lishu rift depression, combined with methods of organic petrology, analysis of major and trace elements as well as biological marker compound, the enrichment conditions and enrichment model of organic matter in the fine-grained sedimentary rocks in volcanic rift lacustrine basin are investigated. The change of sedimentary paleoenvironment controls the vertical distribution of different lithofacies types in the K1sh2L and divides it into the upper and lower parts. The lower part contains massive siliceous mudstone with bioclast-bearing siliceous mudstone, whereas the upper part is mostly composed of laminated siliceous shale and laminated fine-grained mixed shale. The kerogen types of organic matter in the lower and upper parts are types II2–III and types I–II1, respectively. The organic carbon content in the upper part is higher than that in the lower part generally. The enrichment of organic matter in volcanic rift lacustrine basin is subjected to three favorable conditions. First, continuous enhancement of rifting is the direct factor increasing the paleo-water depth, and the rise of base level leads to the expansion of deep-water mudstone/shale deposition range. Second, relatively strong underwater volcanic eruption and rifting are simultaneous, and such event can provide a lot of nutrients for the lake basin, which is conducive to the bloom of algae, resulting in higher productivity of types I–II1 kerogen. Third, the relatively dry paleoclimate leads to a decrease in input of fresh water and terrestrial materials, including Type III kerogen from terrestrial higher plants, resulting in a water body with higher salinity and anoxic stratification, which is more favorable for preservation of organic matter. The organic matter enrichment model of fine-grained sedimentary rocks of volcanic rift lacustrine basin is established, which is of reference significance to the understanding of the organic matter enrichment mechanism of fine-grained sedimentary rocks of Shahezi Formation in Songliao Basin and even in the northeast China.

Study on the relationship between shale organic matter development and paleowater depth—A new understanding of the condensed section

Recent studies dispute the long-standing belief that highly organic-rich shales are predominantly linked to condensed section (CS) at the maximum flooding surface (MFS), proposing instead that they more commonly develop in shallow waters along basin margins. This hypothesis, however, has not been thoroughly investigated. Our study, focusing on the pivotal regions of the Longmaxi Formation and the Chang 7 Member for marine and continental shale exploration in China, integrates sequence stratigraphy with analyses of total organic carbon (TOC) and elemental geochemistry to pinpoint the environments conducive to the formation of organic-rich shales. The most organic-rich intervals, specifically the bottom of the Longmaxi Formation and the mid-to-lower Chang 73 sub-member, are located within the transgressive systems tract (TST), showing higher TOC levels in the middle to lower TST, compared to the CS of the MFS. The formation of high organic matter shale is related to warm, humid, and dysoxic shallow waters with minimal terrigenous influx and high primary productivity. We found a pronounced inverse relationship between organic matter concentration and water depth, with deeper waters promoting the decomposition and recycling of organic matter during sedimentation, thereby diminishing the TOC content. Moreover, volcanic activity also affected the organic enrichment in the Chang 7 Member. With high primary productivity, organic matter tends to accumulate in dysoxic slope zones. Consequently, high-organic content shale intervals are generally situated in shallow, dysoxic water environments, suggesting that the CS of the MFS may actually represent organic-poor intervals with limited source rock potential. The results of this study are of great significance for shale oil and gas exploration in China, and provide a new direction for the discovery of shale oil and gas sweet spots.

Paleo-Sedimentary Environment and Formation Mechanism of the Organic-Rich Shale of the Permian Lucaogou Formation, Jimsar Sag, Junggar Basin, China

The Jimsar Sag is an important shale oil exploration target area in the Junggar Basin, northwestern China. The Permian Lucaogou Formation, with a thickness of 200–300 m, is the primary exploration target. High-frequency variation in lithology is a typical feature of the Lucaogou Formation, reflecting the fluctuation of the depositional environment and organic matter enrichment. The evolution of the depositional environment and accumulation mechanism of organic matter still need to be elucidated for the Lucaogou Formation. High-resolution sampling of the entire Lucaogou Formation was applied to a 248 m long core from Well JX in the Jimsar Sag to examine the depositional environment and organic matter enrichment. The findings unveiled that the Lucaogou Formation was deposited under a hot and arid climate, within the confines of a closed saline paleo-lake, where sediments endured an extended period of anoxic conditions, displayed periodic oscillations in paleo-temperature and paleo-salinity values over time, alongside a continuous rise in paleo-water depth. The predominant source lithology of the Lucaogou Formation is felsic igneous rock. Small-scale transgression and hydrothermal sedimentation occurred during the deposition of the Lucaogou Formation. The prevailing hot climate and enduring reducing environment fostered ideal circumstances for the enrichment of organic matter in the Lucaogou Formation. Due to different sedimentary environments and enrichment mechanisms, organic matter is enriched in two modes in the Lucaogou Formation.

Elemental geochemistry of Cretaceous deposits in the Dahomey Basin (Nigeria): Implications for paleoclimatic and paleoenvironmental reconstructions

The Dahomey Basin, located in southwestern Nigeria, is an energy-rich frontier basin with sedimentary successions consisting of both (onshore) terrestrial and (offshore) marine deposits. Since the discovery of several bitumen seeps, tar sands, and oil and gas shows offshore the basin and the successful production of the Aje Field, the basin has gained immense interest from academia and industry. Nevertheless, surface and subsurface research focusing on paleoclimatic conditions, primary productivity, anoxic conditions, paleowater depth, and hydrodynamic influence in the Dahomey Basin is less documented. To reconstruct paleoenvironmental conditions, subsurface data from the Araromi-Obu (A1) and Idiobiolayo (A15) cores were used. The average chemical index of alteration (CIA) values ranging from 63.01 to 94.88 reflect varying degrees of weathering intensity in the sediment source area of the Cretaceous Araromi Formation. These values show a transition from relatively low to considerably high degrees of weathering in the source region of the Araromi Formation. The analysis of CIA values and Ga/Rb, Sr/Cu, Sr/Ba, Ca/Al, and 100*Mg/Al ratios suggests that the Araromi Formation was deposited in a brackish to marine environment.Furthermore, chemical alteration-sensitive geochemical indicators revealed prevailing warm, humid tropical climates during the late Cretaceous period, with likely minor arid intervals. In addition, the geochemical markers used (Ba/Al, U/Th, Ni/Co, authigenic uranium (Uau), P/Ti, and [(Fe2O3 + CaO + MgO)/(SiO2 + Al2O3)] indicate poor paleoproductivity for the Araromi Formation deposited in an oxygen-rich marginal to shallow marine environment. The upper Cretaceous period was characterised by fluctuating hydrodynamic circulation, with a weak hydrodynamic regime in core A1 and transitioning to strong hydrodynamic conditions in core A15. The inferred paleowater depth corresponds to the salinity measurements, with the A1 core displaying elevated salinity, indicating a greater depth compared to the A15 core, which was deposited in a shallower environment and displays lower salinity.

Paleoenvironmental reconstruction and organic matter accumulation of the paleogene shahejie oil shale in the Zhanhua Sag, Bohai Bay Basin, Eastern China

The controlling factors of organic-rich shale accumulation is essential for the exploration and development of shale oil and gas resources. The sedimentary environment plays a vital role in the formation of organic-rich sediments in lacustrine facies. This article unravels the mineralogy, geochemistry, and paleoenvironmental evolution during the deposition of the Paleogene Shahejie Formation (Es3L). It discusses the effects of paleoclimate, paleosalinity, paleoredox conditions, paleowater depth, and paleoproductivity on organic matter (OM) enrichment. Finally, the OM enrichment model was established. The results show that the mineralogical compositions are mainly composed of calcite (avg. 40.13%), quartz (avg. 21.64%) and clay minerals (avg. 24.07%), accompanied by dolomite (avg. 7.07%), feldspar (avg. 6.36%) and pyrite (avg. 2.95%). The Es3L shale has a high abundance of OM, with total organic carbon (TOC) ranging from 1.07% to 5.12%. The organic matter type is mainly composed of type Ⅰ-Ⅱ1 kerogen, which is generally considered a good-quality source rock. The source of OM is a mixture of lower bacteria, algae, and plants. During the early sedimentary period, the paleoclimate was dry and cold, with high salinity, intense reducibility, and relatively low productivity. During the late sedimentary period, the climate became warmer and more humid. As a result, the salinity decreased to a level that was suitable for biological reproduction, and productivity increased gradually due to the input of terrigenous plants. Paleosalinity and paleoclimate determined the environment of the sedimentary period, in addition, paleoproductivity and paleoredox condition indicated the formation and preservation conditions of OM. The warm and humid climate, brackish water, suitable reduction conditions and high productivity are the favorable conditions for the generation and preservation of organic matter. The research results may have implications for the genetic mechanisms of organic matter accumulation. They will provide theoretical and technical insights into the exploration and development of shale oil.

Lithostratigraphy and Sedimentation Models of TMG-C4 Well Offshore Eastern Niger Delta

Analysis was carried out in TMG-C4 well by integrating well logs data that revealed three depositional sequences within the Agbada Formation of about 400ft thick. Three systems tracts identified include highstand systems tract, transgressive systems tract and lowstand systems tracts. The highstand systems tract is majorly stacked regressive shoreface sands that occurred in two parasequence sets, whereas the transgressive systems tract is a transgressive unit with both source rock and sealing potentials. The lowstand systems tract shows a boxcar log motif and is identified to be probably channel or barrier bar sand. Major shale units (maximum flooding surfaces) with regional significance mapped are designated MFS. This division is relevant to identifying genetic depositional units. Three genetic sequences occurred in the well. The transgressive shales represent interruptions in the overall regressive sequence that is related to sea-level rise. Three of these shales have been mapped and two of them correspond to the eleven genetic megasequences that occur delta wide. A predominantly marine and deltaic sequences strongly influenced by clastic output from the continent is inferred from the well logs. Paleo water depth is interpreted to fluctuate considerably and deposition occurred within a variety of littoral and neritic environments ranging from nearshore barrier sand complexes to fully marine outer shelf mudstones. The sediments are rapidly deposited within the shallow marine realm and reworked into longitudinal bars by wave action, strong longshore drift and tidal effects. The dominant depositional trend observed in the well shows progradation

Restoration and Evolution of the Paleogene (E1f2) Shale Sedimentary Environment in the Subei Basin, China.

Restoring the sedimentary environment of paleolakes is of great significance to the formation of laminated calcareous shale deposited in paleolakes and the prediction of shale oil reservoir distribution. This article focuses on the second section shale of the Paleogene in the Funing Formation in the Gaoyou Sag, Subei Basin, China, and uses X-ray fluorescence diffraction technology and core lithology analysis methods to obtain the content datum of major and trace elements such as Sr, Cu, Ba, Ga, V, and Ni in shale at different depths. Based on the empirical values of Sr/Cu, Sr/Ba, V/(V + Ni), and total organic carbon, paleoenvironmental evolution of the continental shale was determined and studied, including the changes in paleoclimate temperature, paleosalinity, paleowater depth, and strong or weak redox intensity. The research results indicate that the sedimentary environment of the paleolake in the Paleogene Funing Formation, second section, in the Gaoyou Sag is mainly characterized by a dry and hot climate; the salinity of paleolake water is that of stable brackish water, and the entire sedimentary period of the Funing Formation, second section, is dominated by a reduction environment, which is conducive to the preservation of sedimentary organic matter. The frequent changes in the depth of sedimentary water and the alternating dry and hot climate are the main reasons for the development of laminated calcareous shale in the second section of the Paleogene Funing Formation of the Gaoyou Sag and have also contributed to the abundant commercial resources of laminated calcareous shale oil in the second section of the Funing Formation.

A subaerial origin for the mid-Ediacaran Johnnie valleys, California and Nevada: Implications for a diachronous onset of the Shuram excursion

Mid-Ediacaran incised valleys in the Johnnie Formation of eastern California are of interest for their broad time concordance with comparable features elsewhere (e.g., the Wonoka canyons of South Australia), and the largest negative carbon isotope anomaly in Earth history (the Shuram excursion). The Johnnie valleys have been previously interpreted as originating through mass wasting in a submarine setting at a continental margin, and perhaps as a result of syndepositional tectonism. New data suggest an alternative scenario: that the valleys were cut by a combination of fluvial incision and subaerial mass wasting concomitant with sea-level lowering in excess of 200 m, and were subsequently drowned and filled by marine sediments representing paleowater depths as great as tens of meters. Critical evidence from a series of outcrops in the southern Nopah Range includes (1) a new estimate of the erosional relief of the valleys of at least 208 m; (2) the presence in the basal fill of polymictic conglomerate/breccia with a quartz sand matrix that is locally associated with stratified quartz sandstone, suggesting both local and far-traveled fill components; and (3) beds and blocks of giant ooid packstone-grainstone indicative of shallow marine sedimentation during the early stages of valley filling. The conglomerate/breccia is channelized and associated locally with cross-stratification. Our interpretation is based upon a total of forty-five measured sections, physical stratigraphic mapping supported by sub-meter scale resolution 3-D drone imagery, as well as carbon isotope analyses. Comparable valleys, arguably of the same age, and likewise attributed to subaerial incision in earlier studies, are present at or near the top of the Caddy Canyon Quartzite of northern Utah and southeastern Idaho (a straightline distance of ∼ 850 km). Our working hypothesis is that both sets of valleys correspond in time with Gaskiers glaciation (∼580 Ma). Given that negative δ13C values have been obtained from both below and (new data) within the basal valley fill, and correlated by most workers with the Shuram excursion, we infer that the Shuram encompasses rather than postdates the Gaskiers event in eastern California, and that the onset of the excursion may be diachronous at a global scale.

Characteristics and Paleoenvironment of High-Quality Shale in the Triassic Yanchang Formation, Southern Margin of the Ordos Basin

A set of high-quality lacustrine shales at the bottom of the Chang 7 member of the Yanchang Formation in the Ordos Basin is one of the main source rocks of tight oil and gas and shale oil in the Yanchang Formation. Based on outcrop, core, drilling and seismic data, by the quantitative characterization of outcrops, fine characterization of logging facies and seismic facies, and geochemical tests, the lithofacies types, geophysical response characteristics and organic geochemical characteristics of this high-quality shale are clarified, and the formation paleoenvironment, including redox conditions, paleoclimate, paleosalinity and paleowater depth, is analyzed. The high-quality shale at the bottom of the Chang 7 member is divided into three lithofacies types: black shale, dark massive mudstone and silty mudstone. The organic matter in black shale is mainly interbedded or stratified, the organic matter in dark massive mudstone is dispersed and the organic matter content in silty mudstone is lower. The shale shows high gamma (more than 260 API), a high acoustic time difference (more than 280 μs/m), a high resistivity (more than 330 Ω m) well-logging phase and strong-amplitude parallel–subparallel seismic phase characteristics. Based on the logging and seismic facies characteristics, the plane distribution range of this set of shales is defined. The sedimentary thickness gradually increases from the edge (5–10 m) to the center of the basin, among which the Jiyuan–Huachi–Yijun black shale has the largest thickness (more than 30 m). This set of high-quality shales was mainly formed under a warm and humid paleoclimate, in water depths of 60–120 m, and in an anaerobic reducing and continental freshwater paleoenvironment. The fine identification, distribution range and formation conditions of black shale lithofacies are of practical significance for predicting the distribution of favorable lithofacies of shale oil and gas and the deployment of horizontal wells.

The Shanxi Formation responses to astronomically forced climate changes during the Upper Paleozoic in the Eastern Ordos Basin, NW China

A clearly defined geochronology and stratigraphic interpretation will enhance our understanding of the evolution of paleo water depth and transitional facies, contributing to hydrocarbon exploration. Transitional deposition is extensively observed in the upper Paleozoic of the Ordos Basin. Nevertheless, the correlation between the variations of paleo water depth and global sea-level changes remains uncertain. In this study, we employed gamma-ray logging (GR) data to analyze the cyclostratigraphy of the upper Paleozoic Shanxi Formation in the Ordos Basin. We characterized the elemental geochemistry of the Shanxi Formation using X-ray fluorescence. We reconstructed changes in Paleo Water Depth in the Ordos Basin during the upper Paleozoic by employing sedimentary noise modeling of the tuned GR logging data. Time series analysis showed evidence of the existence of 405 kyr long eccentricity, 125 kyr short eccentricity, and 35.5 kyr obliquity in sedimentary records, which was supported by a sediment accumulation rate model. Using the tuned GR data, we constructed a ∼3.07 Myr astronomical time scale in the Shanxi Formation of the sampling well. Moreover, we used the volcanic ash age (298.18 ± 0.32) Ma at the boundary of the Shanxi Formation and Taiyuan Formation as the anchor point to provide an absolute astronomical time scale from 295.11 Ma to 298.18 Ma for the study area. The variation of Paleo Water Depth in the upper Paleozoic transitional facies was confirmed by both the sedimentary noise model and the X-ray fluorescence (XRF) data, thus supporting each other. Additionally, we discovered a long-period obliquity of approximately 1.2 million years in this model, which is consistent with the global sea-level fluctuations observed during the same period. These results reveal a novel approach for evaluating the Paleo Water Depth variations in transitional facies. Furthermore, our study offers a detailed explanation of the relationship between the evolution of the sedimentary environment and the climate change resulting from astronomical orbit during the glacial period.

Sequence stratigraphy and reservoir quality of the Gulf of Suez syn-rift deposits of the Nukhul formation: Implications of rift initiation and the impact of eustacy and tectonic on deposition

The Nukhul Formation represents the sediments deposited during the earliest phase of the syn-rift period in the Gulf of Suez rift. It is the primary hydrocarbon reservoir for fifteen fields in the region. However, the timing of rifting needs to be better constrained, and the influence of local tectonics and global eustatic sea-level fluctuations on reservoir quality and deposition is still being determined. This research aims to establish a stratigraphic sequence framework, determine the age of the Suez Rift commencement, use changes in paleobathymetry, and assess reservoir quality. The methods employed involve the analysis of four boreholes in the Abu Rudeis-Sidri field, incorporating biostratigraphy, well-logs, and 3D seismic data. The integrated paleo-structure and biostratigraphic analyses indicate that the beginning of rifting in the Gulf of Suez occurred at 23.53 Ma at the first common occurrence (FCO) of Trilobatus primordius. The paleobathymetric analysis of the Nukhul Formation reveals deeper paleo-water depths during certain intervals, indicating deposition during a phase of high subsidence and rift-climax. The Nukhul stratigraphic sequence reflects sea-level changes leading to shifts in accommodation, and facies changes, erosion, and sequence formation during the late Chattian and early Burdigalian stages. However, the reservoir characterization of the Nukhul Formation is significantly affected by various factors, including cementation, pore system properties, sediment sorting, rift system geometry, and relative sea-level fluctuations. These factors must be carefully considered in any regional exploration or development efforts. The study also reveals that tectonic activity played a role in forming fault-bounded basins, and eustatic sea-level changes influenced the rate and timing of sediment accumulation. These findings can aid in predicting the distribution of the Nukhul sedimentary deposits and improve the ability to explore and develop their resources.

Quartz Origins and Paleoenvironmental Controls on Organic Matter Accumulation of Marine Shale in the Ordovician Wulalike Formation, Northwestern Ordos Basin, China: Significance for Shale Gas Exploration

The Ordovician Wulalike marine siliceous shale is a notable hydrocarbon source rock in the Northwestern Ordos Basin. However, the causes of quartz and organic matter enrichment are still a mystery to experts. In this study, the organic geochemistry (maceral compositions, Rbitu, and TOC) and elemental geochemistry (major and trace elements) with mineralogy (XRD) and petrography were jointly acquired to systematically investigate the quartz origins and the paleoenvironment, and the main controlling factors for organic matter enrichment in the Wulalike shale. The results show that the organic matter is type I kerogen with low TOC concentrations (average 0.51%), and that the thermal evolution has reached mature and high mature stages (mean Requ is 1.08%). Three types of quartz are developed in Wulalike shale: biogenic quartz (average 63%) is the most dominant, followed by clastic quartz (average 31%) and microcrystalline quartz (average 6%). Babio and Ba/Al values indicate the low paleoproductivity, which is the primary cause for the low TOC found throughout the area. Redox indexes show the anoxic or dysoxic to oxic conditions from the bottom to the top of the section. Paleoclimate and paleowater depth proxies also changed from the bottom to the top. Various paleoenvironments and sedimentological evidence show that Wulalike shale went through the changing environments, and the early sedimentary environments were conducive to organic matter enrichment, leading to a relatively high TOC. The paleoproductivity and preservation conditions have an impact on organic matter enrichment. Based on the results of biogenic quartz distribution and sedimentary environments, it is considered that the bottom of the Wulalike Formation is the most favorable for shale gas exploration and development.

Paleoenvironmental reconstruction of the Eocene sediments in the Baiyun sag of the Pearl River Mouth Basin

The Baiyun sag of the Pearl River Mouth Basin has become a focus for deepwater exploration with the highest hydrocarbon potential in northern South China Sea. The Eocene organic-rich mudstones are the main source rocks in the Baiyun sag, and the evolution of their depositional environment remains unclear. Based on the core and geochemical data, we investigated the depositional environments of the Eocene Wenchang and Enping sediments in the Baiyuan sag and revealed the sedimentary environment influences on the formation of the source rocks in the study area. The results indicate that the sediments of the Wenchang and Enping formations were deposited in dysoxic to oxic environments. Moreover, according to the concentrations of salinity-sensitive trace elements, and δ13C (-10.9‰ to -7.2‰, average -8.58‰) and δ18O (-18.4‰ to -14.5‰, average -17.1‰) values of carbonate cement in the sandstone samples, the Wenchang and Enping formations were in fresh water to brackish water conditions during their deposition, with increasing salinity from bottom to top, which suggests that the two formations were mainly deposited in lacustrine environment and the Enping formation experienced a relatively extensive lake transgression. The vertical variations of geochemical characteristics suggest that the Wenchang and Enping formations were deposited under a semi-arid/warm condition and all Eocene sediments experienced mild to moderate chemical weathering, relatively large detrital input and stable paleowater depth. Both primary productivity and preservation conditions played essential roles in controlling the enrichment of organic matters in the Baiyun sag, and the preservation conditions resulted from the restricted water setting. The present work provide a comprehensive analysis of depositional environments based on the abundance of high-resolution geochemical data. The results obtained reveal the dominated factors controlling organic matter enrichment in the Baiyun Sag of the Pearl River Mouth Basin. Those findings are not only significant for understanding the region’s paleogeography, but also provide guidance for future profitable oil and gas exploration in the northern South China Sea and other similar areas.

Late Jurassic rift physiography of the Froan Basin and Frøya High, offshore Mid‐Norway: Development of a syn‐rift shallow marine system

AbstractThe Froan Basin and Frøya High are two major structural elements located on the Mid‐Norwegian Continental Shelf and are separated from the Halten Terrace by major west‐dipping normal fault zones. Compared to the Halten Terrace, the Froan Basin and Frøya High are relatively under‐explored and remain poorly understood in terms of their Late Jurassic tectono‐stratigraphic evolution. Upper Jurassic, shallow marine, syn‐rift deposits (i.e. Rogn Formation) are present locally, but their source, delivery system and depositional environment are not yet well understood. Improving our understanding of how fault activity and rift‐shoulder uplift influenced rift physiography and paleowater depths is crucial when developing depositional models in this region. In this study, we present a model of the Late Jurassic rift physiography of the Froan Basin and Frøya High based on seismic reflection and well data and reverse subsidence modelling. We show that during the Late Jurassic to Early Cretaceous, major footwall uplift caused the Frøya High and the western margin of the central Froan Basin to be subaerially exposed, forming an intra‐rift footwall island. Shallow marine areas to the east, immediately adjacent to the footwall island, accumulated sediment supplied from the uplifted and partially eroded footwall. In contrast, the Trøndelag Platform, north of the Froan Basin and Frøya High, remained submerged throughout the rift episode. We therefore suggest that the extent of the shallow marine system was controlled by the magnitude of footwall uplift along the western margin of the basin and that sediment dispersal was influenced by the coastal paleogeomorphology of the back‐tilted footwall.

Forecast of lacustrine shale lithofacies types in continental rift basins based on machine learning: A case study from Dongying Sag, Jiyang Depression, Bohai Bay Basin, China

Lacustrine shale in continental rift basins is complex and features a variety of mineralogical compositions and microstructures. The lithofacies type of shale, mainly determined by mineralogical composition and microstructure, is the most critical factor controlling the quality of shale oil reservoirs. Conventional geophysical methods cannot accurately forecast lacustrine shale lithofacies types, thus restricting the progress of shale oil exploration and development. Considering the lacustrine shale in the upper Es4 member of the Dongying Sag in the Jiyang Depression, Bohai Bay Basin, China, as the research object, the lithofacies type was forecast based on two machine learning methods: support vector machine (SVM) and extreme gradient boosting (XGBoost). To improve the forecast accuracy, we applied the following approaches: first, using core and thin section analyses of consecutively cored wells, the lithofacies were finely reclassified into 22 types according to mineralogical composition and microstructure, and the vertical change of lithofacies types was obtained. Second, in addition to commonly used well logging data, paleoenvironment parameter data (Rb/Sr ratio, paleoclimate parameter; Sr %, paleosalinity parameter; Ti %, paleoprovenance parameter; Fe/Mn ratio, paleo-water depth parameter; P/Ti ratio, paleoproductivity parameter) were applied to the forecast. Third, two sample extraction modes, namely, curve shape-to-points and point-to-point, were used in the machine learning process. Finally, the lithofacies type forecast was carried out under six different conditions. In the condition of selecting the curved shape-to-point sample extraction mode and inputting both well logging and paleoenvironment parameter data, the SVM method achieved the highest average forecast accuracy for all lithofacies types, reaching 68%, as well as the highest average forecast accuracy for favorable lithofacies types at 98%. The forecast accuracy for all lithofacies types improved by 7%–28% by using both well logging and paleoenvironment parameter data rather than using one or the other, and was 7%–8% higher by using the curve shape-to-point sample extraction mode compared to the point-to-point sample extraction mode. In addition, the learning sample quantity and data value overlap of different lithofacies types affected the forecast accuracy. The results of our study confirm that machine learning is an effective solution to forecast lacustrine shale lithofacies. When adopting machine learning methods, increasing the learning sample quantity (>45 groups), selecting the curve shape-to-point sample extraction mode, and using both well logging and paleoenvironment parameter data are effective ways to improve the forecast accuracy of lacustrine shale lithofacies types. The method and results of this study provide guidance to accurately forecast the lacustrine shale lithofacies types in new shale oil wells and will promote the harvest of lacustrine shale oil globally.

Coralline algal and foraminiferal records of the Pliocene paleoclimatic conditions and water-depth changes in the northern South China Sea

Detailed paleontological and sedimentological data from the Pliocene interval of core XK-1 allow for a detailed documentation of the carbonate system in the northern South China Sea (SCS). Six facies are identified. Based on a comprehensive analysis of facies changes, the paleoclimatic conditions and paleowater depths during the Pliocene in the Xisha area in the northern SCS are reconstructed. The changes in the abundance of the algal genus Sporolithon and the planktonic foraminifera species Orbulina universa, Globigerinoides quadrilobatus, Globorotalia menardii, and Globigerinoides obliquus indicate that the climate in the SCS gradually became warmer from 5.3 to 4.3 Ma and then cooled from 4.3 to 2.6 Ma, which was attributed to the changes in atmospheric pCO2. The high abundance of encrusting acervulinids and coralline algae and the low coral abundance suggest periods of elevated nutrient level, possibly linked to increases in nutrient input from the higher latitudes. Distinct changes in coralline algal and foraminiferal facies help the reconstruction of paleowater depths during the Pliocene in the Xisha area. The water depth was generally rising from 5.3 to 4.3 Ma and declining afterward, which was similar to that in the Yinggehai–Qiongdongnan Basin (located on the northern part of the Xisha area). This similarity indicates that there was a common factor controlling the long-term variation in water depth in the northern SCS. The comparison between paleowater depth changes and global eustatic sea-level fluctuations indicates that the regional tectonic subsidence was important from 5.3 to 4.7 Ma. Such subsidence was probably related to thermal subsidence after the magmatism intrusion activity beneath the basement at the initial stage of the Pliocene. This study indicates that the comprehensive analysis of algal and foraminiferal facies has great potential to reconstruct the climatic conditions and water depths in the ancient times.

Sedimentary features of the gravity flow influenced shale deposit: A case study of Chang 7 member in the Ordos Basin, China

Black shale deposits are usually regarded to be deposited in a relatively slack and deep water environment. The gravity flows that can greatly affect the shale depositional process are well documented in many marine basins, with much less attention paid to the counterpart in lacustrine settings. The Upper Triassic Chang 7 Member in the Ordos Basin, which contains thick black shale and well developed gravity flow deposits provides a good opportunity to investigate the influence of gravity flows on shale development in lacustrine settings. We integrate X-ray diffraction, thin section petrography analysis, major and trace element analysis, total organic carbon testing, electron microscopy, and energy spectrum analysis on shale to reveal such complicated process. Based on the development of silty laminas and thickness of the laminaset/bedding, we classified the Chang 7 shale into five lithofacies. Each lithofacies was unique in terms of laminar morphology, texture, and composition, indicating different depositional processes. Proxies of paleo-water depth, paleo-settling rate, paleo-productivity, paleo-redox, and clastic influx also revealed that these five lithofacies deposited in different depositional settings. We propose new depositional models to explain such heterogenous shale deposits and suggest the paleo-productivity has the strongest influence on the accumulation of organic matter.

MODELING OF HYDROCARBON GENERATION ZONES AND MIGRATION PATHWAYS OF AN OIL FIELD OF PRECASPIAN REGION, KAZAKHSTAN

This article highlights the features of geological (basin) modeling of various zones and migration routes and presents the assessment of resources of the oil field in the Precaspian region in western Kazakhstan. A brief overview of geochemical features of the basin is presented using modern assaying and visualization software packages. The preferred method of constructing 1D models is indicated, and the results of such 1D modeling derived from the results of basin modeling are presented. Additional data points (including paleowater depths, sediment-water interface temperatures and heat flows) as boundary conditions were collected and included in the modeling software for data calibration and cross-checking purposes. For all 1D models produced in this study, Jurassic source rocks were deemed to have entered the oil window in Cretaceous time with the critical moment occurring in Late Cretaceous and Early Paleogene. The implied tectonic history of the basin suggests two possible periods of trap formation: tectonic inversion in the Middle Jurassic and possible folding during late Alpine orogeny which began in Late Cretaceous or early Eocene. While the Middle Jurassic traps can undoubtedly be filled with hydrocarbons, there is no such high degree of certainty with regards to traps formed after Late Cretaceous, as those traps were formed after peak migration periods.