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Lacustrine fine-grained sedimentary rocks in the Dongying Sag of the Bohai Bay Basin in China exhibit significant potential for hydrocarbon exploration. This study investigates the lithofacies types and sedimentary evolution of the Paleogene Shahejie Formation’s lower third member (Es3l) and upper fourth member (Es4u), integrating petrological and geochemical analyses to explore the relationship between lithofacies characteristics and sedimentary environments. The results show that the fine-grained sedimentary rocks in the study area can be classified into 18 lithofacies, with seven principal ones, including organic-rich laminated carbonate fine-grained mixed sedimentary rock lithofacies and organic-rich laminated limestone lithofacies. In conjunction with analyses of vertical changes in geochemical proxies such as paleoclimate (e.g., CIA, Na/Al), paleoproductivity (e.g., Ba), paleosalinity (e.g., Sr/Ba), paleo-redox conditions (e.g., V/Sc, V/V + Ni), and terrigenous detrital influx (e.g., Al, Ti), five stages are delineated from bottom to top. These stages demonstrate a general transition from an arid to humid paleoclimate, a steady increase in paleoproductivity, a gradual decrease in paleosalinity, an overall reducing water body environment, and an increasing trend of terrestrial detrital input. This study demonstrates that the abundance of organic matter is primarily influenced by paleoproductivity and paleo-redox conditions. The variations in rock components are predominantly influenced by paleoclimate, and sedimentary structures are affected by the depth of the lake basin. Special depositional events, such as storm events in Stage II, have significantly impacted the abundance of organic matter, rock components, and sedimentary structures by disturbing the water column and disrupting the reducing conditions at the lake bottom. The present study offers crucial insights into the genesis mechanisms of continental lacustrine fine-grained sedimentary rocks, facilitates the prediction of lithofacies distribution, and advances the exploration of China’s shale oil resources in lacustrine environments.

Organic matter (OM)-hosted pores play a crucial role in unconventional shale reservoirs, with their development influenced by OM type and thermal maturity across terrestrial, transitional, and marine deposits. In this study, a comparative analysis of porosity and pore structures is presented using organic petrographical, petrophysical, and mineralogical methods on organic-rich samples from diverse depositional environments. A pore evolution model for these sediments in different settings is proposed. Results show that kerogen particles in terrestrial shales at low and moderate thermal maturity (Dameigou Formation and Qingshankou Formation) are mostly nonporous. Transitional shales (Longtan Formation) contain vitrinite and inertinite, with only some inertinite exhibiting visible primary pores. In marine shales at higher maturity (late oil window; Dalong Formation), the interparticle pore space is occupied by solid bitumen, and secondary porosity is present at higher maturity, approaching the thermal gas generation stage. In over-mature marine shales (Wujiaping and Daye Formations), secondary pores are densely distributed within pyrobitumen. A negative correlation between organic carbon content and pore volume is observed in low-maturity lacustrine and transitional shales due to poorly developed kerogen-bound pores and interparticle pore occlusion by solid bitumen. However, over-mature marine shales exhibit a strong positive correlation due to extensive secondary porosity in pyrobitumen. Thus, pore evolution within OM is controlled by kerogen type and maturity. In oil-prone marine and lacustrine shales, secondary porosity in solid bitumen and pyrobitumen increases with thermal maturity. In contrast, terrestrial kerogen rarely forms solid bitumen and mainly develops micropores rather than mesopores at high maturity.

The sedimentary environment is inextricably linked to the macroscopic and microscopic characteristics of shale reservoirs, which influences shale gas accumulation significantly. This study discusses how the sedimentary environment affects the organic-matter-rich shale reservoirs that have been deposited in typical marine, marine-continental transitional, and continental basins in China. The following four aspects were analyzed including shale rock type and thickness distribution, organic matter abundance and distribution, mineral composition and pore structure, and kerogen type and hydrocarbon generation potential. From continental to marine facies, the sedimentary setting of shales with high organic content generally ranges from shore-shallow lakes to deep lakes, deltas, tidal flat lagoons, shallow sea shelves, and deep or semi-deep seas. In deeper water, the clay mineral content decreases, but the brittleness index and siliceous content increase with darker shale color. Thick shales mostly were deposited in deep or semi-deep lakes, delta fronts, prodeltas, tidal flat lagoons, and deep or semi-deep seas from continental to marine basins. The primary factors influencing organic matter enrichment in deep-sea and deep-lake shales are redox conditions and high biological productivity under favorable sedimentary environments, whereas favorable factors for organic matter enrichment in transitional facies include warm-humid palaeoclimates and abundant debris inputs. Continental shale is characterized by the presence of intergranular and intragranular pores, a low pore volume and specific surface area, and a high average pore size and hydrocarbon potential. The kerogen types are complex in continental shales, with type I in deep lake shales and type III in lakeshore shales. Transitional shales occur mostly in coal-bearing strata with type III organic content, medium pore sizes, and hydrocarbon generation potential. The high specific surface area and pore volume, small pore size, and high brittle mineral content of marine shale facilitate the production of dissolution pores. Marine shales are mainly kerogen type I-II 1 with relatively high maturity and low hydrocarbon production potential. By constructing an intrinsic link between the sedimentary environment and reservoir parameters, a sedimentary model of organic-rich shale under different depositional context should be summarized in the future, which can provide a foundation to analyze the geological circumstances of shale gas accumulation.

Trace elements in sedimentary rocks are highly sensitive to palaeoaquatic environmental changes in a sedimentary environment, making them an effective means for studying the paleoclimate and paleoenvironment during the deposition of sediments. The trace elements and major elements of mudstone cores sampled in the Binnan Oilfield in China were tested by inductively coupled plasma mass spectrometry (ICP–MS). Strontium (Sr), barium (Ba), vanadium (V), nickel (Ni) and boron (B), which are all sensitive to the sedimentary environment, were selected as discriminant indicators, and the sedimentary environment of the Shahejie Formation in the Binnan Oilfield was studied by combining with sedimentary indicators. The results show that the equivalent B content and the Sr/Ba ratio discriminate the research area for salt water and freshwater sedimentary environments. The V/(V + Ni) ratio is between 0.65 and 0.81, meaning that this area has a highly reductive sedimentary stratum. The trend of the Rb/Sr curve indicates that the paleoclimate of the Shahejie Formation changed from dry to humid and then back to dry.

Liu, J.; Xing, L.; Shao, C.; Li, S.; Huang, M.; Weng, J., and Fu, C., 2023. Update to the Quaternary stratigraphic division in Xiaoshao Plain, Zhejiang, China. Journal of Coastal Research, 39(5), 984–997. Charlotte (North Carolina), ISSN 0749-0208. Establishing a reliable chronological framework of Quaternary sedimentary stratigraphy contributes to a more comprehensive understanding of the evolutionary patterns of sedimentary environments and predicts their trends. Local exploration standards are essential indicators in the construction of a chronological framework for Quaternary sedimentary stratigraphy. However, many of the local standards introduced today lack detailed field investigations and comprehensive and in-depth analyses of geological investigations in local areas and still need further improvement. For these areas, there are significant differences in understanding between different researchers, and the absence of appropriate and uniform local standards will lead to misjudgment of geological conditions and cause much inconvenience in the design, construction, and management of engineering and construction projects. On the basis of this, Xiaoshao Plain in Zhejiang Province, China serves as a research subject and samples were collected by drilling and coring. The Quaternary stratigraphy of Xiaoshao Plain is tentatively divided into nine major layers and 22 sublayers following the local standard of Zhejiang Province's Code for Geotechnical Engineering Investigation of Engineering Construction (DB33/1065-2009) introduced in 2009. The geological age of some of the strata in the area and changes in the depositional environment were further explored through accelerator mass spectrometry (AMS) 14C testing and microsomal paleontological fossil identification. Thus, the large stratigraphy of the Quaternary sedimentary strata of Xiaoshao Plain in the Zhejiang local standard is supplemented. Finally, in combination with the existing regional engineering geological survey report, the stratigraphic substratum of the area has been reclassified (the study divides it into 36 substrata). This study provides new ideas for the improvement of local standards for geological surveys and guidance for engineering construction development in Xiaoshao Plain.

Due to the limitations of analytical methodologies, the geochemical significance of C 5 alkylated benzenes has largely been overlooked. Thirty-four light oils and condensates were collected from the Tarim Basin and Beibuwan Basin for analysis using comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (GC×GC-TOFMS). The concentrations of twelve C 5 alkylated benzenes exhibit distinct distribution patterns in light oils and condensates originating from varying sedimentary environments and organic matter sources. The concentrations of 1-E-3,4,5-TMB, 1-E-2,3,4-TMB, and 1-E-2,4,5-TMB are likely influenced by the source of organic matter, whereas those of 1-E-2,4,6-TMB, 1-E-2,3,5-TMB, and 1,2,3,4,5-PMB are potentially governed by the sedimentary environment. Based on the differential sensitivity of C 5 alkylated benzenes to sedimentary environments and organic matter sources, the l-ethyl (E)-2,4,6-trimethylbenzene (TMB)/1-E-2,3,6-TMB (C5-R1) and 1-E-2,4,6-TMB/1-E-3,4,5-TMB (C5-R2) have been proposed as geochemical indicators for distinguishing the sedimentary environments and organic matter sources of crude oils from their respective source rocks. Oils derived from their respective source rocks that formed in oxic/disyoxic sedimentary environments are characterized by relatively high C5-R1 values (greater than 0.5), while those formed under reduced sedimentary conditions exhibit relatively low C5-R1 values (less than 0.5). Oils originating from terrestrial higher plants are characterized by relatively high C5-R2 values (greater than 1.0), whereas those derived from a mixed input of lower aquatic organisms and terrestrial higher plants display relatively low C5-R2 values (less than 1.0). The C5-R1 and C5-R2 ratios are likely to remain unaffected or only minimally influenced by secondary alteration processes (evaporative fractionation, biodegradation, and thermal maturity). The C5-R1 and C5-R2 ratios can serve as supplementary parameters for identifying sedimentary environments and organic matter sources, particularly in light oils and condensates where conventional biomarkers are significantly depleted.

The lacustrine mudstones and shales of the Triassic Yanchang Formation in the Ordos Basin serve as critical hydrocarbon source rocks. However, previous studies predominantly focus on individual lithologies, with comparative investigations into the sedimentary environments of dark mudstones and black shales remaining relatively limited. The study systematically compares sedimentary environment parameters (e.g., paleoclimate, paleosalinity, paleoredox conditions, paleowater depth, and paleoproductivity characteristics) between mudstones and shales, and how these distinct environmental factors governed the differential enrichment mechanisms of organic matter within the depositional aquatic system has been elucidated. Geochemical proxies (e.g., CIA, Sr/Cu, Rb/Sr, Sr/Ba, V/Ni, U/Th, V/Cr, Rb/Zr, P/Ti, Cu/Ti) reveal marked contrasts: In comparison with the Chang 7 and Chang 8 dark mudstones, the Chang 7 black shales exhibit (1) warmer–humid paleoclimatic regimes, (2) higher paleosalinity, (3) intensely anoxic conditions, (4) deeper paleowater depth, and (5) elevated paleoproductivity. These environmental divergences directly govern the significant total organic carbon content disparity between black shales and dark mudstones. Organic enrichment in the Chang 7 dark mudstones and black shales is primarily controlled by paleoproductivity and paleoredox conditions, with secondary influences from paleoclimate and paleowater depth. Based on the above studies, this research established a differential organic matter enrichment model. This research is of significant importance for guiding oil and gas exploration and development in the Ordos Basin.

The 2021 Mw 7.4 Maduo (Madoi) earthquake that struck the northern Tibetan Plateau resulted in widespread coseismic deformation features, such as surface ruptures and soil liquefaction. By utilizing the unmanned aerial vehicle (UAV) photogrammetry technology, we accurately recognize and map 39,286 liquefaction sites within a 1.5 km wide zone along the coseismic surface rupture. We then systematically analyze the coseismic liquefaction distribution characteristics and the possible influencing factors. The coseismic liquefaction density remains on a higher level within 250 m from the surface rupture and decreases in a power law with the increasing distance. The amplification of the seismic waves in the vicinity of the rupture zone enhances the liquefaction effects near it. More than 90% of coseismic liquefaction occurs in the peak ground acceleration (PGA) > 0.50 g, and the liquefaction density is significantly higher in the region with seismic intensity > VIII. Combined with the sedimentary distribution along-strike of the surface rupture, the mapped liquefaction sites indicate that the differences in the sedimentary environments could cause more intense liquefaction on the western side of the epicenter, where loose Quaternary deposits are widely spread. The stronger coseismic liquefaction sites correspond to the Eling Lake section, the Yellow River floodplain, and the Heihe River floodplain, where the soil is mostly saturated with loose fine-grained sand and the groundwater level is high. Our results show that the massive liquefaction caused by the strong ground shaking during the Maduo (Madoi) earthquake was distributed as the specific local sedimentary environment and the groundwater level changed.

This study integrates ichnological and sedimentological data to interpret depositional environments of the carbonate sediments of the Tirgan Formation (Lower Cretaceous) in the eastern Kopet-Dagh Basin, north-east Iran. Lithofacies analysis shows that these sediments were deposited in inner ramp, middle ramp and offshore (outer ramp) environments. Five ichnoassemblages are identified in the sediments that consist of Thalassinoides, Thalassinoides–Rhizocorallium, Planolites–Rhizocorallium, Arenicolites–Diplocraterion, and Arenicolites. Th, Th-Rh and Pl-Rh with low diversity and abundance of the trace fossils formed during waning phase of storms in a predominantly medium to high-energy hydrodynamic regime. High sedimentation rate and mobile substrate condition featuring a shallow-marine setting. Ar–Di ichnoassemblage, consisting of horizontal and vertical traces of deposit and suspension feeders, respectively, portray two different phases. A predominantly high energy phase with instable substrate is displayed by the vertical traces, while a minor omission phase, associated with a decrease in sedimentation rate or non-deposition, is indicated by the horizontal structures. Arenicolites ichnoassemblage with low bioturbation index and low ichnodiversity is related to a semi-sheltered area of lagoon environments with periodically marine water circulation. The study of the ichnological attributes in the studied successions indicates the presence of a shallowing up-ward trend in the storm‒tide-dominated ramp sequence. Ichnoassemblage development is largely controlled by depositional and ecological conditions, e.g., the stability of substrate, hydrodynamic regime (wave and tide), and food abundance, which altogether control the substrate colonization. Based on an integrated ichnological and sedimentological approach, we characterize the depositional environment, deciphering allogenic and autogenic environmental controls on the trace fossil distribution on a passive margin depositional setting.

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.