Search Results (128)

The Lower Submember of the Second Member of the Lianggaoshan Formation (LGS2-LS) in the Fuling area, southeastern Sichuan Basin, represents the deepest lacustrine depositional stage of the formation and constitutes an important target for shale oil and gas exploration. Based on core observations, thin-section petrography, X-ray diffraction, geochemical analyses, and sedimentary facies interpretation from representative wells, this study characterizes the lithofacies types, sedimentary environments, and depositional evolution of the LGS2-LS. Results show that the LGS2-LS is dominated by clay–quartz assemblages, with average clay mineral and quartz contents of 44.6% and 38.8%, respectively, and can be subdivided into shallow and semi-deep lacustrine subfacies comprising eight microfacies. Geochemical proxies indicate alternating warm-humid and hot-arid paleoclimatic phases, predominantly freshwater conditions, variable redox states, and fluctuations in paleoproductivity. Sedimentary evolution reveals multiple transgressive–regressive cycles, with Sub-layer 6 recording the maximum water depth and deposition of thick organic-rich shales under strongly reducing conditions. The proposed sedimentary model outlines a terrigenous clastic lacustrine system controlled by lake-level fluctuations, transitioning from littoral to shallow-lake to semi-deep-lake environments. The distribution of high-quality organic-rich shales interbedded with sandstones highlights the LGS2-LS as a favorable interval for shale oil and gas accumulation, providing a geological basis for further hydrocarbon exploration in the southeastern Sichuan Basin. Full article

Glycerol Dialkyl Glycerol Tetraethers (GDGTs) are microbial membrane lipids that can provide crucial information for identifying organic carbon sources and understanding paleoenvironments. Despite numerous studies reporting the presence of GDGTs in various terrestrial and marine environments, there is a paucity of reports concerning GDGTs in mangrove wetlands that are characterized by unique hydrological conditions and disproportionately high accumulation rates of blue carbon (i.e., carbon sequestered in coastal ecosystems, where tidal flooding and anaerobic sediments facilitate exceptional long-term carbon storage). This study investigates GDGTs in 81 sediment samples from 5 sediment cores collected from three Asian mangrove wetlands in Bangladesh, Hong Kong, and Guangxi Province, China. The Hong Kong mangrove sediments had the highest GDGT concentration (370.18 ± 58.00 ng·g⁻¹ dws), followed by Bangladesh mangrove sediments (136.70 ± 41.70 ng·g⁻¹ dws), while Guangxi mangrove sediments had the lowest (100.80 ± 28.71 ng·g⁻¹ dws). All samples demonstrated high BIT index values (>0.8), low IIIa/IIa index values (0.09–0.19) and the predominance of tetramethylated brGDGTs (70.38 ± 2.21%), indicating that terrestrial inputs are the primary source of organic carbon. Despite overall low methylation index (MI) values (0.15–0.35) and GDGT-0/Cren ratios, deeper sediment samples in the lower part of HK exhibited GDGT-0/Cren > 2, likely reflecting enhanced contributions of methanogenic archaea under distinct redox conditions compared to upper sediments. This in situ production may complicate the application of GDGT-based paleo-proxies, as indicated by the substantial deviations between CBT’-pH (MBT’_5ME-temperature) and measured pH (instrumental temperature). The dominant bacterial phyla in the mangrove sediments of Guangxi and Bangladesh were Proteobacteria, Actinobacteriota, Chloroflexi, Acidobacteriota, and Firmicutes (>70% relative abundance). However, correlations between microbial community compositions and brGDGT isomers are different among sampling sites. Our study emphasizes that site- and depth-specific microbial activity may significantly contribute to organic matter cycling and the in situ production of GDGTs in mangrove sediments. These factors should be taken into account for organic carbon sequestration and the validity of GDGT-based paleo-proxies in mangrove wetlands. Full article

The eastern Tianshan region in the Central Asian Orogenic Belt (CAOB) is characterized by multiple complex tectonic activity of uncertain historical contribution to the construction of the CAOB. This study utilizes a multi-proxy geochemical approach to characterize I-type monzogranite pluton rocks and their associated hornblende-rich dioritic enclaves to decipher the tectonic and magmatic evolution of the Xigebi area, eastern Tianshan. Zircon geochronology indicates a Triassic and Permian crystallization age of ca. 224.2 ± 1.7 Ma and ca. 268.3 ± 3.0 Ma for the host monzogranites and the dioritic enclaves, respectively. Major, trace and rare earth element distribution, together with Hf isotope systematics displaying noticeable positive εHf(t) anomalies for both rock types, point to partial melting of meta-mafic rocks in an intraplate extensional setting. The diorite was formed by the melting of lower crustal meta-igneous rocks mixed with mantle melts, and the monzogranite, predominantly from deep crustal meta-basalts contaminated by shallow metasedimentary rocks, with some degree of mixing with deeply sourced mantle magma. While both the host monzogranites and their dioritic enclaves are the products of upwelling magma, the younger Triassic monzogranites captured and preserved fragments of the dioritic Permian lower continental crust during crystallization. These multiple stages of magmatic underplating and crustal reworking associated with vertical stratification of the juvenile paleo-continental crust suggest the monzogranites and diorites indicate a change from a post-collisional setting to a regional intraplate regime on the southern margin of the CAOB. Full article

The Raohe subduction–accretion complex (RSAC) in the Wandashan Block, NE China, comprises ultramafic rocks, gabbro, mafic volcanic rocks, deep-sea and hemipelagic sediments, and trench–slope turbidites. We investigate the basalts within the RSAC to resolve debates on its origin. Zircon U-Pb dating of pillow basalt from Dadingzi Mountain yields a concordant age of 117.5 ± 2.1 Ma (MSWD = 3.6). Integrating previous studies, we identify three distinct basalt phases. The Late Triassic basalt (210 Ma–230 Ma) is characterized as komatites–melilitite, exhibiting features of island arc basalt, as well as some characteristics of E-MORB. It also contains high-magnesium lava, suggesting that it may be a product of a juvenile arc. The Middle Jurassic basalt (around 159 Ma–172 Ma) consists of a combination of basalt and magnesium andesite, displaying features of oceanic island basalt and mid-ocean ridge basalt. Considering the contemporaneous sedimentary rocks as hemipelagic continental slope deposits, it is inferred that these basalts were formed in an arc environment associated with oceanic subduction, likely as a result of subduction of the young oceanic crust. The Early Cretaceous basalt (around 117 Ma) occurs in pillow structures, exhibiting some characteristics of oceanic island basalt but also showing transitional features towards a continental arc. Considering the regional distribution of the rocks, it is inferred that this basalt likely formed in a back-arc basin. Integrating the formation ages, nature, and tectonic attributes of the various structural units within the RSAC, as well as previous research, it is inferred that subduction of the Paleo-Pacific Ocean had already begun during the Late Triassic and continued into the Early Cretaceous without cessation. Full article

Atmospheric pressure gradients determine the dynamics of the southwest monsoon (SWM) and northeast monsoon (NEM), resulting in rainfall in the Indian subcontinent. Consequently, the surface salinity, mixed layer, and thermocline are impacted by the seasonal freshwater outflow and direct rainfall. Moreover, seasonally reversing monsoon gyre and associated currents govern the northern Indian Ocean surface oceanography. This study provides an overview of the impact of these dynamic changes on sea surface temperature, salinity, and productivity by integrating more than 3000 planktonic foraminiferal censuses and bulk sediment geochemical data from sediment core tops, plankton tows, and nets between 25° N and 10° S and 40° E and 110° E of the past six decades. These data were used to construct spatial maps of the five most dominant planktonic foraminifers and illuminate their underlying environmental factors. Moreover, the cured foraminiferal censuses and the modern oceanographic data were used to test the newly developed artificial neural network (ANN) algorithm to calculate the relationship with modern water column temperatures (WCTs). Furthermore, the tested relationship between the ANN derived models was applied to two foraminiferal censuses from the northern Bay of Bengal core MGS29-GC02 (13°31′59″ N; 91°48′21″ E) and the southern Bay of Bengal Ocean Drilling Program (ODP) Site 758 (5°23.05′ N; 90°21.67′ E) to reconstruct the WCTs of the past 890 ka. The reconstructed WCTs at the 10 m water depth of core GC02 suggest dramatic changes in the sea surface during the deglacial periods (i.e., Bolling–Allerǿd and Younger Dryas) compared to the Holocene. The WCTs at Site 758 indicate a shift in the mixed-layer summer temperature during the past 890 ka at the ODP Site, in which the post-Mid-Brunhes period (at 425 ka) was overall warmer than during the prior time. However, the regional alkenone-derived sea-surface temperatures (SSTs) do not show such a shift in the mixed layer. Therefore, this study hypothesizes that the divergence in regional SSTs is most likely due to differences in seasonality and depth habitats in the paleo-proxies. Full article

Numerous uncertainties persist regarding the differential enrichment mechanisms of shale gas reservoirs in southern China. This investigation systematically examines the sedimentary environments and reservoir characteristics of the Wufeng–Longmaxi formations in the Changning area of the Sichuan Basin, through the integration of comprehensive drilling data, core samples, and analytical measurements. Multivariate sedimentary proxies (including redox conditions, terrigenous detrital influx, basinal water restriction, paleoclimatic parameters, paleowater depth variations, and paleo-marine productivity) were employed to elucidate environmental controls on reservoir development. The research findings demonstrate that during the depositional period of the Wufeng Formation in the Changning area, the bottom water was characterized by suboxic to anoxic conditions under a warm-humid paleoclimate, with limited terrigenous detrital input and strong water column restriction throughout the interval. Within the Longmaxi Formation, the depositional environment evolved from intensely anoxic conditions in the LM1 through suboxic states in the LM3 interval, approaching toxic conditions by the LM2 depositional phase. Concurrently, the paleoclimate transitioned towards warmer and more humid conditions, accompanied by progressively intensified terrigenous input from the LM1-LM6, while maintaining semi-restricted water circulation. Both paleowater depth and paleoproductivity peaked from the Wufeng Formation to the LM1 interval, followed by gradual shallowing of water depth and declining productivity during the LM3–LM6 depositional phases. Comparative analysis of depositional environments and reservoir characteristics reveals that sedimentary conditions exert a controlling influence on multiple reservoir parameters, including shale mineral composition, organic matter enrichment, pore architecture, petrophysical properties (e.g., porosity, permeability), and gas-bearing potential. Full article

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. Full article

Haigou deposit, located in Dunhua City, southeast Jilin Province, NE China, is a large-scale gold deposit. The gold ore body is categorized into two types: quartz-vein type and altered rock type, with the quartz-vein type being predominant. The vein gold ore body primarily occurs within the monzonite granite and monzonite rock mass in the Haigou area and is controlled by fault structures trending northeast, northwest, and near north-south. In order to constrain the age and tectonic setting of quartz vein-type gold mineralization, we conducted a detailed underground investigation and collected samples of monzonite granite and pyroxene diorite porphyrite veins related to quartz-vein-type gold mineralization for LA-ICP-MS zircon U-Pb dating and whole-rock main trace element data testing to confirm that monzonite granite is closely related to gold mineralization. Pyroxene diorite porphyry and gold mineralization were found in parallel veins. The zircon U-Pb weighted mean ages of monzonite and pyroxene diorite porphyrite veins are 317.1 ± 3.5 Ma and 308.8 ± 3.0 Ma, respectively, indicating that gold mineralization in monzonite, pyroxene diorite porphyrite veins, and quartz veins occurred in the Late Carboniferous. The monzonite granite and pyroxene diorite porphyrite veins associated with quartz vein-type gold mineralization have high SiO₂, high K, and high Al₂O₃ and are all metaluminous high-potassium calc-alkaline rock series. Both of them are relatively enriched in light rare earth elements (LREE) and macroionic lithophile elements (LILE: Rb, Ba, K, etc.), but deficient in heavy rare earth elements (HREE) and high field strength elements (HFSE: Nb, Ta, P, Ti, etc.), the monzonitic granite Eu is a weak positive anomaly (δEu = 1.15–1.46), the pyroxene diorite porphyre dyke Eu is a weak positive anomaly (δEu = 1.09–1.13), and the Nb and Ta are negative anomalies. The Th/Nb values are 0.28–0.73 and 1.48–2.05, and La/Nb are 2.61–4.74 and 4.59–5.43, respectively, suggesting that diagenetic mineralization is the product of subduction in an active continental margin environment. In recent years, scholarly research on Sr, Nd, and Pb isotopes in Haigou rock masses has indicated that the magmatic source region in the Haigou mining areas is complex. It is neither a singular crustal source nor a mantle source but rather a mixed crust-mantle source, primarily resulting from the partial melting of lower crustal materials, with additional contributions from mantle-derived materials. In summary, the metallogenic characteristics, chronology data, geochemical characteristics, and regional tectonic interpretation indicate that at least one phase of magmatic-hydrothermal gold mineralization was established in the Late Carboniferous as a result of the subduction of the Paleo-Asian ocean plate at the northern margin of the North China Craton. Full article

The enrichment of organic matter in marine shale is a complex process involving tectonic–sedimentary interactions. The tectonic setting exerts critical control over sediment provenance, marine biota, and subaqueous environmental conditions in shale deposition. To unravel the mechanisms and differential controls of organic matter accumulation in marine shales across distinct tectonic regimes, this study systematically examines the Lower Cambrian Niutitang Formation and Lower Silurian Longmaxi Formation shales in the Upper Yangtze Block, SW China. Through comprehensive geochemical analyses encompassing total organic carbon (TOC) contents, as well as major and trace elements conducted on 31 shale samples from the Niutitang Formation and 30 samples from the Longmaxi Formation, we characterized their depositional environmental features and compared the distinctions between them. The results indicate that both the Cambrian Niutitang Formation and Silurian Longmaxi Formation shales exhibit high TOC contents, which range from 1.04% to 8.83% (average 4.73%) and from 0.29% to 6.14% (average 3.35%), respectively. Paleoenvironmental proxies demonstrate that the Cambrian Niutitang shales developed under suboxic–anoxic to even sulfidic conditions, with moderate water restriction and high paleoproductivity levels, while the Silurian Longmaxi Formation was deposited under suboxic–anoxic environments with strong water restriction and low-to-moderate paleoproductivity. Organic matter enrichment in the Cambrian Niutitang Formation followed a “productivity + preservation model”, whereas the Silurian Longmaxi Formation primarily adhered to a “preservation-dominated model”. The differentiation in organic enrichment mechanisms between these two marine sequences is attributed to the distinct tectonic settings during their deposition. During the Early Cambrian, the Upper Yangtze Block was in a rift trough tectonic setting influenced by upwelling currents, which triggered algal blooms and subsequent bacterial sulfate reduction (BSR) coupled with marine anoxia and sulfidation. In contrast, the Early Silurian period featured a semi-restricted marine basin with weaker upwelling activity, where organic matter enrichment was predominantly controlled by a restricted, reducing water column. Our findings demonstrate that tectonic settings exert fundamental controls on nutrient availability for algal communities and water column retention levels, serving as critical determinants for organic enrichment processes in marine shale systems. Full article

Late Mesozoic magmatism in Southeast China extensively reworked the Cathaysia Block’s crust, linked to the Paleo-Pacific Plate subduction beneath East Asia. The northeastern Cathaysia Block, largely covered by Cretaceous volcanic-sedimentary basins, has limited Jurassic exposure to Early Cretaceous intrusions, which provides critical insights into deep crust-mantle processes. In this study, we present zircon U-Pb geochronology and Hf isotope, whole-rock geochemistry, and Sr-Nd isotopes of the Middle Jurassic syenite-granite suites and Early Cretaceous granites with enclaves in the Qingyuan area (SW Zhejiang Province) to constrain their petrogenesis and tectonic significance. Middle Jurassic syenites and alkali-feldspar granites (169–167 Ma) exhibit calc-alkaline to shoshonitic affinities and weakly peraluminous compositions. Early Cretaceous granites (134 Ma) and their enclaves (136 Ma) are high-K calc-alkaline and weakly peraluminous to metaluminous. All samples show LILE and LREE enrichment, HFSE depletion, and negative Eu and Sr anomalies, with only syenites displaying negative Ce anomalies. We suggest that the Middle Jurassic syenites originated from the partial melting of an enriched lithospheric mantle influenced by subduction-related metasomatism. Alkali-feldspar granites derived from partial melting of the basement of the Cathaysia Block. Early Cretaceous granites formed by partial melting of lower crustal mafic rocks, with enclaves representing earlier crystallization products, which were then mechanically mixed with granites. We propose the NE Cathaysia Block underwent significant reworking from the Middle Jurassic to the Early Cretaceous. Middle Jurassic syenites formed in a compressional setting linked to Paleo-Pacific Plate subduction, while Early Cretaceous magmatism reflects lithospheric extension and crust-mantle interaction triggered by slab rollback. Full article

A variety of variables, such as organic matter input, redox conditions, depositional rates, and terrigenous input, affect the deposition of black shale. Furthermore, because of the significant regional variations in paleodepositional environments, these factors have a complex role in organic matter enrichment. Global geological events influenced sedimentary conditions, organic enrichment, and the development of organic-enriched shales during the Late Ordovician to Early Silurian. The Wufeng–Longmaxi Formation black shales in Southeastern Chongqing were analyzed for X-ray diffraction (XRD), major and trace element geochemistry, and total organic carbon (TOC) data; this led to further analysis of the relationship between the depositional environment and organic matter aggregation and rock type evolution. The primary minerals found in the Wufeng–Longmaxi shale are quartz, feldspar, carbonatite (calcite and dolomite), and clay. The high index of compositional variability (ICV) values (>1) and the comparatively low chemical index of alteration (CIA) values (52.6–72.8) suggest that the sediment source rocks are juvenile and are probably experiencing weak to moderate chemical weathering. The selected samples all show negative Eu anomalies, flat heavy rare earth elements, and mildly enriched light rare earth elements. The ratios of La/Th, La/Sc, Th/Sc, ΣREE-La/Yb, TiO₂-Ni, and La/Th-Hf suggest that acidic igneous rocks were the main source of sediment, with minor inputs from ancient sedimentary rocks. The correlations of paleoclimate proxies (Sr/Cu, CIA), redox proxies (V/Cr, V/Ni, V/(V + Ni), Ni/Co, U/Th), paleoproductivity proxies (Ba_xs, Cu_EF, Ni_EF), and water mass restriction proxies (Mo/TOC, U_EF, Mo_EF) suggest a humid–semiarid, anoxic, moderate–high paleoproductivity, and moderate–strongly restricted environment. On the basis of the aforementioned interpretations, the paleoenvironment of the Wufeng–Longmaxi Formations was established, with paleoredox conditions and restricted water masses likely being the primary factors contributing to organic matter enrichment. Full article

The interglacial period of the Cryogenian glaciation is a pivotal interval in geological history, marked by two “Snowball Earth” events and the emergence of early animals. Currently, there is considerable debate regarding the paleo-oceanic environment and the dominant factors controlling organic matter enrichment. Here, based on inorganic geochemical data and mineral composition from the Datangpo Formation in Xiangtan (South China), combined with previous research, we have analyzed the paleo-climate, redox condition, seawater restriction, and primary productivity across different sedimentary facies during this critical interval. The results exhibit that the Datangpo Formation can be divided into three members (Da1–Da3) based on lithology. Paleoclimatic proxies suggest the environment was relatively cold during the deposition of the Da-1 Member, while it was relatively warm and humid during the deposition of the Da 2–3 members. Compared to shallow water areas, deep-water areas experienced a more rapid transition in paleotemperature following the Sturtian glaciation event. Combining Mo-U elements, Ce_N/Ce*_N, and C_org/P ratios, the environment was characterized by an oxic environment during the early deposition period of the Datangpo Formation, then gradually transitioned to suboxic, and finally anoxic conditions. Furthermore, the decompression of terrestrial magma chambers led to intense volcanic/hydrothermal activity during the deglaciation period. Hydrothermal activity was most intense during the Da-1 depositional period, followed by Da-2, and gradually declined during Da-3 depositional period. Hydrothermal activity not only provided essential materials for the formation of Mn carbonate ores but also significantly enhanced the primary productivity by introducing large amounts of nutrients in the paleo-ocean. The primary productivity indicators (Ni/Al, Cu/Al) exhibited an obvious coupling with Ce_N/Ce*_N and C_org/P ratios in the Datangpo Formation, indicating that oxygen-rich environments were favorable for biological proliferation, thereby providing abundant organic matter. Anoxic conditions further facilitated the preservation of organic matter, which may be the primary factor driving organic matter enrichment in the Datangpo Formation. Full article

There are two sets of hydrocarbon source rock formations developed in the Paleogene of the Zhu-1 Depression: the Wenchang Formation of semi deep lacustrine facies and the Enping Formation of lacustrine facies. Their basic geochemical characteristics, chemical structures, kerogen components, sedimentary paleoenvironments, etc., are not the same. High quality hydrocarbon source rocks are the basic conditions for oil and gas generation. This article comprehensively evaluates the key depression Paleogene hydrocarbon source rocks in the Zhu-1 Depression, and studies the development mechanism and controlling factors of hydrocarbon source rocks in this area, which is of great significance for understanding the development conditions, quality, and predicting potential high-quality hydrocarbon source rocks. After conducting rock pyrolysis, major and trace element analysis, and infrared spectroscopy experiments on the samples, it was found that the main source rock type of the Wenchang Formation is type II₁, which has a high HI value; the Enping Formation is mainly composed of II₂-III types with low HI values (with a small number of II₁ types), and the source rocks of the Wenchang Formation have a strong hydrocarbon producing aliphatic structure, with the sapropelic and shell formations being larger than the Enping Formation source rocks. By using methods such as CIA values, C values, and Mo-U covariant models, it can be concluded that during the Wenchang to Enping periods, the climate changed from warm and dry to cool and humid, and the overall environment was characterized by freshwater, weak oxidation weak reduction, and gradually decreasing paleo-productivity. At the same time, it was analyzed that the formation of organic rich sediments in the source rocks of the Zhu-1 Depression played an important role in the relative oxygen phase. The ratio of V/(V + Ni) to V/Cr can better indicate the redox environment of the water body and show a good correlation with TOC. Two sets of development models of source rocks controlled by paleooxygen phase were initially established, providing sufficient scientific basis for oil and gas exploration in the area. Full article

In recent years, substantial hydrocarbon discoveries have been documented in Member 4 of the Yingcheng Formation (Ying IV) in the northern Songliao Basin. To fully understand the developmental characteristics of the Yingcheng Formation source rocks and clarify the main controlling factors of their formation, this study focuses on typical wells in the Xujiaweizi Fault Depression. By integrating core data, organic geochemistry, elemental geochemistry, and paleoenvironmental parameter reconstruction, we determined the paleoenvironment, key controls, and organic matter enrichment model during the deposition of Ying IV. The results show that the total organic carbon (TOC) content of the source rocks ranges from 1.46% to 4.34% (average 2.65%). The paleoclimate during deposition was predominantly warm and humid, with low oxygen, reduced water conditions, freshwater-to-brackish salinity, and a deep-lake environment. Paleoproductivity was moderate to high. Relationship analysis indicates that TOC content was jointly controlled by paleoclimate (warm and humid conditions promoting biological proliferation) and paleoproductivity (nutrient supply from volcanic activity and terrigenous clastics). The positive feedback between elevated productivity under warm-humid conditions and deep-lake reducing environments led to organic matter enrichment in the source rocks of Ying IV, following a productivity-controlled model. This study provides critical geological insights for deep natural gas exploration in the Songliao Basin. Full article

The lower Cambrian Yurtus Formation (Є₁y) in the Tarim Basin, characterized by its high organic matter content, serves as a critical source rock for oil and gas exploration in the platform basin. This study presents a high-resolution geochemical analysis of a geological section located near the Aksu Cement Plant in the northwest margin of the Tarim Basin. The focus is on elucidating the sedimentary environment, mechanisms of organic matter enrichment, and the depositional history of the Є₁y source rock. The Є₁y exhibits distinctive geochemical signatures, including elevated concentrations of Mo, Ba, and U, with an average rare earth element (REE) content of 155.75 μg/g. The formation shows significant light REE enrichment (LREE/HREE = 1.74–5.57), a moderate Ce negative anomaly (δCe = 0.4–0.71), and a notable Eu positive anomaly (δEu = 0.94–2.14), indicative of a unique depositional environment influenced by hydrothermal processes. Geochemical evidence suggests that the Є₁y siliceous shales were deposited in a highly reducing, anoxic, and sulfide-rich environment, promoting organic matter preservation and enhancing sedimentary productivity. The presence of hydrothermal trace elements, likely introduced by hydrothermal fluids from volcanic activity along fractures and faults, played a critical role in enriching the sedimentary system, preserving organic matter, and boosting paleoproductivity. The model of organic matter enrichment proposed in this study underscores the dynamic interplay between hydrothermal influences and high primary productivity. These findings provide important insights into the formation of high-quality source rocks and have significant implications for the exploration of deep and ultra-deep oil and gas reserves in the Tarim Basin. Full article