Zhanghu Wang

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.

Recently, significant breakthroughs have been made in the exploration of shale gas in the Lower Cambrian black shale of the Sichuan Basin, indicating a promising commercial extraction potential. However, there remains considerable controversy regarding the pore structural characteristics for this shale formation, especially in the deep-water region. To address this, this paper focused on core samples from two shale gas wells (Xa1 and Xb1) located in the slope-basin facies zone during the Early Cambrian. The analysis involved X-ray diffraction (XRD) analysis, microscopic imaging, and N₂-CO₂ adsorption-desorption experiments to qualitatively and quantitatively characterize the pore structural characteristics, pore size distribution, and pore types in the deep-water region. The results show that Niutitang shales can be classified into four lithofacies based on mineral composition: siliceous shale, muddy siliceous shale, mixed siliceous shale, and siliceous calcareous shale. Meanwhile, the black shale exhibits diverse pore types, predominantly organic pores, along with significant development of interparticle pores, intraparticle pores, intercrystalline pores, and a few microfractures. In the Niutitang Formation shale of deep-water regions, mesopores contribute the most to the pore volume, followed by macropores, exceeding 89%. Micropores contribute the most to the specific surface area, followed by mesopores, accounting for more than 98%. Each lithofacies shale displays a similar multipeaked distribution pattern, with siliceous shale generally having the most significant pore volume and specific surface area and the highest heterogeneity. Notably, the total organic carbon (TOC) content is the primary factor controlling the micropore structure of the Niutitang Formation shale in the study area, with a less significant impact on mesopores and macropores. Siliceous shale, which has a high average TOC content of 9.58 wt %, exhibits the greatest pore volume and specific surface area. Quartz contributes to the development of micropores in the shale of the study area, whereas clay minerals somewhat inhibit pore development. Overall, the mineral composition has a minor impact on pore development. These findings provide theoretical support for the evaluation of deep shale gas reservoirs and the prediction of favorable areas in the Lower Cambrian Niutitang Formation in Western Hunan.

Molecular and stable carbon isotopic compositions of 32 crude oils from the Kekeya area of the Southwest Depression, Tarim Basin, were analyzed comprehensively to clarify oil groups and trace oil sources. The results indicate that lacustrine shale sequences within the Upper-Middle Permian Pusige Formation (P_3–2p) are the major effective oil sources; the thermal maturation effects exert the crucial impact on geochemical compositions of crude oils. In the Kekeya structural belt, crude oils produced from the Lower-Neogene, Middle-Paleogene and Middle-Cretaceous sandstone reservoirs were generated mainly from deeply buried P_3–2p at the late-to-high maturity stage. These condensates are depleted in terpanes, steranes and triaromatic steranes and enriched in adamantanes and diamantanes. The evaluated thermal maturity levels of crude oils by terpanoids and steranes are generally lower than that of diamondoids, implying at least two phases of oil charging. In the Fusha structural belt, oils produced from the Lower-Jurassic reservoirs (J₁s) of Well FS8 were generated from the local P_3–2p at the middle to late mature stage. On the contrary, these oils are relatively rich in molecular biomarkers such as terpanes and steranes and depleted in diamondoids with only adamantanes detectable. The P_3–2p-associated oils can migrate laterally from the Kekeya to Fusha structural belt, but not to the location of Well FS8. The Middle-Lower Jurassic (J_1–2) lacustrine shales as the major oil sources are limited to the area around Well KS101 in the Kekeya structural belt. Crude oils originated from J_1–2 and P_3–2p can mix together within the Cretaceous reservoirs of Well KS101 by presenting the concurrence of high concentrations of terpane and sterane biomarkers and diamondoids as well as 2–4% ¹³C-enriched n-alkanes than those of P_3–2p derived oils. This study provides a better understanding of hydrocarbon sources and accumulation mechanisms and hence petroleum exploration in this region.