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The paleo-environment and organic matter accumulation after the Marinoan "Snowball Earth" event is of great significance to understand the Earth system and petroleum geology during this critical period. However, the present studies in South China are not well constrained due to the lack of continuous sedimentary records. Here, we present an integrated petrological and geochemical study of a unique, continuous, and complete outcrop in the northeastern Sichuan Basin, SW China. Results show that the postglacial Doushantuo shales in the study area can be divided into three intervals (from bottom to top: Unit1–Unit2–Unit3) according to geochemistry evolutionary trend. Multiple redox proxies (V EF , U EF , Mo EF , C org /P, Th/U, and U Mo covariation pattern) indicate the bottom redox condition changed from oxic to suboxic, and back to weakly oxic stratigraphically. Ba bio and Ba/Al paleoproductivity proxies suggest that the primary productivity was middle–high and it was slightly higher in Unit 2. The CIA corr values of Doushantuo shales range from 72.2 to 86.8 and gradually increase from Unit 1 to Unit 3, indicating relatively intense weathering conditions and global warm and humid paleoclimatic conditions. The Doushantuo shales in our studied outcrop is approximately 100 m in thickness. They are at over mature stage with average random reflectance of 1.86–2.14%, which makes the recovered original organic matter abundance up to ~2.0 wt%. The kerogen of the shales is type II. The accumulation of organic matter was mainly controlled by the preservation environments, with the primary productivity level as a secondary control. These data were further compared with other outcrops reported in previous studies for a comprehensive understanding. The variation of organic carbon contents in the Doushantuo shales hold similar features across the entire South China, with the middle part producing the highest-quality source rocks with conventional and unconventional hydrocarbon geological significance. This suggests that changes in sea level and paleo-weathering after the Marinoan glaciation controlled the enrichment of organic carbon in shales. The enrichment is a large-scale geological event across basins rather than a local one. This might be general and referenced for the postglacial and/or interglacial shales. [ABSTRACT FROM AUTHOR]

Inorganic geochemical approaches (i.e., trace elements) for hydrocarbon–source rock correlations in sedimentary basins are a frontier area of research in petroleum geochemistry. This study used inorganic geochemistry to the over mature petroleum systems in the Sinian (Neoproterozoic) Dengying Formation in the Sichuan Basin of China. Our results show that the Ba N –Ni N –Cu N ternary diagram, V N –Ni N –Mo N ternary diagram and V/Ni–U/Th diagram (related to paleo-productivity and redox conditions), rare earth element (REE)–Y patterns, Ce/Ce*–La/La* diagram, and Ce/Ce*–La/La*–(LREE/HREE) N ternary diagram (related to fluid interactions) can robustly distinguish different source rocks. In particular, V, Ni, and Mo have a strong relationship to organic matter in source rocks, while their occurrence is most likely related to porphyrins in petroleum. Therefore, the V N –Ni N –Mo N ternary diagram can be used for hydrocarbon–source rock correlations. In contrast, Ba, Cu, U, Th, and REE–Y have different and complex geochemical behaviors, limiting the application in hydrocarbon–source rock correlations. The application in the Sichuan Basin shows that the hydrocarbons in the Dengying reservoir were mainly derived from the lower Cambrian Qiongzhusi Formation with minor contribution from the lower Cambrian Maidiping and Sinian Doushantuo formations. Compiled data from Lower Paleozoic crude oils in the Tarim Basin show that the V N –Ni N –Mo N ternary diagram can also be applied to low-maturity crude oil systems. Therefore, V, Ni, and Mo have strong relationship to organic matter in petroleum systems and the potential for hydrocarbon–source correlation, while other elements should be used under given conditions. [ABSTRACT FROM AUTHOR]

The Neoproterozoic provides an important record of glacial–postglacial paleoenvironments, organic carbon cycling, and hydrocarbon accumulation. Here, to better understand the environmental, sedimentary, and petroleum geological potential of the Neoproterozoic, we summarize and review research on paleoenvironment and hydrocarbon prospectivity during the era for regions worldwide and also present a case study of the Marinoan postglacial Ediacaran Doushantuo Formation in the Sichuan Basin, upper Yangtze region, China. Sediments that were deposited during the Neoproterozoic postglaciations generally have hydrocarbon-generating potential. In some places, the sediments have reached stages of burial and thermal evolution appropriate for hydrocarbon generation, with some sites having been exploited for commercial production. In the Sichuan Basin, the distribution and thickness of Doushantuo Formation sediments were controlled by glacially eroded topography and/or extension-related subsidence. Abundant organic matter was produced through paleo-productivity (Ba/Al and Cu/Al proxies can reach up to 181.08 and 8.94, respectively) and anoxia (U-EF and Mo-EF proxies can reach up to 5.45 and 14.00, respectively) governed by marine transgression. In South China, many depocenters of the Doushantuo Formation sediments have a strip-shaped geometry in the plane and are expected to be prospective for oil and gas. The reported findings can be used as a reference for further studies of glacial and postglacial deposits with organic matter accumulation of the Neoproterozoic and other periods. [ABSTRACT FROM AUTHOR]