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The determination of the original organic matter type and content of source rock in over-mature petroleum systems is critical for accurately evaluating the original hydrocarbon-generation capacity, but has challenged researchers for decades. Here we present a new method for such analyses using inorganic geochemistry, which was applied to a case study of over-mature shales of the Lower Cambrian Niutitang Formation (Upper Yangtze region, South China). Multiple redox proxies (e.g., V EF , U EF , Mo EF , and Th/U ratio) indicate that the Lower and Upper members were deposited under anoxic bottom water and suboxic conditions, respectively. This implies different proxies suitable for recovering the original hydrogen index (HI o) values, i.e., Cu XS and P org for the Lower (anoxic) and Upper (suboxic) members, respectively. Then, the original total organic carbon (TOC o) content of the shales were calculated by using the formula TOC o = (TOC pd – (S 1 +S 2) ×0.085 – HI o × 0.0004)/(1 – HI o /1177), of which TOC pd refers to the measured present-day TOC data and S 1 and S 2 are the Rock-Eval pyrolysis hydrocarbons. Excellent original hydrocarbon generation capacity was revealed in that HI o and TOC o are 600–893.4 mg hydrocarbon (HC)/g TOC (average = 758.4) and 0.2–32.2 wt% (average = 10.2), respectively. Combined with six lithofacies identified from mineralogy and petrology, heterogeneities of the hydrocarbon-generation potential in different lithofacies of the Niutitang Formation shales of this study were revealed. The laminated siliceous shale and laminated carbonaceous shale facies had higher HI o and TOC o than the other facies, and can thus be regarded as the lithofacies with the highest hydrocarbon-generation potential. Our study provides a new method using inorganic geochemistry to restore the original kerogen type and TOC content of over-mature shales with a high degree of accuracy, which can be used to robustly evaluate the hydrocarbon generation potential of conventional and unconventional hydrocarbon resources in over-mature source rocks. [ABSTRACT FROM AUTHOR]

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]

The Permian Junggar Basin was the site of deposition of a large set of organic-rich sediments, among which the Lucaogou Formation is considered an excellent petroleum source bed. High-frequency variation in lithology is a typical feature of the Lucaogou Formation, reflecting the fluctuation of the paleoclimate and depositional environment. The relationship between climate fluctuation and organic matter enrichment still needs to be elucidated for the Lucaogou Formation. High-resolution sampling was applied to an 86.2 cm long core from the Lucaogou Formation from the Jimsar Sag in the Junggar Basin to examine the effects of climate fluctuation on organic carbon burial. Petrographic, mineralogical, and elemental analyses of the core indicated that the paleoclimate for the core went through a relatively wet-dry-wet sequence, with changes occurring on timescales of thousands of years. The influence of climate fluctuation on organic matter accumulation is multilevel and multifactorial, involving chemical weathering, sediment supply, lake level, paleosalinity, redox, and primary productivity. High-resolution geochemistry portrays the evolution of these factors and how they collectively determine the burial of sedimentary organic carbon. The Lucaogou epoch was generally characterized by dry/cold climatic conditions, low weathering intensity, and limited detrital input. Moreover, low temperatures and scanty nutrient input resulted in low to moderate productivity. The enrichment of organic matter in the Lucaogou Formation was primarily controlled by preservation. • Paleoclimate mediated organic matter enrichment through a complex multilevel and multifactorial process. • Chemical weathering, sediment supply, salinity, redox, and primary productivity changed along with fluctuating climates. • Organic matter accumulation was mainly controlled by preservation rather than primary productivity and mineral dilution. [ABSTRACT FROM AUTHOR]

β -Carotene, the precursor of β -carotane, is commonly rich in brackish sediments and would become unstable theoretically under the exposure to high temperatures, bright light conditions, and acidic or oxdizing environments. However, the specific process and mechanism of the absence of such compounds are still not clear sufficiently. To fill the knowledge gap, in this study we explore the origin of the absence of β -carotane in a 50-m-thick section of the Middle Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin, northwest China, based on a combined mineralogical, petrographic, organic and inorganic geochemical study. Biomarker analyses indicate that the β -carotane-free section was deposited in a brackish, reducing, and semi-deep lacustrine environment. Hydrothermal fluid effects during sediment deposition are inferred to have caused the breakdown of β -carotene. The hydrothermal (<100 °C) effects are evidenced by mineralogy and geochemistry, e.g., the hydrothermal-indicative minerals (reedmergnerit, tuff and pyrite), high Fe/Ti and low Al/(Al + Fe) ratios, rare earth elements (REEs) distribution. Associated biomarker variations were found, e.g., low i -alkanes/ n -alkanes, high C 24 Tetracyclic/C 26 Tricyclic terpanes, high (C 19 + C 20)/(C 21 + C 23) tricyclic terpanes, high C 29 /C 30 hopanes, and very low gammacerane/C 30 hopane ratios. Our data suggest that hydrothermal fluids can prevent the preservation of β -carotene, and the absence of β -carotane indicates a period of hydrothermal activity during deposition of the Lucaogou Formation in a 50-meter-thick sequence. • β -Carotanes were found to be absent in a brackish lacustrine sequence. • Hydrothermal fluids have caused the breakdown of β -carotene. • Anomalously low abundance of isomerization alkanes and high C 24 Tet/C 26 TT ratios were also found. • Hydrothermal activity was present in the Permian Lucaogou Formation, Junggar Basin. • Absence of β -carotane can be used as a proxy for hydrothermal activity. [ABSTRACT FROM AUTHOR]