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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 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]