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A preliminary sequence stratigraphic geochemical investigation was carried out to assess the areal distribution and hydrocarbon generation potential of marine mudstones of the Upper Cretaceous Yogou Formation in the Termit Basin, Niger. A total of three third‐order sequences (YSQ1, YSQ2, and YSQ3, from bottom to top) and nine systems tracts were identified in the Yogou Formation. Mudstones of each sequences are widely distributed throughout the study area with their maximum cumulative thickness of ~600–900 m in the Dinga and Moul depressions. The ΔlogR method was applied to establish a prediction model for predicting the total organic carbon (TOC) content of the Yogou Formation mudstones. The results of this source rock evaluation indicate that thick offshore mudstones with fair to good hydrocarbon generation potential and dominated Type II to Type III kerogen widely occur in YSQ1, YSQ2, and in the LST (lowstand systems tract) and TST (transgressive systems tract) of YSQ3, while the thin lower shoreface mudstone intervals in the HST (highstand systems tract) of YSQ3 are characterized by good to excellent hydrocarbon generation potential, dominated by Type II kerogen. The present‐day thermal maturation level of source beds from the YSQ3, YSQ2, and YSQ1 has overpassed the main zone of oil generation in the depocenter of Dinga Depression, whereas the YSQ3 and part of YSQ2 source beds were still within the oil window in the depocenter of Moul Depression. The organic matter richness and hydrocarbon generation potential of source beds varies within the Yogou Formation because of changes in organic matter source during evolution of Western Africa rift basins and global sea‐level fluctuations. This study provides a practical sequence stratigraphic geochemical framework which will give a better understanding for the geochemical characteristics of source rock and support improved assessment of petroleum resources.

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Background Gas hydrate-bearing subseafloor sediments harbor a large number of microorganisms. Within these sediments, organic matter and upward-migrating methane are important carbon and energy sources fueling a light-independent biosphere. However, the type of metabolism that dominates the deep subseafloor of the gas hydrate zone is poorly constrained. Here we studied the microbial communities in gas hydrate-rich sediments up to 49 m below the seafloor recovered by drilling in the South China Sea. We focused on distinct geochemical conditions and performed metagenomic and metatranscriptomic analyses to characterize microbial communities and their role in carbon mineralization. Results Comparative microbial community analysis revealed that samples above and in sulfate-methane interface (SMI) zones were clearly distinguished from those below the SMI. Chloroflexota were most abundant above the SMI, whereas Caldatribacteriota dominated below the SMI. Verrucomicrobiota, Bathyarchaeia, and Hadarchaeota were similarly present in both types of sediment. The genomic inventory and transcriptional activity suggest an important role in the fermentation of macromolecules. In contrast, sulfate reducers and methanogens that catalyze the consumption or production of commonly observed chemical compounds in sediments are rare. Methanotrophs and alkanotrophs that anaerobically grow on alkanes were also identified to be at low abundances. The ANME-1 group actively thrived in or slightly below the current SMI. Members from Heimdallarchaeia were found to encode the potential for anaerobic oxidation of short-chain hydrocarbons. Conclusions These findings indicate that the fermentation of macromolecules is the predominant energy source for microorganisms in deep subseafloor sediments that are experiencing upward methane fluxes.

The Paleogene lacustrine mudstone is one of the most important sets of source rocks in Termit Basin, Niger. However, studies on the Paleogene source kitchen are scarce. In this study, the source rock types, spatial distribution and their hydrocarbon generative potential within the Paleogene Sokor-1 and LV formations are systematically evaluated. A total of two third-order sequences (SSQ1 and SSQ2, from the bottom to top) and six systems tracts were identified in the Sokor-1 Formation, while the LV Formation mainly comprises pure shale and is regarded as a compositive stratigraphic sequence with no need of subdivision. Six types of source rocks could be distinguished within the three depositional environments of the sequence stratigraphic framework: (1) deep lake mudstones and shale deposited in the lacustrine deep-water facies, (2) shallow lake mudstones and carbonaceous mudstones occurring in the shallow lake environment, and (3) deltaic-front mudstones and prodeltaic mudstones developed in the deltaic facies. Deep lake mudstones/shale, which mainly occurred in the SSQ1-TST (transgressive systems tract), SSQ2-TST and LV Formation, are considered organic-rich source rocks with oil-prone generative potential. The shallow lake mudstones and deltaic mudstones were predominantly distributed in the tectonic slope and marginal areas of the Agadem Block within the lowstand systems tract and high stand systems tract of the SSQ1 and SSQ2. Nevertheless, geochemical results indicated that the deltaic mudstones are good gas-prone source rocks but the shallow lake mudstones were classified as poor potential source rocks. The variance in organic matter accumulation and preservation conditions within different depositional facies and the fluctuation of relative lake level are the controlling factors for the occurrence and distribution of high-quality source rocks. The effective Paleogene source kitchen is limitedly distributed around the depocenter of the Dinga Depression within a small scale, and therefore exploration for oil and gas resources from the Paleogene source kitchen should focus on the eastern Dinga fault-step zone adjacent to the depocenter of the Dinga Depression.

The distribution of phenyldibenzofurans (PhDBFs) and methyldibenzofurans (MDBFs) and their potential implications in petroleum organic geochemistry have been investigated for sediment extracts and crude oils derived from lacustrine shale, marine shale, marine carbonate, and terrestrial (fluvial/deltaic/fresh water) mudstone. PhDBF isomers in a set of lacustrine shales from the Liaohe Basin (East China) are identified in the m/z 244 mass chromatograms of the aromatic fraction by coinjection of internal synthetic standards on a high resolution capillary column (HP-5MS). The results show that the relative abundance of 4-PhDBF/2-PhDBF and 4-PhDBF/(2-PhDBF+3-PhDBF) increase gradually with increasing burial depth and maturity (R 0 ≥ 0.6%), and have a good correlation with vitrinite reflectance in lacustrine shales from wells S202 and SG1 in the Liaohe Basin. These two ratios, defined as phenyldibenzofuran ratio-1 and -2 (PhFR-1 and PhFR-2), may be potential maturity indicators for mature sediments in this study. Four MDBF isomers are also ubiquitously present in all samples studied, and show regular distribution patterns in the m/z 182 mass chromatograms of the aromatic fraction. In rocks and oils derived from the terrestrial depositional environments of the Liaohe and Beibuwan Basins (China), there is a marked predominance of 2- and 3-MDBF over 4- and 1-MDBF, and three peaks formed by 4-, 2-+3-, and 1-MDBF show a reversed “V-shaped” distribution pattern in the m/z 244 mass chromatograms. However, for samples derived from the marine environment in the Termit (Africa) and Tarim (China) Basins, the abundances of 4-MDBF generally exceed or approach those of 2- and 3-MDBF. A cross-plot of (1+4)-/(2+3)-MDBF versus Pr/Ph ratios measured on rocks and oils from various depositional environments is used here to investigate the effect of the depositional environment and lithology on the distribution patterns of four MDBF isomers. This cross-plot classified the samples in this study into the following five environmental/lithological zones: marine carbonate; marine carbonate and shale; marine shale; lacustrine shale; and fluvial/deltaic/fresh water lacustrine shales. Samples from various depositional environments in this study can be basically distinguished.