Sedimentary environmental constraints on organic matter enrichment in the coal-bearing Permian Longtan Formation: A case study from NT1H Well, central Sichuan Basin, China

Yongjun Yao¹Xuesong Tian^2,3,4Maoyao Liu^3,4Yisha Liao¹Junfei Peng^2,3,4Dongxin Guo^2,3Ye Zhang^3,4Yuelei Zhang^2,3,4Cong Tuo¹Dishu Chen^2,3,4*Chaogang Chen⁵Jinxi Wang^3,4*Tingpeng He^3,4

• ¹Chongqing Gas Field, PetroChina Southwest Oil & Gas Field Company, Chongqing, China
• ²Chongqing Huadi Resources and Environment Technology Co., LTD, Chongqing, China
• ³Key Laboratory of Shale Gas Exploration, Ministry of Natural Resources, Chongqing Institute of Geology and Mineral Resources, Chongqing, China
• ⁴National and Local Joint Engineering Research Center of Shale Gas Exploration and Development, Chongqing Institute of Geology and Mineral Resources, Chongqing, China
• ⁵Sichuan Energy Investment Oil and Gas Exploration and Development Co. LTD., Chengdu, China

The Permian Longtan Formation (burial depth >2000 m) is a major hydrocarbon-producing unit in central Sichuan Basin, China, and an important heterogeneous unconventional gas reservoir. However, the control of sedimentary environment on organic matter enrichment in its coal-bearing strata remains unclear, hindering deep coalbed methane (CBM) exploration in this region. This study analyzed core samples from NT1H Well using petrological and geochemical methods (e.g., thin-section, X-ray diffraction, trace elements, total organic carbon, FE-SEM, and macerals) to characterize the middle-lower Longtan Formation. It examined how sedimentary environments influence organic matter enrichment and generating-gas capacity (abundance, maturity, and component), revealing key controls. Results show the lithologies in the middle-lower Longtan Formation include mudstone/shale, silty mudstone, and tuffaceous-muddy siltstone-standstone interbedded with multiple thin coal seams, mainly containing clay minerals, quartz, pyrite, and siderite. Horizontal bedding, horizontal-wavy bedding, and parallel bedding are common. Coal seams and their surrounding mudstones/shales have higher organic matter abundances than muddy siltstones-sandstones, indicating strong capacities of organic matter accumulation and gas generation. Coal seams, dominated by vitrinite with minor inertinite, show the highest organic abundance and maturity (TOC 43.76 wt% to 77.97 wt%, R₀ >2.5 %) than surrounding mudstones/shales. The coal-bearing strata is mainly assigned to lagoon deposits, with two microfacies: sandy-muddy lagoon and peat swamp. The peat swamp is formed by a rapid transformation of muddy lagoon, with sufficient supply of terrestrial materials. These results reveal that the peat swamp microfacies, characterized by weak hydrodynamics and anoxic-to-reducing conditions, is the primary control on the development of multiple thin coal seams, mudstone/shale, and carbonaceous shale, constraining the degree of organic matter enrichment. This study clarifies the sedimentary environmental constraint on organic matter enrichment in the coal-bearing strata, providing a geological basis for evaluating deep CBM resources in central Sichuan Basin.