Genesis and sources of natural gas in fold-and-thrust belt: The Middle Permian in the NW Sichuan Basin

Abstract

Fold-and-thrust belts are important locations for oil and gas exploration, but due to the complex sources, migration, and charging histories the exploration risk is extremely high. The Longmenshan belt in the NW Sichuan Basin offers an excellent opportunity to study the genesis and sources of natural gas in fold-and-thrust belt. We synthesize existing geochemical data from ∼1100 samples from various strata in the Sichuan Basin. Among them, the bitumen biomarkers and the natural gas molecular compositions and stable carbon isotopes in the Middle Permian of different structural belts were analyzed to investigate the gas genetic types and sources. Then, they were compared with natural gas from other strata in the Sichuan Basin. Results indicate that the Middle Permian natural gases in the NW Sichuan Basin are dominated by hydrocarbon gases, with a small amount of CO2 and N2. The identification of the gas origin and gas-source correlation indicate that there are significant differences between the Middle Permian natural gas in different structural belts of the Longmenshan belt. The natural gas in the thrust nappe belt is mainly oil-cracking gas from the Qiongzhusi Formation (Fm) source rock, and the natural gas in the thrust front belt shows obvious mixed-source characteristics, with both kerogen-cracking gas of the Permian source rocks and oil-cracking gas from the Qiongzhusi Fm source rock. However, the natural gas in the unfaulted belt mainly comes from the kerogen-cracking gas in the Permian source rocks. Overall, from the thrust nappe belt to the thrust front belt, and then to a belt that is largely unfaulted, the contribution of deep source rocks controlled by fault migration channels decreases with gradually weakening deformation. This study will help to better understand the petroleum system in the fold-and-thrust belt and optimize the future exploration strategy.