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The Northwestern Sichuan Basin is a hot spot for natural gas exploration, and in recent years, many wells have obtained high-yield commercial gas flows in the Qixia Formation. However, due to the influence of foreland orogeny, the oil and gas in the Qixia Formation often have the characteristics of multisource filling, multistage accumulation and multistage transformation, which results in an unclear understanding of the gas source and the accumulation process. This paper uses the concept of sub-block research through carbon isotope and biomarkers analyses, to carry out gas-gas, gas-source and bitumen-source rock comparisons in different structural belts to determine the natural gas sources. On this basis, basin simulation and fluid inclusion analysis technology are used to determine the stage of hydrocarbon accumulation, and then the accumulation process is determined. Studies have shown that the gas source of the Qixia Formation are different in the different structural belts: (1) The natural gas from the Qixia Formation in the thrust nappe belt mainly originates from the Qiongzhusi Formation source rock; (2) The natural gas from the Qixia Formation in the hidden fault belt is a mixture of the source rock of the Qixia Formation and the source rock of the Qiongzhusi Formation; and (3) The natural gas from the Qixia Formation in the gentle belt mainly originates from the Middle Permian source rock and the Upper Permian Longtan Formation source rock. The Indosinian, Yanshania, and Himalayan movements all have great influence on the formation of the Qixia Formation reservoir in northwestern Sichuan, and the accumulation process of the different structural belts is quite different, but generally, the accumulation process of the different structural belts have experienced three stages: a primary oil pool stage, gas pool stage and reformation and definition stage. • Through sub-block research, gas source of the foreland thrust belt is studied. • Different gas sources cause differences in carbon isotopes. • Accumulation process of each structural belt is distinct. • The results have implications for exploration in complex tectonic areas. [ABSTRACT FROM AUTHOR]

Industrial gas flows have recently been discovered from the Guanwushan Formation (D 2 g) in the Northwestern Sichuan Basin for the first time, thus ushering in the prelude of Devonian natural gas exploration in the Sichuan Basin. However, the source and origin of the Devonian natural gas is not clear and this seriously restricts the next exploration and development stages. Based on geochemical experimental analyses, this study aimed to comprehensively research the geochemical characteristics of the Devonian natural gas, reservoir solid bitumen, and source rocks. The results showed that (1) the Devonian natural gas is mainly composed of hydrocarbon gas and is oil cracking gas. (2) The Devonian natural gas has relatively light ethane carbon isotopes ranging from −31.82‰ and −29.93‰, between the Cambrian and Permian natural gas values, indicating it is a mixed gas, but is more similar to the Cambrian natural gas. (3) The carbon isotopes of the Devonian reservoir solid bitumen were less than −29‰ and C 29 sterane had an absolute predominance among the sterane compounds, thus showing that it is derived from the source rocks of the Qiongzhusi Formation. (4) During the late Permian, the Lower Cambrian source rocks entered the high peak period of hydrocarbon generation and the crude oil migrated down the fractures to the Devonian reservoirs. During the later stage of deep burial, the crude oil cracked into the dry gas and the heavy contents formed pyrobitumen through polycondensation. Meanwhile, gas from the Permian source rocks mixed to form the present Devonian reservoirs. These research results play an important guiding role in the natural gas exploration and development in the marine stratum of the ultra-deep Devonian System in the Sichuan Basin. • Devonian natural gas in the Northwestern Sichuan Basin belongs to crude oil cracking gas. • Devonian natural gas in the Northwestern Sichuan Basin was mainly derived from source rocks of Qiongzhusi Formation. • The research is favorable for the next exploration and development in Devonian natural gas. [ABSTRACT FROM AUTHOR]