The origin and source of the Devonian natural gas in the Northwestern Sichuan Basin, SW China

Abstract

Industrial gas flows have recently been discovered from the Guanwushan Formation (D2g) 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 C29 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.