Geochemical characteristics and genesis of tight gas in Shahezi Formation, Xujiaweizi Fault Depression, North Songliao Basin, China

Abstract

Along with exploration of unconventional natural gas in North Songliao Basin, the tight gas in Shahezi Formation has become a new hotspot, followed by continual breakthroughs in terms of gas exploration. In order to understand the generation and distribution of tight gas in deep-layer of North Songliao Basin, this study focuses on the geochemical characteristics and accumulation period of tight gas, based on analysis of geochemical data such as gas compositions, carbon isotopes and homogenization temperature. According to the result of these analysis, gas in Shahezi Formation is dominated by methane with a light portion of heavy hydrocarbons and generally non-hydrocarbon, such as carbon dioxide and nitrogen. There is a widely distribution of δ13C1 which is ranges from −33.2‰ to −27.8‰, while δ13C2 is higher and have a relatively intensive distribution from −29.2‰ to −22.4‰. Carbon isotopic distribution of tight gas is characterised by a normal pattern, some partial reversal of carbon are observed. The identification of tight gas shows that the main type of tight gas in Shaheizi Formation are coal-formed gas and there are two accumulation periods including Nengjiang period and Qingshankou period. The mix of high maturity coal-formed gas in Nengjiang period and relatively low maturity coal-formed gas in Qingshankou Period is the major cause of carbon isotope reversal. In addition, it is notable that carbon dioxide in study area are inorganic origins, resulted from magma activity in Yingcheng Period.