The origin and secondary alteration of dissolved gas in oil: A case study from the western Tu-Ha Basin, China

Abstract

The Tu-Ha Basin is a typical Jurassic coal-derived petroliferous basin in western China. Seventeen natural gas samples were collected from the Jurassic Sanjianfang and Qiketai Formations, the Cretaceous Tugulu Formation, and the Paleogene Shanshan Formation to analyze the source and secondary alteration of natural gas. The natural gas samples were characterized by molecular and stable carbon (δ13C) and hydrogen (δD) isotope compositions. The results indicate that the natural gases are wet gases, in which methane is the main alkane component. Nitrogen (N2) and carbon dioxide (CO2) are the two non-hydrocarbons. The N2 values (>12%) are relatively higher, which increase rapidly at the burial depth of 1500–2000 m. Some natural gases have suffered secondary alteration at this depth. The natural gases are thermal genetic coal-derived gases (humic gas), coming from the Jurassic coal measure source rocks, which are chiefly in the low mature to early mature stage. Nitrogen in the shallow is chiefly derived from the atmosphere, while nitrogen at the deeper burial depth is mainly derived from the thermal decomposition of organic matter (OM). The shallow natural gas samples have the heaviest carbon isotope composition of ethane (δ13C2) and a carbon isotope sequence of (δ13C1 < δ13C2 > δ13C3 > δ13n-C4 < δ13i-C4), which is mainly caused by the biodegradation. The higher nitrogen content illustrates that a large amount of surface water is running into the reservoirs. Water washing occurs in the reservoirs, which decreases the molecular methane content and increases the amount of heavier hydrocarbons at 1500–2000 m depth.