Origin and accumulation model of the AK-1 natural gas pool from the Tarim Basin, China

Abstract

The AK-1 gas pool represents the first commercial gas discovery in the Kashi Depression, northwest Tarim Basin. The pool is characterized by dry hydrocarbon gas (dryness index > 0.995), heavy methane carbon isotopic value ( δ 13C 1 = −25‰), and a lighter δ 13C 2 ( δ 13C 2 − δ 13C 1 = 3.9‰). High CO 2 content ( δ 13C CO 2 = −8.6‰) is observed in the main production reservoir. These characteristics indicate that the hydrocarbon gas originated from multiple sources, and that the CO 2 is of mixed inorganic and organic origin. There are two possible source areas: the lower block of the AK-1 overthrust structure and the central area of the Kashi Depression. Two possible source rocks are present in both areas: Lower Carboniferous mudstone and Lower and Middle Jurassic coal measures, typically with type II and type III kerogens, respectively. Both reservoir fluid inclusion data and trap structure evolution indicate that the gas pool was formed during the Pliocene–Quaternary. We constructed geological models of methane generation and carbon isotopic fractionation for the two potential source rocks using different geothermal histories of the two possible source areas. By comparing modeled results with the geochemical characteristics of the gas pool, we conclude that the source rock of the gas pool is the Lower Carboniferous mudstone, and the main source area is in the lower block of the AK-1 overthrust structure with a secondary source area in the center of the Kashi Depression. The pool gas is interpreted as a late stage cumulative gas. The structure trapped gas generated from source rock in the lower block of the AK-1 overthrust at a methane fractional conversion of 0.64–1.0 together with inorganic CO 2 from the thermal decomposition of Carboniferous carbonate rocks in the central area of the Kashi Depression.