Origin of the Gas Hydrate and Free Gas in the Qilian Permafrost, Northwest China: Evidence from Molecular Composition and Carbon Isotopes

The Qilian permafrost of the South Qilian Basin (SQB) has become a research focus since gas hydrates were discovered in 2009. Although many works from different perspectives have been conducted in this area, the origin of gas from gas hydrate is still controversial. Molecular composition and carbon isotope of 190 samples related to gas hydrates collected from 11 boreholes allowed exploration of genetic type, thermal maturity, biodegradation, as well as gas‐source correlation of alkane gases from gas hydrates and free gases. Results indicate that alkane gases biodegraded after the formation of natural gas. According to differences in carbon isotopes of methane and their congeners (CH4, C2H6, C3H8), the thermal maturity (vitrinite reflectance, VRo) of most alkane gases ranges from 0.6% to 1.5%, indicating a mature to high mature stage. The thermal maturity VRo of a small part of alkane gas (in boreholes DK5 and DK6) is higher than 1.3%, indicating a high mature stage. Alkane gases were mainly produced by secondary cracking, consisting of crude oil‐cracking gases and wet gases cracking to dry gases. Genetic types of alkane gases are primarily oil‐type gases generated from shales and mudstones in the upper Yaojie Formation of Jurassic, with less coal‐type gases originated from the mudstones in the Triassic Galedesi Formation and the lower Yaojie Formation of Jurassic. Carbon dioxides associated with alkanes from gas hydrates and free gases indicate the thermal decomposition and biodegradation of organic matter. The origins of natural gases from gas hydrates and free gases shed light on the evaluation of petroleum resource potential, deeply buried sediments, and petroleum resource exploration in the SQB.