Origin of Paleogene natural gases and discussion of abnormal carbon isotopic composition of heavy alkanes in the Liaohe Basin, NE China

Chemical components, stable isotopic compositions and light hydrocarbons are investigated to determine the origin and isotopic anomalies of natural gases in the Paleogene of the Liaohe Basin of northeastern China. Natural gas in Paleogene is dominated by alkanes. Dry gas predominantly occurs in the shallow layers of the Paleogene having burial depth less than 1900 m, specifically in the Dongying Formation (E3d) and the first member of the Shahejie Formation (E3s1); wet gas mostly occurs in the deep layer, primarily in the third member of the Shahejie Formation (E2s3). The relative content of methyl cyclohexane in the Western Depression (WD) of the Liaohe Basin is less than that in the Eastern Depression (ED), while the relative contents of alkanes in the C5-7 light hydrocarbons of the WD are higher than those of the ED. Natural gases are of multiple origins, including microbial gas (gas generated by microbial action on immature source rock), coal-derived gas (gas generated by humic source rock) and oil-associated gas (gas generated by sapropelic source rock). In the WD, oil-associated gas is dominant, while in the ED, coal-derived gas dominates. The abnormally heavy carbon isotopic composition of Paleogene heavy alkanes is primarily caused by bacteria oxidation, rather than by being mixed with deep source gas. Natural gases from the E3d and E3s1 suffer the most severe bacterial oxidation, the intensity of which gradually declines with the increasing depth. The low average heptane and isoheptane values indicate that natural gases are mostly generated in the early mature stage of the source rock. Natural gases in E2s3 of the WD are mainly derived from E2s4 and E2s3 source rocks, while those of E3d and E3s1 are predominantly generated by E2s3 source rock. Gases in the ED come principally from E2s3 source rock.