Multiple origins of the Paleogene natural gases and effects of secondary alteration in Liaohe Basin, northeast China: Insights from the molecular and stable isotopic compositions

The molecular and isotopic (C and H) compositions of Paleogene [Shahejie Formation (E2-3s) and Dongying Formation (E3d)] natural gases from 11 new gas samples as well as the data of 71 gases published from previous studies in the Liaohe Basin were analyzed, and the results were considered in combination with geochemical and distribution characteristics of the source rocks. The genetic type, gas-source correlation and secondary alteration of the natural gases were studied in detail, and the source rock maturities of the gases were estimated. The following conclusions were reached: (1) with the exception of dry gases in the major part of the third member of the Shahejie Fm. (E2s3) reservoirs, the gases in other reservoirs are mainly wet; (2) variable carbon and hydrogen isotopes of the alkane gases are observed in different reservoirs of the basin with δ13C1 values ranging from − 60.7‰ to − 34.2‰, δ13C2 from − 47.3‰ to − 22.3‰, δ2HCH4 from − 261‰ to − 188‰, and δ2HC2H6 from − 269‰ to − 148‰; (3) the natural gases of the basin are of multiple origins, and mixing of thermogenic and microbial is observed - the mixing ratio in the first member of the Shahejie Fm. (E3s1) is the highest among all the reservoirs; (4) gases of different reservoirs suffered varying extents of biodegradation, which caused partial reversal of the carbon isotope series of methane and its homologues (δ13C1 < δ13C2 < δ13C3 > δ13C4); the reversal of the gases in the shallow reservoirs (E3s1 and E3d) is greater, while those in the deep reservoir (E2s3) is less; (5) humic natural gases of the ED were mainly generated by source rocks at early maturity, while the sapropelic ones of the ED and WD were generated during the mature and high mature stages; (6) both the source rocks of E2s3 and E2s4 strata could contribute to the gas reservoirs of E2s3 and E2s1 in the west depression. Sapropelic gases in east depression were generated by E2s3 source rocks, while the other gases reservoired in the E2s3, E3s1, and E3d strata are probably sourced from the source rock strata of E2s3 and E3s1-2.