The molecular and isotopic (C and H) compositions of Paleogene [Shahejie Formation (E(2-3)s) and Dongying Formation (E(3)d)] 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. (E(2)s(3)) 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 delta C-13(1) values ranging from -60.7% to -34.2%0, delta C-13(2) from 47.3% to -22.3%, delta H-2(CH4) from -261% to -188%, and delta H-2(C2H6) 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 (delta C-13(1) < delta C-13(2) delta C-13(4)); the reversal of the gases in the shallow reservoirs (E(3)s(1) and E(3)d) is greater, while those in the deep reservoir (E(2)s(3)) 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 E(2)s(3) and E(2)s(4) strata could contribute to the gas reservoirs of E(2)s(3) and E(2)s(1) in the west depression. Sapropelic gases in east depression were generated by E(2)s(3) source rocks, while the other gases reservoired in the E(2)s(3), E(3)s(1), and E(3)d strata are probably sourced from the source rock strata of E(2)s(3) and E(3)s(1-2). (C) 2017 Elsevier B.V. All rights reserved.