Geochemistry of deep coal-type gas and gas source rocks in Songliao Basin

The composition and carbon isotope composition of about 150 natural gas samples reveal that natural gases of deep strata in Songliao Basin are derived from multiple sources and the coal-type gases are widely distributed in the basin. Several gas fields show features of coal-type gases or contain coal-type gases, with δ13C2 being larger than −28 ‰ and δ13CMCC6 being larger than −22‰, such as Shengping gas field in the north of the basin, Xiaohelong and Xiaochengzi gas fields in the south. Organic matter of source rocks in the deep strata is dominantly Type III, with a small portion of Type II. The average TOC of source rocks in the Shahezi (K1sh) Formation is 1.99%. Sedimentary facies studies of single wells, which penetrated coals of the Shahezi Formation, indicate that these coals are developed in peat swamps of deltaic plain, depressions between fan deltas, coastal shallow lakes, flooding plain etc, and structures are mainly distributed in the boundary of fault depressions, especially in their slope, some in the center of lakes.