Petroleum potential and history of Yitong Basin, China

The Yitong Basin in Northeast China is an asymmetric, elongate pull-apart graben. It underwent intense subsidence during the Tertiary, forming four sub-depressions filled with thick alluvial-lacustrine clastic sequences. The sedimentation of the basin is characteristic of rapid near-source deposition during intermontane flooding. The source rock consists of dark claystones and shales deposited in the Eocene. Kerogens are mainly of Types IIb to III with abundant non-fluorescent amorphous organic matter. The average total organic carbon is 0.8–1.4%; the hydrogen index is about 80–150 mg/g TOC. Biomarker distributions in the extracts and oil show high proportions of C29 hopane and moretane, Tm, sesquiterpanes and 4-methyl steranes, but little gammacerane. These data indicate important contributions from higher plants. The Yitong Basin has an average thermal gradient of 38°C/km. Reconstruction of its burial history shows that oil generation begins at 1900–2200 m and condensate is formed deeper than 4000 m. The Yitong Basin produces condensate and paraffinic oils with volumetric oil/gas ratios of 200–600. These oils are mature to overmature with C29 sterane ratios of 0.45, sterone ββ/αα + ββ ratios of 0.45 and 20S/R + S sterane ratios of 0.55. Compared with the nearby coal-bearing Shulan Depression, the Yitong Basin has a higher subsidence rate, but similar source organic matter.