Source and significance of Cretaceous oil seepage in the northern Ordos Basin, China

It has long been believed that the region on the north of the Dongsheng gas field has only limited exploration potential due to the lack of effective source rocks. However, several areas of Cretaceous oil seepage have been discovered in the area and their sources are unclear. In this study, the organic geochemical characteristics of these oil seepages and potential source rocks (coals and carbonaceous mudstones of different formations) are examined using TOC analysis, pyrolysis, gas chromatography-mass spectrometry, and carbon isotopes. The results show that the oil seepages are low-to-moderately mature (0.63 %-0.78 % R-c) and slightly biodegraded. The molecular biomarker compositions of the oil seepages are characterized by C-29 >> C-28>C-27 normalized relative abundance of regular steranes, high values of Pr/Ph, (C19TT + C20TT)/C23TT and C24TeT/C26TT ratios, low values of Ga/C30H, and low abundance of dibenzothiophenes. The biomarker compositions, as well as the stable carbon isotopes, indicate that the source of the oil seepages was deposited in a lacustrine fluvial/deltaic environment under oxic to sub-oxic conditions, with organic matter input predominantly from terrigenous higher plants. Oil-source correlation revealed that the oil seepages were closely related to the coals in the Shanxi formation in the south source kitchen, implying that the hydrocarbons generated in the south source kitchen have been able to migrate to the north edge of the Ordos Basin. The oil seepages are accompanied by significant gas leakage. The zone between the Dongsheng gas field and the Wulangar Uplift is therefore likely to have very good accumulation and preservation conditions and may offer a promising exploration prospect for natural gas.