Origin of crude oils from the paleogene Xingouzui formation in the Jiangling depression of Jianghan basin, central China

Abstract

The Paleogene Xingouzui Formation is one of the exploration targets in the Jiangling Depression from Jianghan Basin, Central China. However, the origin and source of crude oils remains controversial. In the present work, a total of 16 fresh source rocks and 23 oil samples were collected and geochemically investigated to clarify the source of organic matters, sedimentary environment and evolution degree. The Xingouzui Formation source rock samples are mainly of fair−good source rocks, with the TOC content, S1 and S2 of 0.36%–1.36% (mean of 0.74%), 0.14–0.38 (mean of 0.27 mg HC/g rock) and (mean of 1.12 mg HC/g rock), respectively. And these rocks are dominated by type Ⅱ2 kerogen within “oil window” maturity stage, with the Ro value of 0.6%–1.2% (mean of 0.9%). The biomarker results indicate that the source rocks of the Xingouzui Formation are mainly deposited in anoxic reducing environment, which main source materials are the mixture of terrestrial higher plants and lower marine organisms. The Pr/Ph values of the oils and source rocks range of 0.47–0.64 and 0.23–1.07, respectively. And the δ13C values of the oils and source rocks range from −34.93‰ to −26.41‰ and from −28.3‰ to −23.8‰, respectively. Family-I and family-Ⅱ oils were separated based on the results of biomarkers and stable carbon isotope, indicating that family-I oils, mainly occurring in the Wancheng fault zone and Gonggan slope zone, are characterized by relatively lower homohopane series, higher mature, and less input of terrestrial higher plants. Comparatively, family-Ⅱ oils mainly occur in the Jingzhou anticlinal zone, with the characterization of small amount of homohopane series, less mature and more terrigenous organic matter input. Family-I oils mainly originate from of the Ⅲ and Ⅱ groups of the lower Xingouzui Formation (Ex1-Ⅲ and Ex1-Ⅱ) source rocks in the Wancheng fault zone and less contribution from Ex1-Ⅱ source rocks of the Shiqiao monoclinal zone; while family-Ⅱ oils are mainly derived from the Ex1-Ⅲ source rocks in the north of Jingzhou anticlinal, and a small amount comes from the Ex1-Ⅱ source rocks of the Shiqiao monoclinal zone. In addition, Ex1-Ⅱ and Ex1-Ⅲ source rocks in the Wancheng fault zone show some contribution to family-Ⅱ2 oils. This work could provide scientific theoretical guidance for petroleum exploration and exploitation in the Jiangling depression.