Geochemical characteristics of solid bitumen in the Jurassic Sangonghe Formation in the central Junggar Basin and its implications for hydrocarbon accumulation process

Abstract

The ubiquitously distributed solid bitumen in the Jurassic Sangonghe Formation reservoir in the central Junggar Basin recorded the important information of hydrocarbon accumulation. In this study, based on lithology, reflectance of bitumen, laser Raman spectroscopy and biomarkers of bitumen, combined with the history of structure evolution and hydrocarbon accumulation, we studied the genesis of the reservoir bitumen and its indication for oil and gas accumulation. Results indicated that these solid bitumen were mainly distributed in structural fractures, the crack surface of the samples have experienced bending deformation, combined with obvious deformation of mineral microstructure. Solid bitumen was then indicated to be derived from the crude oil evolution after the destruction of the ancient reservoir due to tectonic activities. According to the parameter characteristics of biomarkers of bitumen, the crude oil was mainly derived from the source rocks of the Permian Fengcheng Formation and the Lower Wuerhe Formation. Therefore, the Sangonghe Formation reservoir has multi-source charging features. Due to the low maturity (equivalent vitrinite reflectance: 0.62%-0.79%) of bitumen and the evidence for biodegradation, we concluded that the bitumen was generated by biodegradation. Whereas the biomarker showed that the bitumen was affected by the late oil accumulation. In conclusion, the ancient oil reservoirs were formed during the early stage of the formation of Che-Mo ancient uplift in the Middle Jurassic, and were destroyed during the Late Jurassic to the Early Cretaceous. Light hydrocarbon components were dissipated because of the reservoir destruction, then bitumen was formed due to the biodegradation. In the Early Cretaceous, there was no strong tectonic activity in the Sangonghe reservoir after the late hydrocarbon accumulation from the Lower Wuerhe Formation. With the reburial of the reservoir and hydrocarbon accumulation, the hydrocarbon reservoirs today were formed. Although the reservoir of the Sangonghe Formation has undergone tectonic activity adjustment in the study area, the recharge of oil and gas makes it still a favorable exploration target.

Keywords

• laser raman spectroscopy
• biomarker
• bitumen in reservoir
• sangonghe formation
• jurassic
• central junggar basin