Tracing Oils and Gas Accumulation in Complex Petroleum Systems Based on Biomarkers, Light Hydrocarbons, and Diamondoids from Concomitant Crude Oils: A Case Study in the Shawan Sag of the Junggar Basin, NW China

Hangxin Cai; Shuang Yu; Jun Jin; Erting Li; Julei Mi; Yuce Wang; Changchun Pan

doi:10.1021/acsomega.4c01844

Tracing Oils and Gas Accumulation in Complex Petroleum Systems Based on Biomarkers, Light Hydrocarbons, and Diamondoids from Concomitant Crude Oils: A Case Study in the Shawan Sag of the Junggar Basin, NW China

ACS Omega. 2024 Apr 26;9(18):20512-20531. doi: 10.1021/acsomega.4c01844. eCollection 2024 May 7.

Authors

Hangxin Cai^{1

2}, Shuang Yu^{1

3}, Jun Jin⁴, Erting Li⁴, Julei Mi⁴, Yuce Wang^{1

3}, Changchun Pan^{1

3}

Affiliations

¹ State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China.
² University of Chinese Academy of Sciences, Beijing 100049, China.
³ CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China.
⁴ Research Institute of Experiment and Detection, Petro China Xinjiang Oilfield Company, Karamay, Xinjiang 834000, China.

Abstract

The Shawan Sag is one of the most promising blocks in the western Junggar Basin for natural gas and oil exploration. To date, the research and exploration degree here is still low, and there is limited understanding of the origin and migration of natural gas and oils, which seriously restricts future exploration and development. At least three oil charging episodes and one gas charging episode could be confirmed in the reservoirs on the western slope of the Shawan Sag based on the analysis of concentrations and ratios of hopanes and steranes, diamondoids, and light hydrocarbons. The first and second charging oils originated from the peak and late oil generation window stages of Lower Permian Fengcheng Formation (P₁f) source rocks, respectively. The third charging oil originated from the late oil generation window stage of source rocks of the Middle Permian lower Wuerhe Formation (P₂w). Combining the carbon isotopic compositions and the gas composition, the fourth charging gas was derived from the postmature source rocks of Carboniferous(C) and Lower Permian Jiamuhe Formation (P₁j) and the high-postmature source rock of Lower Permian Fengcheng Formation (P₁f). The preservation conditions of nature gas in the Mesozoic (T-K) cap rocks in the Shawan Sag, as indicated by methyladamantanes (MAs)/methyldiamantanes (MDs) ratio, are better than those in the Mahu Sag. In this study, the methyladamantane maturity parameters of oils in the reservoirs were proposed to evaluate the maturity and further determine the migration direction of late-charging natural gas. The results suggest that late postmatured natural gases were charged from the southern part to the northwest of the Shawan Sag. Therefore, the southeastern direction of Well SP1 is a favorable area for natural gas exploration in the Shawan Sag. Meanwhile the northern part of the sag is favorable for the exploration of high-mature oil generated from source rocks within the Middle Permian Lower Wuerhe Formation.