Hydrocarbon generation potential evaluation of high-over mature Permian source rocks in the Southwestern part of the central depression, Junggar Basin

Abstract

A comprehensive study was conducted on the hydrocarbon generation potential of the deeply buried Permian source rocks with high-over mature in the southwestern part of the Central Depression, Junggar Basin. Using rock-eval pyrolysis, carbon-sulfur analysis, kerogen macerals analysis, adamantane quantification, molecular geochemistry, and major and trace element analysis, coupled with organic carbon recovery methods. The evaluation primarily focused on thermal maturation, types, and abundance of organic matter, with an emphasis on summarizing methods for evaluating high-over mature source rocks. The results demonstrate that the Permian source rocks in the Shawan Sag (Well Zhengshen-101) and the Penyijingxi Sag (Well Zhuangshen-1) have reached a high-over mature stage, as evidenced by vitrinite reflectance (Ro), adamantane parameters, methyl phenanthrene indices (MPI), and Laser Raman spectroscopy. Horizontally, for the Fengcheng (P₁f) and Lower Wuerhe (P₂w) formations, the Well Zhengshen-101 in the Shawan Sag is thermally more matured than the Well Zhuangshen-1 in the Penyijingxi Sag, while vertically, both wells show that the P₁f is thermally more mature than the P₂w. Raman and Fourier transform infrared (FTIR) spectroscopy confirm that Type II kerogen is the dominant organic matter type. Through material balance, degradation ratio, and inorganic element methods, obtain the original content of organic carbon (TOC₀) and hydrocarbon generation potential (S₁+S₂). After restoration, the P₁f samples from Well Zhengshen-101 in the Shawan Sag show moderate to high-quality organic matter abundance, predominantly high-quality; the P₂w samples are classified as high-quality source rocks. The Xiazijie Formation (P₂x) and the P₂w samples from Well Zhuangshen-1 in the Penyijingxi Sag range from poor to high-quality, predominantly good to high-quality. Horizontally, the P₂w samples from Well Zhengshen-101 in the Shawan Sag exhibit higher organic matter abundance compared to those from Well Zhuangshen-1 in the Penyijingxi Sag. Notably, the P₂x source rocks in the Penyijingxi Sag demonstrate superior potential compared to the P₂w, highlighting their underexplored significance. Comprehensive analysis of the depositional environment in the study area indicates that the Well Zhengshen-101 in the Shawan Sag may experience a deeper, stable reducing environment with moderate sedimentation rates, stable water stratification, and a more abundant nutrient supply, all of which favor organic matter enrichment. This systematic evaluation advances methodologies for assessing high-over mature source rocks and underscores the exploration potential of high-over mature Permian source rocks in the Junggar Basin. Meanwhile, comparison with the highly mature marine shales of the Sichuan Basin further accentuates the methodological innovation of this study.

Keywords

High-over mature source rocks / Hydrocarbon generation potential / Laser Raman spectroscopy / Fourier transform infrared spectroscopy / Organic matter recovery / Adamantane quantification

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