Restoration of Original Organic Carbon and Evaluation of Hydrocarbon Generation Potential for High-Mature to Overmature Source Rocks: A Case Study of the Permian Source Rocks in the Junggar Basin

High-mature to overmature source rocks constitute a critical material basis for deep oil and gas exploration in ancient petroliferous basins. However, intense thermal evolution induces extensive kerogen cracking and depletion of residual organic matter, resulting in the failure of conventional evaluation indicators and thereby severely limiting the accurate assessment of source rock effectiveness and resource potential.Taking the high-mature to overmature source rocks (with vitrinite reflectance Ro > 1.3%, locally reaching up to 3.0%) from the Fengcheng Formation and Lower Urho Formation of the Permian System in the Junggar Basin as the research objects, this study is grounded in geological constraints. Firstly, regional geological surveys, outcrop observations, and limited drilling core analyses were conducted to provide basic parameters for original organic carbon restoration. Meanwhile, preliminary method selection was performed by integrating the geological-geochemical differences among various formations and sags. On this premise, three methods—namely the degradation rate method, forward modeling method, and chemical kinetics method—were applied to restore the original organic carbon of the two sets of source rocks (Fengcheng Formation and Lower Urho Formation). The original organic matter types were inferred from the maceral characteristics of kerogen, which was employed to compare the adaptability and restoration accuracy of the three methods. Eventually, the original organic carbon restoration coefficients corresponding to each formation and sag were determined.Additionally, utilizing single-well logging data, a logging TOC (Total Organic Carbon) prediction model was established based on linear regression. Combined with the optimized restoration coefficients, the original organic matter abundance of single wells was calculated, realizing the accurate assessment of the original organic matter abundance of high-mature source rocks. The results indicate that after restoration, the TOC, hydrocarbon generation potential, and HI (Hydrogen Index) of the two sets of source rocks in the study area are significantly enhanced compared with those before restoration. Specifically, for the Lower Urho Formation in the Fukang Sag, the average post-restoration TOC is 1.73%, representing a 10% increase from the pre-restoration value; for the Fengcheng Formation in the Shawan Sag, the average post-restoration TOC is 0.99%, a 67% rise compared with the pre-restoration level. This evaluation method clarifies the original organic matter abundance and hydrocarbon generation potential of high-quality source rocks in sparsely explored sags, and effectively addresses the key challenges in the assessment of high-mature to overmature source rocks.

Keywords: High-mature to overmature source rocks; Organic carbon restoration; Logging prediction; Junggar Basin; Permian