Abstract: The high-quality source rocks in the Shahejie Formation of the Gegangji area, Dongpu Sag, exhibit extremely strong distribution heterogeneity. Conventional logging methods struggle to accurately identify and predict their spatial distribution, which significantly hinders the effectiveness of oil and gas exploration in the sag zone. In response, this study integrates geochemical data with logging response characteristics and employs a random forest algorithm to develop a high-precision TOC prediction model. This model achieves effective prediction of total organic carbon (TOC) content in highly heterogeneous geological conditions, revealing the distribution patterns of thinlayer high-quality source rocks and their controlling effect on hydrocarbon accumulation. The results indicate that the high-quality source rocks in the study area are predominantly gray-black shales with high organic matter abundance and favorable kerogen types. However, their thin single-layer thickness, rapid vertical and lateral variations, and complex logging responses lead to insufficient identification accuracy using traditional methods. The random forest algorithm significantly improves the prediction capability of heterogeneous organic matter distribution in complex lithological associations by integrating multiple types of logging parameters. Prediction results show that high-quality source rocks in the study area are mainly distributed at the bottom of the lower sub-member of the third member of the Shahejie Formation (Es₃^l) and the middle-upper part of the upper sub-member of the fourth member (Es₄^u). Moreover, their thickness gradually increases from west to east and from south to north in the near-sag to deep-sag areas. The distribution of high-quality source rocks exhibits a clear controlling effect on hydrocarbon accumulation, demonstrating typical“near-source accumulation”characteristics. High-yield oil and gas reservoirs as well as hydrocarbon-bearing intervals are all located near high-quality source rocks, and the degree of hydrocarbon enrichment is closely related to the thickness of these rocks. This research provides an important geological basis for oil and gas exploration planning in the sag zone of the Dongpu Sag.

Key words: High-quality source rock, Heterogeneity, Random Forest, Controlling effect on hydrocarbon accumulation, Shahejie Formation, Dongpu Sag