This study aims to resolve the genetic origin of crude oils accumulated in the D Subsag and to assess the potential cross-sag hydrocarbon migration from the adjacent Haizhong Sag. The D Subsag, situated on the western margin of the Weixinan Sag in the Beibuwan Basin, is a significant petroleum province with proven reserves exceeding 10 million tons in the Weizhou Oilfield. However, the origin of these oils and the contribution from the Haizhong Sag source kitchen remain poorly constrained, hindering accurate resource assessment. To address this, we integrated organic geochemical analyses of nine source rock samples from the Haizhong Sag (Well H1) and eight crude oil samples from the D Subsag reservoirs. Bulk geochemical and biomarker signatures reveal distinct organic facies within the Paleogene succession. Type III kerogen, characterized by terrigenous higher plant input (high C19+20 tricyclic terpanes and C29 regular steranes, Pr/Ph > 2.5) deposited under oxic freshwater conditions, dominates source rocks from the third member of the Weizhou Formation (EWZ3). In contrast, the second and third members of the Liushagang Formation (Els2 and Els3) contain mixed Type II2-III kerogen with elevated contributions from lacustrine algae and aquatic organisms (elevated C23 tricyclic terpanes and C27 regular steranes). Thermal maturity assessment (with Tmax of 436 to 448 °C) confirms that all source intervals are within the oil generation window. Two genetically distinct oil groups are identified in the EWZ3 reservoirs. Group 1 oils (Well W4) exhibit a lacustrine algal signature (C27/C29 sterane > 1.15; low Pr/Ph 1.54–1.68) that does not correlate with the analyzed Haizhong Sag source rocks, suggesting localized, intra-sag source contributions. In contrast, Group 2 oils (Wells W6 and W6-2) display strong geochemical affinities with the Els2 and Els3 source rocks, evidenced by mixed terrestrial/aquatic signatures (∑nC21−/∑nC22+ < 1.0). These findings confirm that fault systems acted as conduits for long-distance migration from the Haizhong Sag, while also highlighting a previously unrecognized contribution from local source intervals. This refined petroleum system model provides critical constraints for delineating remaining hydrocarbon potential and reducing exploration risk in the Beibuwan Basin.
Yang et al. (Mon,) studied this question.