Biodegradation-Corrected δ 13 C for Oil-Source Correlation in the Western Qionghai Uplift

The western segment of the Qionghai Uplift is a major oil-bearing block in the Zhu III depression. However, diverse crude oil phases and types occur within this area, characterized by complex geochemical signatures, and the origin and genesis of these crude oils remain unclear. Through the analysis of crude oil physical properties, biomarker compounds, and carbon isotope data, this study clarifies the geochemical characteristics and genetic origins of the crude oils. The results indicate that (1) response characteristics of typical biomarkers (C 25 -norhopane) to biodegradation indicate PM 1 to PM 3 biodegradation levels in the study area’s crude oils. Correction of whole-oil δ 13 C values to account for biodegradation effects enables a more accurate oil-source correlation. (2) Crude oils in the study area can be classified into two types: A and B. Type A crude oil is derived from sapropelic-humic organic matter, exhibiting characteristics of both the shallow lacustrine and mid-deep lacustrine source rocks of the Wenchang formation, indicating mixed contributions from these two sets of source rocks. In contrast, Type B crude oil is predominantly sapropelic and is sourced solely from the mid-deep lacustrine source rocks of the Wenchang formation. The source rocks of the Enping formation do not contribute to the oil reservoirs in this area. (3) Two distinct phases of hydrocarbon charging, occurring at 15–4.8 Ma and 3.5–0 Ma, respectively, have been identified in the study area. The initial charge was solely supplied by the basal mid-deep lacustrine source rocks, leading to the accumulation of Type B crude oil. During the late stage, as the shallow lacustrine source rocks entered the oil window, both source rock units contributed simultaneously, resulting in the formation of Type A crude oil. The present-day variation in crude oil properties in the Q area of the western Qionghai Uplift is attributed to spatiotemporal differences in hydrocarbon charging from the two source rock units and their coupling with subsequent tectonic activity consequently, and an accumulation model is established, summarized as “dual-source hydrocarbon supply from the Wenchang B Sag, fault-sandstone composite migration, differential charging, and late-stage adjustment”. These results provide critical insights for evaluating hydrocarbon accumulation mechanisms and optimizing exploration targets.