关键词: 开平凹陷, 地球化学, 油源对比, 原油成因, 烃源岩, 珠江口盆地

Abstract:  To analyze the geochemical characteristics of crude oil in the Kaiping sag of the Pearl River Mouth basin and to ascertain the hydrocarbon sources for more scientific and rational exploration and development strategies, this study conducted an analysis of geochemical features, crude oil genesis, and sourcing based on geochemical experimental analysis and testing data of crude oil and source rocks from the middle-deep layers in the southern region of the Kaiping sag. The results indicate that the crude oil in the southern Kaiping sag is characterized by high wax content and light-to-medium density. The crude oil from the K18-1 and K11-4 hydrocarbon-bearing structures in the southern Kaiping sag exhibits similar geochemical features, including a dominance of C21TT or C23TT, low oleanane content, a preference for C27 and C29 regular steranes, and the presence of 4-methyl C30 sterane, suggesting a contribution from both algal aquatic organisms and terrestrial higher plants. However, there are still certain differences in organic matter input and depositional environments between the hydrocarbon-generating precursors of the crude oil from the two hydrocarbon-bearing structures. The crude oil from the K18-1 hydrocarbon-bearing structure is relatively unique, differing from that of K11-4 and other regions. It has low oleanane content and a high C21TT/C19+20TT index, indicating it is a typical lacustrine crude oil. In contrast, the crude oil from the K11-4 hydrocarbon-bearing structure is similar to that from other regions in the Kaiping sag, with relatively high oleanane content and a lower C21TT/C19+20TT index, suggesting it is more fluvial-deltaic in nature. An evaluation of source rocks and an analysis of molecular geochemical characteristics indicate that there are significant differences in the geochemical features of source rocks from different intervals in the southern Kaiping sag. The lower member of the Wenchang Formation consists of Type Ⅰ-Ⅱ2 organic matter lacustrine source rocks, while the upper member of the Wenchang Formation to the Enping Formation comprises Type Ⅱ1-Ⅲ deltaic coal-measure source rocks. The former exhibits superior organic matter abundance and hydrocarbon generation potential compared to the latter. Based on an analysis of the depositional environments, organic matter sources, and maturity levels of crude oil and source rocks, combined with a comparison of oil-source biomarker compound parameters and an analysis of the hydrocarbon generation potential of source rocks, it is believed that the crude oil in the southern Kaiping sag primarily originates from the lower member of the Wenchang Formation underlying the respective hydrocarbon-bearing structures. The southern Kaiping sag possesses relatively high-quality hydrocarbon source conditions and exhibits favorable exploration prospects.

Key words:  , Kaiping sag, geochemistry, oil-source rock correlation, origin of crude oil, source rock, Pearl River Mouth basin