关键词: 珠三坳陷, 文昌10洼, 神狐隆起, 天然气特征, 油源对比, 油气来源, 成藏特征

Abstract: Complex hydrocarbon resources are detected in the southern fault zone of Wenchang10 subsag and Shenhu uplift of Zhu Ⅲ depression. However, there are still disputes on the hydrocarbon sources and origins. Using hydrocarbon geochemical methods, combined with regional tectonic setting, sedimentary evolution features, and thermal evolution characteristics of source rocks, a systematic study has been performed on the hydrocarbon sources and genetic origins. The results show as follows. (1) Three types of source rocks contribute to hydrocarbon accumulation, i.e., humic-sapropelic source rocks in semi-deep lacustrine subfacies of the Member 2 of Wenchang Formation, humic-sapropelic source rocks in shallow to semi-deep lacustrine subfacies of the Member 1 of Enping Formation near the sag zone, and humic-dominated source rocks in shore-shallow lacustrine facies of the Member 1 and 2 of Enping Formation. (2) Crude oils are classified into four types. Type Ⅰ is characterized with a high oleanane contents, high bicadinane content, absence of 4-methylsterane, high C₁₉-C₂₀ tricyclic terpene (TT) contents, high rearranged drimane content, low 8β(H)-homodrimane content, and high phytane/pristane (Ph/Pr) ratio. It is derived from humic-dominant source rocks in the Member 1 and 2 of Enping Formation. Type Ⅱis characterized by low oleanane content, low bicadinane content, a relatively high 4-methylsterane content, low C₁₉-C₂₀TT contents, high C₂₃TT content, low rearranged drimane contents, high 8β(H)-homodrimane content, and relatively low Ph/Pr ratio. It is originated from the humic-sapropelic source rocks in the Member 1 of Enping Formation within the subsag. Type Ⅲ exhibits low to medium oleanane content, relatively high bicadinane content, low 4-methylsterane content, relatively high C₁₉-C₂₀TT contents, low rearranged drimane contents, high 8β(H)-homodrimane content, and moderately high Ph/Pr ratio, indicating a hybrid origin of Type Ⅰ and Type Ⅱ crude oils, dominated by the latter. Type Ⅳ exhibits low to high oleanane and bicadinane contents, high 4-methylstane content, relatively high C₁₉-C₂₀TT contents, high rearranged drimane contents, low 8β(H)-homodrimane content, and moderately high Ph/Pr ratio, demonstrating a hybrid origin of Type Ⅰ crude oil and a small amount of semi-deep lacustrine subfacies crude oil from the Member 2 of Wenchang Formation, dominated by the former. (3) The light hydrocarbon parameters and carbon isotope characteristics indicate that the natural gas in the study area was dominated by oil-type gas, mixed with minor coal-type gas. It is of mixed origin, consisting of high-maturity oil-type gas from the Member 2 of Wenchang Formation and the lower part of Member 1 of Enping Formation, and mature coal-type gas from the Member 1 and 2 of Enping Formation. (4) Oil and source rock correlation and the coupling of source rock thermal evolution process with both trap development and hydrocarbon accumulation history reveal that source rocks in the Member 2 of Wenchang Formation have made a limited contribution to oil accumulation in the uplifted zone, and can only be responsible for hydrocarbon charging in reservoirs proximal to the subsags. During hydrocarbon migration from the subsag center towards the uplifted zone, the increasing migration distance and the presence of structural troughs as barriers result in a decrease in the proportion of oil-type gas, thus leading to the lower maturity of gas reservoirs in the uplifted zone compared to those in the Wenchang10 subsag.

Key words: Zhu Ⅲ depression, Wenchang10 subsag, Shenhu uplift, natural gas characteristics, oil and source rock correlation, hydrocarbon origin, accumulation characteristics