Abstract: Early low-mature oil is formed by specific organic parent materials with low activation energy for hydrocarbon generation under low temperature and early mature conditions [vitrinite reflectance (R_o)of 0.3 % to 0.6 % , geothermal temperature of 60 ℃ to 100 ℃]. It has higher maturity and better oil quality than immature oil, and is easy to form commercial oil reservoirs and natural industrial capacity. This type of oil was discovered early in China, but there is less understanding of its hydrocarbon generation mechanism and reservoir potential. In recent years, the Exploration and Development Research Institute of PetroChina Huabei Oilfield Company has been exploring early low-mature oil and obtained high-yield oil flows and large-scale cost-effective reserves in the middle-shallow strata of Dongying Formation of Baoding sag in Jizhong depression, thus achieving a breakthrough in traditional cognition. The research on the hydrocarbon generation mechanism, including the organic geochemical analysis of crude oil and source rocks, analysis of hydrocarbon inclusions in reservoirs, and thermal simulation experiments of hydrocarbon generation and expulsion in Baoding sag, reveals that under the environment of strong-reduced saline water bodies and high geothermal field, the bacteria and algae with low activation energy and soluble organic matter in the organic-rich strata of the lower submember of Member 1 of Shahejie Formation have generated large amounts of early low-mature oil by biochemical action and low-temperature thermocatalysis in the late diagenesis stage, with the oil rate of 190 mg/g and oil expulsion of 86 mg/g, respectively. They account for 46.4 % and 37.3 % of the oil generation and expulsion of kerogens in source rocks during the peak stage of oil generation (R_o of 0.8 % to 1.3 % , geothermal temperature over 120 ℃), providing sufficient oil sources for the formation of early low-mature oil reservoirs. The early generated oil completed the migration and accumulation process from source to reservoir by fault networks and sand bodies, showing a regularity of near source enrichment.

Key words: Baoding sag, early low-mature oil, geochemical characteristics, formation mechanism, enrichment regularity

摘要： 早熟低熟油是生烃活化能低的特定有机母质在低温早熟条件[镜质体反射率(R_o)为0.3%~0.6%,地温为60~100℃]下生成的石油,较未成熟油的成熟度高、油质好,易于形成商业油藏及自然工业产能。这类石油在中国发现早,但其成烃机制和成藏潜力并未被充分认识。近年来,中国石油华北油田公司勘探开发研究院对早熟低熟油进行了探索并在冀中坳陷保定凹陷中-浅层东营组钻获高产油流和规模效益储量,突破了传统认知。对保定凹陷原油和烃源岩开展的有机地球化学分析、储层含烃包裹体分析和生/排烃热模拟实验等成烃机制研究揭示,沙河街组一段下亚段有机质富集层中的低活化能菌藻类和可溶有机质在成岩晚期的强还原咸化水体、高地温场演化环境下,经生化作用、低温热催化作用生成了大量的早熟低熟油,产油量达190 mg/g,排油量达86 mg/g;生油量和排油量分别占干酪根生油高峰阶段(R_o为0.8%~1.3%,地温＞120℃)生油量和排油量的46.4%和37.3%,这为早熟低熟油藏的形成提供了充足的油源。早期生成的石油经通源断裂网络和砂体输导,完成了从源到藏的运聚过程,呈现出近源富集的规律。

关键词: 保定凹陷, 早熟低熟油, 地球化学特征, 形成机制, 富集规律