珠江口盆地陆丰凹陷在洼陷迁移控制下的油气成藏规律

陆丰凹陷经历了古近纪文昌组沉积期和恩平组沉积期2幕裂陷活动,形成了文昌组和恩平组辫状河三角洲-湖泊为主的沉积体系。从文昌组沉积期到恩平组沉积期,随着构造和沉积中心由南向北逐渐迁移,烃源岩的发育层位也发生迁移,陆丰凹陷南部的优质烃源岩发育在文昌组而北部的优质烃源岩发育在恩平组。地球化学资料分析显示,陆丰凹陷南部3个洼陷的优质烃源岩的发育层位也具有由南向北迁移和向上迁移的特征。凹陷内油藏的分布明显受烃源岩控制,不同的烃源岩发育特征导致不同洼陷中石油生物标志物组成、成藏组合、成藏时间和输导特征存在明显的差异。陆丰凹陷次级洼陷的成藏模式可按缓坡带、陡坡带、洼陷带和凸起带进行划分。缓坡带的油气运移以阶梯状和"S"状输导为主,砂体和断层是油气运移的主要途径,运移距离较远;陡坡带紧邻生烃洼陷,油气通过控洼断裂经垂向输导成藏;洼陷带的油气依赖洼陷内部的通源断层垂向输导成藏;凸起带距离烃源灶较远,油气由断裂输导至凸起后,通过砂体侧向运移在合适的圈闭中成藏。

Hydrocarbon accumulation patterns controlled by the migrated subsags in Lufeng sag,Pearl River Mouth Basin

In the Paleogene, Lufeng sag experienced two episodic rifting periods, i.e., Wenchang period and Enping period, thus forming the braided river delta-lake dominated sedimentary system in Wenchang Formation and Enping Formation. From Wenchang period to Enping period, due to tectonic and sedimentary center migration from the southern to northern area, the development horizons of source rocks also migrated. The high-quality source rocks in the southern Lufeng sag developed in Wenchang Formation, but those in the northern part developed in Enping Formation. Geochemical data analysis showed that the developmental horizons of high-quality source rocks in three subsags of the southern Lufeng sag also migrated from south to north. Source rocks significantly controlled the distribution of petroleum accumulation in Lufeng sag. Different development characteristics of source rock result in obvious differences in biomarker composition, accumulation assemblage, accumulation time and migration characteristics among different subsags. The accumulation pattern of subsag in Lufeng sag can be subdivided into gentle slope belt, steep slope belt, inner subsag belt and uplift belt. Hydrocarbons in the gentle slope belt migrate through faults and sand bodies, characterized by stair-step and "S" type migration patterns and a long migration distance. The steep slope belt is close to the source kitchen, and hydrocarbon accumulated by means of vertical transport through subsag controlling faults. Hydrocarbons accumulated in the inner subsag belt rely on vertical transport through the fault connecting with the source to form reservoirs. Since the uplift belt is far from the source kitchen, hydrocarbons migrate to the uplift through faults and laterally through sand bodies to accumulate in proper traps.