文昌A凹陷油气成藏特征与分布规律探讨

A discussion on the hydrocarbon accumulation characteristics and distribution in Wenchang A sag

文昌A凹陷位于珠江口盆地西部,是一个发育文昌组和恩平组两套湖相烃源岩复合成藏的油气区。根据特征生物标志化合物与碳同位素组成的油岩对比结果,可将文昌A凹陷发现的原油划分为两类,即文昌Ⅱ型和恩平型,前者来自文昌组浅湖相烃源岩,后者源于恩平组湖相烃源岩;天然气主要源于恩平组烃源岩,也可能有文昌组烃源岩凝析气或裂解气的贡献。文昌A凹陷油气两期成藏,早期成油,晚期聚气及轻质油,其油气分布主要受烃源岩有机相、热演化程度和运移分馏作用等因素的控制,凹陷中部以气为主,凹陷边缘气油共存,而凸起上以油为主。

Wenchang A sag, located in the west part of Pearl Mouth basin, is a multiple hydrocarbon-generation area where two lacustrine source rocks, Wenchang and Enping Formations, have developed. Based on the oil-source correlation of biomarkers and stable carbon isotopic data, the crude oil in the sag can be divided into two types, i.e. Wenchang II and Enping, in which Type Wenchang II oil is generated from the shallow lake source rocks in Wenchang Formation, and Type Enping oil from the lacustrine source rocks in En- ping Formation. In addition, gas is mainly generated from the Enping Formation source rocks, with some contribution of condensate gas and oil- cracked gas from Wenchang Formation. There are two stages of hydrocarbon accumulation in Wenchang A sag, with the early stage characterized by oil accumulation and the late stage by gas and light oil accumulations. These hydrocarbon distribution is mainly controlled by organic facies of source rocks, their thermal evolution and hydrocarbon migration and fractionation, with gas principally in the central sag, both gas and oil along the sag margin, and oil dominated in the highs.