珠江口盆地白云凹陷裂陷期沉积环境演化与控烃作用

Sedimentary environment evolution and hydrocarbon control in Baiyun Sag of Pearl River Mouth Basin during rifting period

珠江口盆地白云凹陷裂陷期发育文昌组与恩平组2套烃源岩,湖盆沉积演化控制烃源岩的形成。通过三维地震资料及深部钻井分析,认为白云主洼裂陷期发育轴向辫状河三角洲体系、断控扇三角洲体系、北缘缓坡带辫状河三角洲体系以及盆内滨浅湖—中深湖体系。裂陷早期,发育多组NE向断阶且呈现多隆洼的地貌特征,物源主要从东西轴向进入凹陷内,形成多点供源水系,发育多洼成烃;裂陷中期,由于拆离断层的伸展作用,湖盆扩张,北部番禺低隆起被强烈抬升,形成缓坡大规模的三角洲体系及大型湖盆;裂陷晚期,湖盆以拗陷作用为主,受华南物源及由南往北的海侵影响,发育煤型及海侵烃源岩。通过沉积相、地球化学指标可将白云凹陷烃源岩划分为3种类型:文昌组中深湖相Ⅰ—Ⅱ_(1)型烃源岩、文昌组浅湖相Ⅱ_(2)型烃源岩以及恩平组浅湖相Ⅱ_(2)—Ⅲ型烃源岩,不同类型烃源岩生烃潜力有差异,以文昌组中深湖相最优。元素地球化学指标揭示湖盆成烃环境整体表现为淡水—微咸水、还原—强还原的水体环境,而古生物揭示湖盆生油母质为湖相浮游藻类及陆源植物,优质烃源岩有机质来源以浮游藻类为主。总之,白云凹陷拆离断裂活动及沉积体系控制了烃源岩的发育厚度和规模,优质的水体环境控制了优质烃源岩的形成。因此,白云凹陷烃源岩具有厚度大、分布范围广的地质条件,具有大的资源和勘探潜力。

Two sets of source rocks, Wenchang Formation and Enping Formation, were developed in Baiyun Sag of the Pearl River Mouth Basin during the rifting period. Through 3D seismic data and deep drilling analysis, it is believed that axial braided river delta system, fault controlled fan delta system, braided river delta system in the north gentle slope zone and shore shallow lake medium deep lake system in the basin are developed during the rifting period of Baiyun Main Depression. In the early stage of rifting, many groups of NE trending fault terraces are developed and the landform is characterized by many ups and depressions. The provenance mainly enters the depression from the east to the west, forming multi-point source water systems and developing multi depressions to generate hydrocarbons;In the middle of rifting, due to the extension of detachment faults, the lake basin expanded, and the Panyu low uplift in the north was strongly uplifted, forming a large delta system and a large lake basin with gentle slopes;In the late rifting period, the lake basin was dominated by depression, and coal type and transgressive source rocks were developed under the influence of South China provenance and transgression from south to north. According to sedimentary facies and geochemical indicators, the source rocks in Baiyun Sag can be divided into three types: Type Ⅰ-Ⅱ_(1)source rocks of middle-deep lacustrine facies in Wenchang Formation, Type Ⅱ_(2)source rocks of shallow lacustrine facies in Wenchang Formation and Types Ⅱ_(2)-Ⅲ source rocks of shallow lacustrine facies in Enping Formation. The hydrocarbon generation potential of different types of source rocks is different, and the middle-deep lacustrine facies source rocks in Wenchang Formation are the best. The element geochemical indicators reveal that the overall hydrocarbon generating environment of the lake basin belongs to freshwater to brackish water, reducing to strong reducing water environment, while the paleontology reveals that the oil generating parent material of the lake basin is Huxiang planktonic algae and terrestrial plants, and the organic matter of high-quality hydrocarbon source rocks mainly comes from planktonic algae. In a word, the detachment fault activity and sedimentary system of Baiyun Sag control the development thickness and scale of hydrocarbon source rock, and the high-quality water environment controls the formation of high-quality hydrocarbon source rock. Therefore, the source rocks in Baiyun Sag have geological conditions of large thickness and wide distribution, and have great resources and exploration potential.