湖相泥页岩中碳酸盐成因及页岩油气地质意义——以东营凹陷沙河街组四段上亚段—沙河街组三段下亚段烃源岩为例

利用岩心、薄片、全岩矿物组分、元素、同位素等资料,对东营凹陷沙河街组四段上亚段—沙河街组三段下亚段深湖相烃源岩中碳酸盐成因机理进行了研究,认为深湖主要发育透镜状粗晶、纹层状微细晶、纹层状隐晶和块状隐晶4种类型碳酸盐。其中,透镜状粗晶碳酸盐为成岩重结晶的产物;纹层状微细晶碳酸盐和纹层状隐晶碳酸盐为藻类光合作用不断从水中萃取CO_2,导致水体CO_3~(2-)浓度升高,与湖水表层携带的Ca~(2+)相互作用的产物,咸水环境有利于碳酸盐晶粒保存,碳酸盐多呈微细晶纹层状分布,微咸水环境水体较深,碳酸盐晶粒在碳酸盐补偿深度面以下通常会发生溶蚀,大多呈隐晶纹层状保存下来;块状隐晶碳酸盐是浅水地带沉积碳酸盐由重力流搬运至深水地带的产物。碳酸盐一定程度上控制了东营凹陷沙河街组四段上亚段—沙河街组三段下亚段泥页岩的储集性和可压性,也将为古湖泊沉积环境恢复提供重要信息。

Carbonate genesis and geological significance of shale hydrocarbon in lacustrine facies mud shale: a case study of source rocks in the upper submember of Member 4 and lower submember of Member 3 of Shahejie Formation,Dongying sag

The genetic mechanism of carbonate in the deep lacustrine facies source rocks of the upper submember of Member 4 and lower submember of Member 3 of Shahejie Formation, Dongying sag is studied based on data involving cores, thin sections, whole-rock mineral components, elements and isotopes. It is considered that there are four kinds of carbonate developed in the deep lake, i.e., lenticular coarse crystalline, lamellar micro-fine crystalline, lamellar cryptocrystalline and massive cryptocrystalline. Among them, the lenticular coarse crystalline carbonate is the product of diagenetic recrystallization. The lamellar micro-fine crystalline carbonate and lamellar cryptocrystalline are generated by algal photosynthesis, i.e., CO₂ is constantly extracted from water, thus leading to that the CO₃^2- is increased in water and then interacts with Ca²⁺ carried by lake water surface. Saline water environment is favorable to the preservation of carbonate particles, where the carbonates are mostly shown as micro-fine crystalline with the lamellar distribution. In the brackish water environment, the water is deeper, and dissolution usually occurs to the carbonate crystalline beneath the carbonate compensation depth surface, where most lamellar cryptocrystalline are preserved. The massive cryptocrystalline carbonate is the product of the sedimentary carbonate in the shallow water migrating to deep water by gravity flow. The reserving property and compressibility of mud shale in the upper submember of Member 4 and lower submember of Member 3 of Shahejie Formation, Dongying sag are controlled by carbonate, providing important information for the restoration of paleo-lake sedimentary environment.