渤海海域中部古近系深埋藏湖相高孔隙度白云岩储层特征及其成因

渤海湾盆地渤海海域中部古近系沙河街组中发育湖相高孔隙度白云岩储层,准确预测高孔白云岩储层的分布对该区的油气勘探具有重大意义。为此,选取石臼坨凸起沙河街组(E2s)碳酸盐岩储层薄片,进行岩石矿物学、偏光显微镜、荧光、扫描电镜、阴极发光等综合分析,确定了高孔白云岩的岩石类型、成岩作用和储集空间特征。分析结果表明:高孔白云岩储层主要是生屑云岩,储集空间以残余粒间原生孔和生物体腔孔为主,成岩作用的主要特征为压实作用弱,发育衬垫白云石。进一步利用同位素地质温度、气液两相包裹体均一温度等确定了自生矿物的形成时期,再以显微荧光观察结合包裹体激光拉曼烃类检测技术,获得了烃类充注的期次。结论认为,沙河街组高孔白云岩储层发育具有3大主控因素:(1)高能沉积环境是高孔白云岩储层发育的基础;(2)渗流带—潜流带形成的早期衬垫白云石有效降低了机械压实对孔隙的破坏强度;(3)早期烃类充注有利于高孔白云岩储层的发育。

Characteristics and genesis of Paleogene deeply-buried high-porosity lacustrine dolomite reservoirs in the central Bohai Sea area

High-porosity lacustrine dolomite reservoirs are developed in the Paleogene Shahejie Formation (E2s) in central Bohai Sea area, Bohai Bay Basin. Accurate prediction on the distribution of these reservoirs is quite significant to the oil and gas exploration in this area. In this paper, 138 thin sections of E2s carbonate reservoirs in Shijiutuo salient were analyzed comprehensively based on petrology & mineralogy, polarizing microscope, fluorescence, scanning electron microscope and cathode luminescence. Based on the analysis, the rock type, diagenesis and reservoir space characteristics of the high-porosity dolomite reservoirs were figured out. Specifically, the reservoirs are dominated by bioclastic dolomite. The reservoir space is mainly composed of residual intergranular primary pores and visceral cavity pores. The diagenesis is mainly characterized by weak compaction and the existence of liner dolomite. Then, the formation period of authigenicminerals was determined by using isotopic geological temperature and hydrocarbon inclusion temperature. Finally, the hydrocarboncharging stages were made clear by means of microscopic fluorescence observation, together with the Laser Raman hydrocarbon detectionon inclusions. It is concluded that the development of theE₂s high-porosity dolomite reservoirs in this area is mainly controlled by three factors. First, high-energy sedimentary environment is the basis for the development of high-porosity dolomite reservoirs. Second, the early liner dolomite formed in the vadose zone and the underflow zone reduces effectively the destruction of mechanical compaction on pores. And third, the early hydrocarbon charging is favorable for the development of the high-porosity dolomite reservoirs.