鄂尔多斯盆地中部气田中奥陶统马家沟组岩溶型储层特征

Characteristics of karst reservoirs in the Middle Ordovician Majiagou Formation of gas fields in the central Ordos Basin

鄂尔多斯盆地中部气田目前累计探明地质储量3377.3×108m3。其中,中奥陶统马家沟组碳酸盐岩油气藏储量占总探明储量的80%以上,而岩溶型储层贡献最大。在对大量起控制作用的钻井岩心进行系统、详细的描述和研究的基础上,结合镜下铸体薄片研究,提出了研究区奥陶系岩溶作用的形成环境、成因类型、作用机理及岩溶型储层的特征。特定环境中沉积的具含量不一晶间孔隙的粗粉晶白云岩和含硬石膏结核白云岩是储层发育的物质基础,表生成岩期和埋藏成岩期岩溶作用所形成的次生孔、缝、洞的发育和保存程度是能否形成良好储层的关键,埋藏成岩期岩溶作用和构造裂隙主要在表生成岩裸露期形成的孔隙系统上叠加发育。储集类型可细分为裂缝-溶孔(洞)型,裂缝-孔隙-溶孔型和孔隙型,其中裂缝-溶孔(洞)型对储层贡献最大。优质储层纵向上主要分布在硬石膏结核粉晶白云岩发育的马五13、马五12小层,平面上分布在中部的硬石膏结核盆缘坪相带。

The accumulative proved geologic reserves of the gas fields in central Ordos Basin is about 337.73 billion cubic meters, of which more than 80 % comes from carbonate reservoirs （ mostly the karst reservoirs） in the Majiagou Formation of the Middle Ordovician. On the basis of a systematic and detailed description and study of the key core samples as well as the microscopic observation of cast thin sections, the authors of the paper put forward the formation environment,genetic types,mechanisms of the Ordovician karstification and the karstic reservoir characteristics. They also suggest that coarse medium crystalline dolomite with intercrystalline pores and the nodular dolomite with anhydrite deposited in particular environment are the material bases of reservoir development, while the development and preservation of the secondary pores, the fractures and the vugs formed by the karstification in the epidiagenetic stages and burial diagenesis stages are crucial for the formation of a good reservoir. The karstification in the burial diagenesis stages and structural fissures mainly overprinted on the pore system formed in the epidiagenetic stage. The reservoirs can be further classified into the fracture-dissolved pore （vug） type, the fracture-pore-dissolved pore type and the pore type, among which the fracture-dissolved pore （vug） type is predominant. Quality reservoirs are mainly distributed vertically in the M1^3 and M1^2 sub-members with well-developed anhydrite nodular medium crystalline dolomite and horizontally in the central basin-marginal fiat facies belt with anhydrite nodules.