混积型致密油储层微-纳米孔隙结构发育演化特征及影响因素

Tight oil stored in the mixed siliciclastic–carbonate sedimentary sequences is an important type of tight oil. It is widely distributed in China and North America, and has a large resource potential. These reservoirs have complex mineral compositions and strong heterogeneities. The characteristics of micro-nano scale pore structure hava not been revealed, and the pore structure evolution and the influencing factors have not been elucidated, which restrict the understanding of tight oil occurrence and enrichment mechanism. Taking the mixed siliciclastic–carbonate tight oil reservoirs of the Xiagou Formation in Qingxi Sag, Jiuquan Basin as the research object, the project conducts a combined application of carbon dioxide adsorption (CO2), nitrogen adsorption (N2), and mercury injection capillary pressure (MICP) to quantitatively characterize the whole pore size distribution, specific surface area and the total pore volume of the micro-nano scale pores. Casting thin section and high resolution field emission scanning electron microscopy (FE-SEM) image analysis are employed respectively to characterize the two-dimensional pore structure. Based on the thin section examination, geochemical testing on source rocks, and whole rock X-ray diffraction (XRD), this project studies the influences and controls on pore development and evolution of micro textures and structures, geochemical features, and mineral compositions. The expected results of this project will clarify the development and evolution characteristics and influencing factors of micro-nano scale pore structures of the mixed siliciclastic–carbonate tight oil reservoirs, which have important theoretical and practical significance to reveal the tight oil occurrence and to identify enriched sweet-spot.

碎屑岩与碳酸盐岩混合沉积层系中的致密油是一种重要的致密油类型，在中国及北美地区分布广、资源潜力大。混积型致密储层具有复杂的岩矿组成和强烈的非均质性，其微观孔隙结构的发育特征未被揭示，孔隙结构演化特征及影响因素尚未阐明，制约了对致密混积岩油的赋存空间和状态及富集机理的深入认识。项目以酒泉盆地青西凹陷下沟组混积型致密油储层为研究对象，对自然演化样品和热压模拟实验获得的不同成熟阶段的样品，联合应用高压压汞—N2低温吸附—CO2低温吸附定量表征微-纳米孔隙的全孔径分布、比表面积及总孔体积；利用铸体薄片和场发射扫描电镜精细观测二维孔隙结构特征；结合岩石薄片鉴定分析、烃源岩地化测试分析及全岩X射线衍射分析，研究微观结构构造、地化特征、岩矿组成等不同影响因素对孔隙发育演化特征的控制作用。成果将阐明该类储层微-纳米孔隙结构的发育演化特征及影响因素，对揭示致密油赋存状态和优选富集甜点具有重要理论和现实意义。

混积型致密储层具有复杂的岩矿组成和强烈的非均质性，其微观孔隙结构的发育特征未被揭示，孔隙结构演化特征及影响因素尚未阐明，制约了对致密混积岩油的赋存空间和状态及富集机理的深入认识。在自然科学基金项目的资助下，研究了混积型致密储层微观孔隙结构、地化特征、岩矿组成等以及它们对孔隙发育演化特征的控制作用。致密混积型储层广泛发育（微）裂缝、粒间溶蚀孔、粒内溶蚀孔及晶间孔多种孔隙类型。这套致密混积型储层孔隙直径分布范围为0~10μm，多显示双峰和三峰。不同岩性纳米和微米级孔隙所占比例差异明显，白云质粉砂岩微米级孔隙最多，但泥质白云岩含有孔径更大的微米级孔隙。孔喉直径范围差别较大，有的全部属于纳米级孔喉，有的发育有小部分的微米级孔喉，有的则含有较多的微米级孔喉，多显示单峰和双峰。与泥页岩、致密砂岩、灰岩相比，致密混积型储层孔隙类型更丰富，孔径分布范围更广，微裂缝更发育。初步分析混积岩的岩性构成、矿物组成的差异及有机质丰度是影响孔喉结构特征的三大主要因素。