惠民凹陷古近系碎屑岩次生孔隙纵向分布规律

Vertical Distribution of Secondary Pores in Paleogene Sandstones in Huimin Depression and Its Genesis Analysis

为了在油气勘探的过程中更加准确地预测有效储层的分布,根据铸体薄片、扫描电镜、碳酸盐含量、镜质体反射率、粘土矿物以及储层物性等数据,研究了惠民凹陷古近系砂岩次生孔隙的纵向分布规律.在小于1400m的深度,砂岩的孔隙主要为压实和胶结之后的原生粒间孔隙;在埋深超过1400m以后开始出现少量次生孔隙,在1400～1500m深度范围形成原生孔隙与次生孔隙并存的混合孔隙带;超过1500m,大量的次生孔隙出现在1500～2300m和2700～4000m的2个深度段.所收集的砂岩孔隙度和渗透率数据在纵向上的变化关系也间接证实了这一点.次生孔隙主要为长石溶蚀成因,碳酸盐胶结物溶蚀是次要的.在次生孔隙发育带内,惠民凹陷烃源岩演化产生大量有机酸和泥岩中的粘土矿物迅速脱水是产生溶蚀作用形成次生孔隙的直接原因.

This paper mainly discusses the vertical distribution of secondary pores in Paleogene sandstones in Huimin depression based on the thin section, SEM, carbonate contents, vitrinite reflectance, clay minerals and reservoir property data in order to make a more precise prediction on the distribution of effective reservoirs in the process of gas exploration. Thin section and SEM observation show that the pores in sandstones are mainly primary pores preserved after being compacted at a depth of less than 1 400 m. There are a few secondary pores at a depth of more than 1 400 m, then comes mixed pore zones, characterized by the coexistence of primary and secondary pores in a depth range of 1 400-1 500 m; a large number of secondary pores occurred at two depth ranges of 1 500-2 300 m and 2 700-4 000 m. The vertical variation pattern of all porosity and permeability data we have collected can also confirm such evolution. It is found from thin section and SEM data that secondary pores mainly formed by feldspar dissolution, then by carbonate cement dissolution. The vertical distribution of authigenic kaolinite and carbonate cements can also confirm it. At the two depth ranges where secondary pores are well developed, feldspar dissolution resulted in the increase of authigenic kaolinite and large amounts of quartz overgrowth, while the amount of carbonate cements has no obvious change. The phenomena of feldspar being dissolved and no or only traces of carbonate cement being dissolved are determined by free energy of chemical reaction between organic acid and feldspar or carbonate cement. Dissolution of feldspar in organic acid is characterized by relatively low chemical energy, but for carbonate cement, that is quite higher. The distribution of secondary pores is closely related to the evolution of source rock and diagenitic evolution of sandstones. In addition, at the 1 400-4 000 m depth range, the interlayer water was rapidly dehydrated from the clay minerals in mudstones, which is directly responsible for further corrosion in sandstones.