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

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.