Abstract:

This paper mainly discusses the vertical distribution of secondary pores in Paleogene sandstones in Zhanhua depression,based on the thin section, SEM,carbonate contents, vitrinite reflectance, clay minerals, and porosity and permeability data. Thin section and SEM observation shows that the pores in sandstones are mainly of primary pores preserved after compaction at a depth of less than 1 500 m, at a depth interval of 1 500～1 900 m, pores are mainly of primary intergranular pores left after cementation made by carbonate, authigenic clay minerals and quartz overgrowth; there are a few secondary pores at a depth of 1 900 m,and then become mixed pore zone,characterized by the coexistence of primary and secondary pores in a depth range of 1 900～2 200 m; a large amount of secondary pores occur at 2 200～3 500 m interval; and over 3 500 m, it is characterized by residual pores preserved after compaction and cementation. The vertical variation pattern of 15 000 porosity and permeability data from more than 200 wells can also directly confirm such evolutionary processes. From the surface to 1 900 m, the poroperm characteristics of sandstones gradually become poorer and poorer; at the 2 200～3 500 m interval, there is a zone with higher porosity and permeability; and over 3 500 m, the poroperm characteristics of sandstones are getting poor again. It is found from thin section and SEM data that secondary pores have mainly been formed by feldspar dissolution, with minor by carbonate cement dissolution. The vertical distribution of authigenic kaolinite and carbonate cements can also give such explaination. At the 2 200～3 500 m interval where secondary pores are well developed, feldspar dissolution has resulted in the increase of authigenic kaolinite and large amount of quartz overgrowth, while the amount of carbonate cements is still relatively high. The phenomena of feldspar being dissolved and no or only trace of carbonate cement being dissolved are determined by free energy of chemical reaction between organic acid and feldspar/carbonate cement. Dissolution of feldspar in organic acids is characterized by relatively low chemical energy,as for calcium feldspar, ΔG=-154.49 kJ/mol;and for K-Feldspar, ΔG=-17.92 kJ/mol,while for carbonate cement, ΔG= 46.89 kJ/mol.Secondary pores are largely distributed in a depth range of 2 200～3 500 m. Such distribution is closely related to the maturation of source rock and diagenitic evolution of sandstones. In Zhanhua depression, the source rock reached the hydrocarbon-generating window at a burial depth of 2 000 m,with the peak of hydrocarbon-generation to be at 2 000～3 000 m. At this interval, the organic acids generated from source rock maturation caused the dissolution of feldspar, precipitation of kaolinite,and quartz overgrowth,as well as the formation of secondary pores. On the other hand, at 1 900～2 800 m interval, the interlayer water has rapidly been dehydrated from the clay minerals in mudstones,which would result in a further corrasion in sandstones. Therefore, a large amount of organic acid generated during source rock maturation,and water dehydrated from clay minerals would cause the dissolution of feldspar, as a result,a large amount of secondary pores have been formed in Paleogene sandstones in Zhanhua depression. These pores are mainly distributed at a depth range of 2 200～3 500 m.

Key words: secondary pores, vertical distribution, genesis, Zhanhua depression