TWO TYPES OF REACTION MECHANISMS BETWEEN K-FELDSPAR, KAOLINITE AND ILLITE AND THEIR IMPACTS ON GENESIS OF SECONDARY PORES. THE CASE OF PERMIAN XIASHIHEZI FORMATION AND CARBONIFEROUS TAIYUAN FORMATION IN DANIUDI GAS FIELD, ORDOS BASIN

The first member of Permian Xiashihezi Formation (P₁x₁) and the second member of Carboniferous Taiyuan Formation (C₃t₂) in Daniudi gas field, Ordos Basin, contain thick sections of low porosity, low permeability sandstones. This paper reveals that different reactions between k-feldspar, kaolinite and illite occurred and made different formation mechanisms of secondary pores as a result of two factors: K⁺ and smectites.

The rock types of P₁x₁ and C₃t₂ are litharenite and litharenite-sublitharenite-quartz arenite. Synsedimentary volcanic materials such as tuffs which contain more smectites in P₁x₁ were found. C₃t₂ was impacted by marine sediments of underlying C₃t₁ and had more K⁺. The two differences impacted profoundly the transformation of smectite to illite which is a low-energy spontaneous reaction from 50°C to 120°C ( Berger et al., 1997 ).

Two formations experienced the dissolution of plagioclases from syndiagenesis to initial stage of burial diagenesis (IBD) and kaolinites precipitated. More smectites in P₁x₁ made the tranformaion of smectice to illite and dissolution of k-feldspar coinciding and reinforcing each other from IBD to 120°C paleogeotherm. And k-feldspars ran out. The reaction produced Mg²⁺, Fe²⁺ and Ca²⁺, SiO₂ and Na⁺, facilitating the cementation of chlorite, calcite and quartz. The deplation of K⁺ led kaolinites not being transformed to illites in later burial diagenesis. The intercrystal pores of kaolinites occupy 19% of porosity.

High salinity of present formation water in C₃t₂ indicates the impact of sea water. Enough K⁺ from sea water facilitated the tranformation reaction of smectite to illite and it lasted from IBD to 120°C paleogeotherm. Smectites were relatively minor and were totally transformed to illites. K-feldspars were not dissoluted because of abundant K⁺. After this stage dissolution of k-feldspars and transformation of kaolinite to illite coincided and reinforced each other. The reaction produced more quartz cements (1.4%) than in P₁x₁ (0.7%). Quite small amount of kaolinites and chlorites supported the conclusion. Quartz, ankerites and authigenic illites are chiefly cements. Mg²⁺, Ca²⁺ from sea water contributed to the formation of ankerites from calcites. The intercrystal pores of illites occupy 14% of the total porosity.