Diagenesis Of Sandstone Reservoir Of Member 2 Of Shanxi Formation In Yulin Area, Ordos Basin

Upper Paleozoic sandstones contain abundant natural gas resource, among the formations, Member 2 of Shanxi Formation is the most important gas-bearing strata and has indicated a promising prospect for exploration. As an important coal-bearing stratum, Member 2 of Shanxi Formation is controlled by acid pore fliud during diagenesis evolution. Petrology, authigenic minerals, pore types and physical properties of reservoir are generally inbolved in the paper based on core description, thin sections under microcope, X-Ray diffraction (XRD), scanning electron microsope (SEM), chemical element analysis and isotopic analysis, and on the basis of the above research, diagenesis and formation mechanism of secondary pore are further discussed.Quartzarenite is the main rock type of sandstones in the study area which has very high composition maturity. It is framework end-member compositon of sandstone that is characterized by extremely deficient feldspar and quite rich quartz. Sandstones are coarse-grained, well-sorted and lower matrix. The primary pore, dominantly residual intergranular pore, holds an important position in the pore type. Kaolinite intracrystalline micropores and secondary dissolution pores take a dominant place in secondary pore types. The average porosity is 5.03%, and permeability of most samples is below 1.0×10-3μm2. There is favorable positive correlation between porosity and permeability, while the changes of the latter are controlled by the development of the former.Diagenesis types include compaction, cemetion, dissolution and metasomatism. According to the diatribution and paragenesis of authigenic minerals, clay minerals transformation and organic matter maturity, the reservoir sandstone is now in middle diagenetic phase B. Compaction is the most important destructive process, which is mainly responsible for porosity loss. Authigenic minerals in sandstones involve siliceous cement, clay minerals (mainly kaolinite) and carbonate minerals, whose contents in rock are respectively 6.30%, 3.72% and 3.28%. As the formost constructive process, dissolution forms abundant secondary porosity and thus improves physical properties of reservoir. Dissolution medium roots in acid pore fluid related with coal-bearing stratum, and dissoluble material are primarily feldspar and metastable rock fragments and other aluminosilicate.Under thin section and SEM, siliceous cement often behaves as overgrowth, which can form during earlier diagenetic stage; as pore-filling cement, kaolinite usually has preferable ntracrystalline micropores, which is associated with quartz cements; calcite and dolomite fill the pores as a granular texture, and the latter is formed in earlier stage, while the former isprecipitated in the late stage, which is also favored by the analysis ofδ18O andδ13C of carbonate cements. Mateiral source of all these cements is provided by dissolution of feldspar via organic acid fluid. Siliceous cement, kaolinite and dolomite formed in earlier stage can enhance mechanical strength and resistance to compaction, and thus compaction is restrained in some degree and primary pore is preserved, which is also the chief reason for abnormmaly high primary pore in deep buried Shan 2 sandstone.