Diagenetic mineralogy and its effect on the reservoir properties of the sandstones of the Permian of S120 block (Sulige gas field), Ordos Basin, NW China

Abstract

The characteristics of diagenetic minerals and their effects on reservoir quality of the tight sandstones of the Permian in Sulige gas field of the Ordos Basin were studied through observations on thin sections, scanning electron microscopy, energy dispersive spectroscopy analysis and electron microprobe analysis. Diagenetic minerals in the Permian sandstones consist of illite, kaolinite, chlorite, siliceous and calcite cements. Large amounts of intercrystalline pores between kaolinite and illite provide channels for acidic fluids flow and thereby were conducive to the formation of clastic solution pores, intergranular solution pores and composite pores. Authigenic chlorite occurs in the form of three morphotypes as grain-coating, pore-lining and pore-filling. Grain-coating and pore-lining chlorite with different crystal shapes occur as coatings on the framework grains. Pore-filling chlorite precipitated as discrete flaky plates in pore spaces. Quartz microcrystals developed but quartz overgrowth did not develop because of the occurrence of pore-lining chlorite. This, in turn, led to the preservation of primary pores by occupying potential nucleation sites for quartz overgrowth and thereby preventing quartz microcrystals from merging into quartz overgrowth. This process is regarded as the most important for influencing the quality of the lithophysical properties. Calcite cement was mainly precipitated during a late diagenetic stage and has a negative effect on the reservoir quality. This study provides important insights into analyzing the relationship between diagenetic minerals and reservoir quality and the results are directly applicable to the exploration and development of tight sandstone reservoirs all over the world.