Origin and impact of diagenetic illite in the Upper Triassic Chang-7 tight-oil sandstone reserviors of the Ordos Basin, China

Diagenetic illite is one of main porosity occluding cements in the Upper Triassic Chang-7 tight-oil sandstone reservoirs of the Ordos Basin, China. Chang-7 sandstone reservoirs is a typical tight oil reservoir with high... [ view full abstract ]

Diagenetic illite is one of main porosity occluding cements in the Upper Triassic Chang-7 tight-oil sandstone reservoirs of the Ordos Basin, China. Chang-7 sandstone reservoirs is a typical tight oil reservoir with high content of illite up to 9.2%, which is the key factor to control the quality of reservoir. This study focuses on the origin of illite and its impact on the tight sandstone reservoirs. Analyses of the composition, distribution and occurrence of illite through SEM, EDS, thin section and X-dliffraction show that Chang-7 sandstones are mostly fine-grained, feldspathic litharenite and lithic arkose, rich in volcanic rock fragment. The pervasive dissolution of K-feldspar yields lots of secondary pore which is the main pore type. The illites are complex in chemical composition and are mainly in scattered, chaotic sheet-like and fibrous shape. They only occur in residue intergranular pores and their content increases with the decreasing of grain size. Volcanic activities were very intense during the deposition of Chang-7 sandstones, which provides the favorable condition for semctite development at the early diagenetic stage. The semctites were transformed to illites subsequently. This process may be the main origin of illite in Chang-7 sandstone reserviors, as illitization of smectite is very common. In addition, the palaeo-geotemperature in the study area is generally lower than 120℃ and there are no external potassium-rich hot fluid input making it impossible for the formation of illite of other origin. The formation of authigenic illite shows a dual effect on the development of relatively high-quality reservoirs in tight sandstones. On the one hand, it can reduce the permeability and lower the quality of reservoirs, and on the other hand, the grain-coating illite can preserve the primary porosity by preventing quartz from cementation, while the transformation of smectite to illite and the illitization of K-feldspar and kaolinite during burial diagenesis can accelerate the dissolution of K-feldspar to form secondary pores. Therefore, the formation of authigenic illite generally plays a positive role in the primary porosity preservation and the development of secondary pores of these tight sandstones, and consequently favors the formation of relatively high-quality reservoirs.