Origins of authigenic minerals and their impacts on reservoir quality of tight sandstones: Upper Triassic Chang-7 Member, Yanchang Formation, Ordos Basin, China

Abstract

Reservoir quality is critical for sweet-spot evaluation in tight sandstone plays, but few studies have focused on the origin of authigenic minerals in tight sandstones and their impact on reservoir quality. This study integrates petrographic analysis, carbon and oxygen isotopic data and mercury injection capillary pressure data of the Upper Triassic Chang-7 tight sand reservoir samples in the Ordos Basin, China to understand the origin of authigenic minerals and the impact of authigenic minerals on reservoir quality. Carbonate minerals, including calcite, ferroan calcite and dolomite, and kaolinite, are the major authigenic minerals in the Chang-7 sandstone. They were derived from the chemical diagenetic alteration of detrital feldspar and biotite, with the involvement of ions that are believed to be primarily from the connate water in interstitial pore space of the interbedded mudstones. Meteoric water and organic fluids from hydrocarbon generation may also have been involved in the alteration processes of the minerals in the Chang-7 tight sandstone. The origin of kaolinite indicates that the porosity was increased by dissolution of detrital grains, offsetting the porosity loss from burial compaction. Authigenic minerals appear to have a weak correlation with pore structure, suggesting that the pore structure of the Chang-7 tight sandstone is not only controlled by major authigenic minerals but also affected by other geological factors.