Diagentic features of illite in Upper Triassic Chang-7 tight oil sandstones, Ordos Basin

The Upper Triassic Chang-7 tight oil sandstones are the main tight clastic reservoirs in the Ordos Basin, central China. Illite is one of the most important cements affecting porosity and permeability of the tight oil sandstones, and this study focuses on different types of illitization and its relative formation time. The Chang-7 tight sandstones are mainly fine-grained lithic arkose and feldspar lithic sandstones, rich in mica and illite (hydrous mica). They are formed in distal delta front to semi-deep and deep lake facies, mainly deposited in a low energy environment. Porosity and permeability are very low (average porosity 8.58% and permeability 0.20 mD). The illite has many morphological characteristics and is multiphase, incorporating six types of illitization: hydrous mica, mica, and smectite illitization at the eodiagenetic stage, and kaolinite and K-feldspar illitization, and neoformation illite particles, at the mesodiagenetic stage. These different types of illitization are mainly controlled by sedimentary environment and provenance. Quartz overgrowth and diagenetic illites appear to be locally mutually exclusive, competing for detrital grain surfaces, but neoformation illite particles appear to form on the quartz overgrowth. The systematic study of Upper Triassic Chang-7 tight oil sandstones carried out here, enhances our understanding of illitization and the diagenetic model of the Ordos Basin, and contributes to reducing the exploration risk of continental tight sandstone reservoirs.