Quantitative identification of diagenetic facies and controls on reservoir quality for tight sandstones: A case study of the Triassic Chang 9 oil layer, Zhenjing area, Ordos Basin

The identification of diagenetic facies is essential for prospecting hydrocarbon reserves and for hydrocarbon exploration in tight oil reservoirs. A comprehensive method for determining quantitative index of reservoir diagenetic facies (RDI) using core analysis techniques, hierarchical cluster analyses (HCA), and grey correlation analyses (GCA) was proposed. The petrological characteristics, physical properties, and pore structures of the Chang 9 tight oil sandstones are analysed based on thin-section analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), cathode luminescence (CL) and mercury injection capillary pressure (MICP) tests. The properties of the Chang 9 sandstones were controlled by different forms of diagenesis: Compaction and cementation of calcite have reduced or occluded the pore volume in the sandstones, but not all cementation is deleterious to reservoir quality. Chlorite cementation helped preserve relict primary porosity and the direct dissolution of feldspars and unstable lithic fragments has resulted in enhanced pore connectivity. Following petrographic observations and quantitative analysis, the sandstones were divided into four diagenetic facies: calcite cementation facies, intense compaction fades, porosity preserved by grain-coating chlorites facies, and dissolution fades. The intense compaction fades and calcite cementation facies are responsible for the evolution of low-quality reservoirs, while the reservoirs with porosity preserved by grain-coating chlorites facies and dissolution facies are of better porosity and permeability. The distribution of fractures improves reservoir quality by enhancing the dissolution of unstable grains and providing convenient channels and storage spaces for oil and gas migration.