Diagenetic controls on reservoir quality and heterogeneity of the Triassic Chang 8 tight sandstones in the Binchang area (Ordos Basin, China)

Abstract

The Upper Triassic Chang 8 sandstone reservoir is an important target for tight oil exploration in the southwestern Ordos Basin (China). The reservoir quality and heterogeneity are the critical issues of concern in tight oil exploration. Based on integrated analyses of petrography, X-ray diffraction (XRD), scanning electron microscopy (SEM), fluid inclusion microthermometry and stable carbon and oxygen isotopes, the diagenetic evolution and their impact on reservoir quality and heterogeneity are analyzed. Results show that the Chang 8 sandstones are mainly composed of silt to fine-grained, moderately sorted feldspathic litharenite and lithic arkose. The reservoir quality is relatively poor with an average porosity and permeability of 7.49% and 0.38 mD, respectively. The main diagenetic alterations controlling reservoir quality of the Chang 8 sandstones are compaction, and carbonate and chlorite cementation. Compaction resulted in a reduction of more than half of the initial porosity in nearly all of the sandstone samples, whereas the porosity distribution is mainly attributed to variable content of carbonate cements and chlorite coatings. Carbonate cements preferentially precipitated along the top or base of sandstone intervals (distance to the sandstone-mudstone boundary mostly <1 m). The sandstones with well-developed chlorite coatings generally have high porosity (greater than 10%) but relatively low permeability (less than 0.4 mD), indicating that chlorite coatings preserve intergranular porosity but reduce the permeability significantly by obstructing pore throats. Feldspar dissolution had little impact on reservoir properties because it provided large amounts of material for cementation in a closed diagenetic system. Relationship between reservoir properties and oil-bearing grades suggested that the reservoir quality can affect oiliness of tight sandstones. It was concluded that sandstones with porosity >7% and permeability >0.4 mD can be regarded as favorable reservoirs. The outcomes of this research may provide references for the exploration and development of tight oil reservoirs in the southwestern Ordos Basin and similar tight sandstone reservoirs elsewhere.