Impacts of sedimentology and diagenesis on pore structure and reservoir quality in tight oil sandstone reservoirs: Implications for macroscopic and microscopic heterogeneities

Integrated analysis of reservoir quality, pore structure, depositional and petrologic characteristics, and types and degree of diagenetic alterations was done in this study by employing petrophysical evaluation, thin section observation, stable isotope analysis, scanning electron microscopy combined with EDS, X-ray computed tomography, and high-pressure mercury intrusion. The purpose was to investigate the macroscopic and microscopic heterogeneities, and the effects of depositional factors and diagenetic alterations responsible for them. The results indicate that the Chang 8 tight oil sandstone of the Longdong area in the Ordos Basin can be divided into three categories, namely the Type I reservoir, with porosity greater than 10% and permeability greater than 1.0 mD, having sparse lenticular distributions in the mouth bar of the braided delta front, the Type II reservoir, with porosity of 6-10% and permeability of 0.1-1.0 mD, mainly distributing in the distributary channel, and the Type III reservoir, with porosity less than 6% and permeability less than 0.1 mD, having a reticular distribution in the distributary bay. The Type I reservoir has an intergranular pore (Inter-P) dominant pore system with a greater pore-throat radius (1-10 mu m), a larger pore volume and better connectivity. The Type II reservoir has a mixed system consisting of intragranular pores (Intra-P) and Inter-P with a pore-throat radius of 0.1-1 mu m, smaller pore volume, and worse connectivity. The Type III reservoir is characterized by an Intra-P dominated system with a radius between 10 and 500 nm, smallest pore volume and worst connectivity. The wide range of porosity, permeability, and pore structure parameters can be attributed to the frequent variations in depositional fades and diagenetic alterations. Larger grain size, higher content of quartz and feldspar, and moderate grain-coating chlorite content (less than 8%) are favorable for the preservation of intergranular pores and dominate the formation of the Type I reservoir. The mesogenetic calcite and ferrocalcite cementation and pore-filling kaolinite precipitation obstruct the intergranular space and are the predominant factors tightening the reservoirs in the distributary channel and distributary bay, leading to multi-scale heterogeneities. Secondary porosity from dissolution plays a much more important role in contributing to the porosity and permeability of the Type II and III reservoirs than of the Type I reservoir. The study provide a comprehensive understanding for the multi-scale heterogeneities and can be used to predict the potential high-quality zones in tight oil sandstone reservoirs.