Diagenetic controls on sweet spots pore types and reservoir quality of Permian tight sandstones in the Ordos Basin, China

Abstract

Identification of sweet spots in tight sandstone reservoirs is crucial to the effective economic development of tight oil and gas. The pore types and porosity-permeability relationships in the sweet spots of tight sandstones are primarily controlled by sedimentary and diagenetic processes. All of the Permian tight sandstone sequences in the northeastern Ordos Basin were deposited under similar depositional environment with sediments supplied from the same source. Understanding the variations of the petrophysical properties of the Permian tight sandstones would thus provide insight into the control of reservoir quality by the diagenetic process. Petrographic and petrophysical analyses were conducted on the Permian tight sandstone sweet spot reservoir interval, revealing the distribution and characteristics of pore types that control reservoir quality. Pore types change from a mix pore type of primary intergranular, secondary dissolution porosity and undifferentiated microporosity in the shallower upper Permian reservoir interval to a secondary dissolution porosity- and microporosity-dominated one in the deeper lower Permian reservoir interval. Porosities decrease from 15.18% in the upper Permian to 10.43% in the lower Permian, while permeabilities decrease from 15.29 mD to 0.85 mD. The primary intergranular porosity decreases from 7.94% to 0%, while the secondary dissolution porosity increases from 4.75% to 6.22%. In contrast, microporosity increases from 3.60% to 4.52%. With increasing burial depth, the intergranular porosity decreases significantly due to mechanical compaction or occlusion by quartz cementation. The precipitation of authigenic clay minerals and ferrous carbonate cementation also contributes to the reduction of primary intergranular porosity. Although the dissolution of feldspars produced more secondary dissolution porosity with increasing burial depth, quartz cementation and authigenic clay minerals resulting from feldspar dissolution limit the porosity increase from mineral dissolution. Changes in pore type proportions and the reduction in total porosity with increasing burial depth alter porosity–permeability relationships. In the shallower upper and middle Permian reservoir intervals, primary intergranular porosity are well developed, typically exhibiting large pore throat sizes. The average medium pore throat radius in the upper and middle Permian reservoirs reaches 0.88 μm and 0.42 μm, respectively, resulting in high permeability. In contrast, the deeper lower Permian reservoir intervals lack primary intergranular porosity and primarily develop secondary dissolution porosity and microporosity, which connect to small pore throats. As a result, the lower Permian reservoir has an average medium pore throat radius of only 0.32 μm, resulting in low permeability. Because the lower Permian reservoirs are dominated by secondary dissolution porosity and microporosity, their porosity–permeability relationships have gentler slope compared to those of the upper and middle Permian reservoirs, where primary intergranular pores are more prevalent.