Pore evolution in tight sandstone and its impact on oil saturation: a case study of Chang 6 to Chang 8 reservoirs in Triassic Yanchang Formation, Ganquan area, Ordos Basin

Abstract

Tight sandstone reservoirs exhibit strong microscopic heterogeneity and significant variations in oil saturation. To investigate the variations in pore and throat size distribution during the diagenesis of tight sandstone reservoirs as well as their impact on oil saturation, the study takes the Chang 6 to Chang 8 tight sandstone reservoirs of the Triassic Yanchang Formation in the Ganquan area of the Ordos Basin as a case study. The influence of diagenesis on porosity was quantitatively calculated using methods such as cast thin sections, scanning electron microscopy (SEM), and high-pressure mercury injection. On the basis of the test results, a pore and throat size distribution model during the main hydrocarbon accumulation period was established, constrained by statistical models of pore and throat parameters. The movable fluid saturation during the main hydrocarbon accumulation period was then calculated using integration methods. The results showed that the Chang 6 to Chang 8 tight sandstone reservoirs experienced strong compaction during the early and middle diagenetic stages, with an average porosity reduction of 81.85% due to compaction. Cementation further reduced porosity by about 11.00% on average. Although dissolution increased pore space, the increase was relatively smaller, with an average value of 4.38%. The average paleoporosity at the beginning (128 Ma) and the end (111 Ma) of the main hydrocarbon accumulation period was 13.82% and 8.68%, respectively. The volume proportion of pore and throat radii greater than the minimum flow throat radius (0.1 μm) was low, and the movable fluid saturation ranged from 35.05% to 93.27%. The low pore and throat radii and movable fluid saturation during the main hydrocarbon accumulation period was one of the reasons for low oil saturation. However, due to the influence of authigenic clay minerals on reservoir rock wettability, the current oil saturation has not decreased sharply. The pore and throat size distribution model during hydrocarbon accumulation provides a feasible method for analyzing the evolution of pore and throat size as well as its relationship with oil saturation in similar reservoirs.

Keywords

• tight sandstone
• porosity evolution
• pore and throat size distribution
• oil saturation
• chang 6 to chang 8 reservoirs
• triassic
• ordos basin