Ternary pore structure characteristics of the tight sandstone: a case study from the Chang 7 Member of the Triassic Yanchang Formation in the Ordos Basin, China

The analysis of pore structure characteristics in the tight sandstone is crucial for the exploration and development of tight oil reservoirs. However, there are few studies on the differences of pore morphological characteristics at different scales, which poses great challenges to the exploration and evaluation of tight reservoirs. Thus, casting thin sections, scanning electron microscopy and mercury intrusion tests were employed to examine the pore structure characteristics of the tight sandstone from the Chang 7 Member of the Triassic Yanchang Formation in the Ordos Basin, China. The results show that the tight sandstone exhibits a ternary pore structure comprised of combined pores, particle pores and intergranular pores. Combined pores form a spiny ball model with large radius and strong connectivity. Particle pores represent a shuttlecock model with medium radius and connectivity. The intergranular pores create an envelope surface filled with particles. These intergranular pores have small radius and poor connectivity. Within the tight sandstone, residual intergranular pores and dissolution pores are the primary contributors to the reservoir space, while permeability is primarily controlled by combined pores. Due to the formation of multiple connecting channels during diagenesis, combined pores exhibit higher connectivity compared to other types of pores. A high proportion of combined pores results in a more uniform fluid flow process, leading to the formation of larger, highly oil-saturated sand bodies. The sandstones that are dominated by combine pores, undergo strong dissolution, and are located near the source rocks represent the most favorable area for the exploitation and development of tight sandstone reservoirs. Overall, the study demonstrates that morphological diversity of tight sandstone pores reveals the varied impact of different pore types on reservoir storage and connectivity. These research findings provide a geological foundation for the exploration and evaluation of tight sandstone reservoirs. © 2024 Central South University of Technology. All rights reserved.