Pore throat heterogeneity of different lithofacies and diagenetic effects in gravelly braided river deposits: Implications for understanding the formation process of high-quality reservoirs

Abstract

Tight gas sandstone is a unique kind of unconventional hydrocarbon resource in that its intricate pore throat structure governs reservoir physical properties and gas reservoir productivity. However, the effect of differential diagenesis of various lithofacies on the heterogeneity of pore throat structure remains unclear. In this paper, a series of experiments were integrated to evaluate pore throat structures and fractal characteristics of the different lithofacies in a tight sandstone gas reservoir, and followed by a study of the impact of diagenetic alteration on them. Results demonstrate that the mineral composition, dominant pore types, pore throat structural parameters, fractal dimension, and physical properties of various lithofacies are notably different, which is mostly attributable to diagenetic alteration variability. There are more primary pores and less clay mineral content in coarser lithofacies. Pebbly coarse-grained sandstone and sandy conglomerate reservoirs that have undergone strong dissolution have more composite pores composed of residual intergranular pores and dissolution pores, which help to form a homogeneous pore throat system with good connectivity, resulting in the best reservoir quality. Due to the strong compaction and high clay mineral content filling pores and throats in medium-grained sandstone and fine-grained sandstone lithofacies, a large number of primary pores were lost, and throats were blocked, which easily formed intercrystalline pores, leading to poor reservoir quality. Coarse-grained sandstone reservoirs with moderate clay mineral content have undergone moderate compaction and dissolution and have moderate reservoir quality. Filamentous illite is the most destructive clay mineral to pore throat connectivity. Intergranular dissolution pores and moldic pores contribute significantly to the physical properties of pebbly coarse-grained sandstone and sandy conglomerate reservoirs. Pebbly coarse-grained sandstone and sandy conglomerate lithofacies are considered high-quality reservoirs with greater development potential for tight gas. This work has important guiding significance for the prediction of high-quality reservoirs in similar tight sandstone reservoirs.