Pore-throat structures of the Permian Longtan Formation tight sandstones in the South Yellow Sea Basin, China: A case study from borehole CSDP-2

Abstract

With good exploration prospects and proven source rocks in Permian, reservoir evaluation has become the main bottleneck hindering petroleum exploration in the South Yellow Sea Basin, China, where the pore-throat structures, which are particularly important for the reservoir quality of tight sandstones, have never been characterized. To solve this, tight sandstones taken from borehole CSDP-2 of the Permian Longtan (P2l) Formation were systemically investigated with high-precision pore-throat characterization methods. The results show that pore-throat structures of tight sandstones in the Longtan Formation can be classified into three categories with synthesis considerations of pore-throat type, size, and connectivity; these pore-throats are denoted lattice-like, tubular, and isolated, respectively. The lattice-like pore throats with relatively larger radii (>0.1 μm) and higher connectivity exhibit the best performance for reservoir quality, followed by tubular pore-throat structures, while isolated pore throats have almost no capacity to migrate or store fluids. In the formation mechanism of various pore-throat structures, both the plastic component content and complex diagenesis play a significant role. Strong compaction results in a loss of nearly half of the initial porosity (av. 49.81%) during the initial diagenetic stage. Then, clay mineral cements partially occlude pores and throats, which are the dominant factors in the transition of pore-throat structures from lattice-like to tubular. Silica and carbonate cementation are the key processes that fill the primary pore throats, secondary dissolution pores, and microfractures, leading to the formation of isolated pore-throat structures and low-porosity, ultra-low-permeability reservoirs. Feldspar dissolution cannot take place on a large scale due to the early densification of the sandstone, and it does not significantly improve the reservoir quality; it increases the porosity by less than 4.0%. This research provides constructive insights into the coupled evolution of pore-throat structures and diagenetic processes, which can aid in further understanding the formation mechanism of tight sandstones. In practice, this work also provides a scientific basis for identifying effective reservoirs, and promotes petroleum exploration in the South Yellow Sea Basin, China.