Making sense of pore systems and the diagenetic impacts in the Lower Triassic porous dolostones, northeast Sichuan Basin

As the heterogeneity of pore systems, carbonate rocks commonly exhibit unpredictable porosity vs. permeability relationship and make the exploitation and prediction of prolific reservoirs difficult. Most of published studies on pore systems were conducted in sandstones and shales, while carbonate rocks, especially dolostones have received little attention. In this study, we investigate the porous dolostone reservoir in the Lower Triassic Feixianguan Formation, northeast Sichuan Basin, of which the helium porosity is up to 28.9% and permeability is up to 3360 mD. High pressure mercury intrusion capillary pressure (MICP) tests and Field emission scanning electronic microscopic (FE-SEM) examinations reveal the coexistence of two types of open pore systems. One consists of interparticle and intercrystalline pores with large throats of >3 mu m radii. The other consists of intraparticle pores with small throats of 0.06-1 mu m radii. Intracrystalline pores in sucrosic dolostones are isolated and invalid for the storage or fluid flow. Two open pore systems are shown as two separated fractal segments, and the fractal dimensions are related to the throat-surface roughness and pore volume. Observed in thin sections, pores in some dolostones are occluded mainly by two sorts of calcite cement and solid bitumen. Furthermore, MICP analyses exhibit the difference between pre-bitumen and post-bitumen calcite cement, i.e., that pre-bitumen calcite destroys both two open pore systems while post-bitumen calcite only harms to the pore system of interparticle and intercrystalline pores with large throats of >3 mu m radii. This difference might be due to the bitumen or its precursor oil.

Automated summary

Carbonate rocks exhibit unpredictable porosity vs. permeability relationship due to the heterogeneity of pore systems, making the exploitation and prediction of prolific reservoirs difficult. This study on porous dolostone reservoirs reveals the coexistence of two types of open pore systems, which are affected by calcite cement and solid bitumen occlusion. MICP analyses show the difference between pre-bitumen and post-bitumen calcite cement affecting the pore systems.