Different pore structure modalities in sandy conglomerate reservoirs and their forming mechanisms

Abstract

The major causes of complicated pore structure modalities in sandy conglomerate have not been documented so far. This study incorporates laboratory core analysis, lithofacies classification, qualitative and quantitative analysis of different pore structure modalities to reveal differences in oil displacement efficiency caused by different pore structure modalities. Analytical methods involved in this study include mercury injection data analysis, constant rate mercury penetration, cast thin section observation, scanning electron microscope, and X-ray-CT scanning. Key findings of this work include the following: (1) sandy conglomerate reservoirs are complex and diverse in lithofacies. According to oilfield development with practical, it can be divided into 3 major types, 7 sub-type, 13 tiny types; (2) Different pore structure modals have different features, a rock with single pore structure modality mostly make up of coarse sand grains, has high pore development degree, 3D pore structure in network shape, and good connectivity between pores; a rock with bimodal pore structure consisted of mostly gravel and middle-coarse sandstone, is characterized by particles of 2 grades, well-developed pores, 3D pore structure in sparse network shape, and average connectivity between pores; a rock with multimodal pore structure dominated by gravel, coarse sand, silt sand, or clay is composed of grains of 3 grades, and it has pores in average development degree, star point scattering distribution, and poor connectivity; (3) same lithofacies but different pore structure modalities, and different lithofacies and same pore structure modality are common in sandy conglomerate reservoirs, the former is caused by different grain arrangement modes and differential reformations of various diagenetic process to pore structure, while the latter is a result of particle sorting, roundness, and arrangement mode, etc. sedimentary and diagenetic factors; (4) different pore structure modalities would lead to quite large differences in oil displacement efficiency. In single modal pore structure, the displacement is dominated by network-connected pattern, and result in high efficiency; in bimodal pore structure, the displacement of oil is dominated by star connected pattern which gives average efficiency; in multimodal pore structure, the dominant oil displacement is sporadic scattered pattern and result in poor efficiency on the whole with minor high permeability zone in local part.