Investigation of the factors that control the development of pore structure in lacustrine shale: A case study of block X in the Ordos Basin, China

Abstract

As important indicators of conditions that affect the occurrence of shale gas reservoirs, pore structures are systematically studied for their significance in shale gas exploration and development. To date, research on the controlling factors of pore structure development in lacustrine shale remains lacking. In this paper, the authors used lacustrine shale samples of the Mesozoic Yanchang Formation in the Ordos Basin, Block X, to investigate the factors that influence the development of pore structure. The qualitative and quantitative study of pore type and size distribution in lacustrine shale was performed using scanning electron microscopy (SEM) on surfaces polished via argon beam milling imaging, nano-computed tomography (CT), nitrogen adsorption isotherm experiments and low-field nuclear magnetic resonance (NMR) imaging. In addition, the main controlling factors for the development of pore structure in lacustrine shale are discussed in detail. This research shows that a variety of pore types develop in lacustrine shale reservoirs, all having different geological significance in the evolution of shale gas accumulation. The results also show that the pore and throat structures are dispersed in three-dimensional space with spherical, tubular and irregular shapes. Both micropores (<2 nm) and mesopores (2–50 nm) provide the main specific surface areas of pores, whereas mesopores provide the primary pore volume. The total organic carbon content, mineral components, and thermal maturity are the main factors that control the development and evolution of pore structure in lacustrine shale.