Nanoscale pore morphology and distribution of lacustrine shale reservoirs: Examples from the Upper Triassic Yanchang Formation, Ordos Basin

Pore structure plays an important role in the gas storage and flow capacity of shale gas reservoirs. Field-emission environmental scanning electron microscopy (FE-SEM) in combination with low-pressure carbon dioxide gas adsorption (CO(2)GA), nitrogen gas adsorption (N(2)GA), and high-pressure mercury intrusion (HPMI) were used to study the nanostructure pore morphology and pore-size distributions (PSDs) of lacustrine shale from the Upper Triassic Yanchang Formation, Ordos Basin. Results show that the pores in the shale reservoirs are generally nanoscale and can be classified into four types: organic, interparticle, intraparticle, and microfracture. The interparticle pores between clay particles and organic-matter pores develop most often, 1 with pore sizes that vary from several to more than 100 nm. Mercury porosimetry analysis shows total porosities ranging between 1.93 and 7.68%, with a mean value of 5.27%. The BET surface areas as determined by N-2 adsorption in the nine samples range from 10 to 20 m(2)/g and the CO2 equivalent surface areas (<2 nm) vary from 18 to 71 m(2)/g. Together, the HPMI, N(2)GA, and CO(2)GA curves indicate that the pore volumes are mainly due to pores <100 nm in size. In contrast, however, most of the specific surface areas are provided by the micropores. The total organic carbon (TOC) and clay minerals are the primary controls of the structures of nanoscale pores (especially micropores and mesopores). Micropores are predominantly determined by the content of the TOC, and mesopores are possibly related to the content of clay minerals, particularly the illite-montmorillonite mixed-layer content. (C) 2015 Science Press and Dalian Institute of Chemical Physics. All rights reserved.