Microscopic pore structure in shale reservoir in the argillaceous dolomite from the Jianghan Basin

We proposed the areal porosity method (APM) to quantitatively characterize the inorganic porosity. The principle is that the areal porosity of the images in rock thin section is almost equal to the porosity of the rocks. So inorganic porosity of shale oil reservoir can be calculated approximately by the ratio between inorganic pore area and viewed area in its SEM images. Different components in SEM images have different gray-levels. Compared with other parts in the rock, the pore has greater gray-level, so porous part and non-porous part can be distinguished from each other in SEM images. Based on the theories above, the ratio between inorganic pore area and viewed area in any one of the sections is nearly the same, assuming inorganic minerals and pores are distributed evenly in the shale oil reservoir. This article focuses on approximate calculation of inorganic porosity in a certain depth under different magnifications in the SEM images. Inorganic porous part in the SEM images can be identified and marked by means of image analyzing software. So inorganic porosity can be obtained by accounting the ratio of pore area and viewed area. [GRAPHICS] Although extensive studies have been conducted on the shale reservoir in recent years, little work has been focused on argillaceous dolomite. In this study, investigation of microscopic pore structure in the argillaceous dolomite reservoir in the Xingouzui Formation from the Jianghan Basin has been undertaken to further the understanding of the oil and gas storage mechanism in these rocks. Based on the understanding of pore characteristics by scanning electron microscope (SEM), areal porosity method (APM) and material balance principle (MBP) were respectively used to calculate the inorganic and organic porosities. Moreover, effective porosity and connectedness of the pores were indicated by mercury intrusion porosimetry (MIP) and focused ion beam-scanning electron microscope (FIB-SEM) respectively. Inorganic pores dominate in the argillaceous dolomite reservoir, and there also exist a small amount of organic pores. The calculated inorganic porosities range from 6.37% to 18.58%, close to the measured porosities. The pore radii are mainly within 2.0 mu m. With the increase of porosity, pores with large pore radii make much more contribution to porosity. Organic porosities are much lower, with an average of 0.15%, which is meaningless to the porosity of the shale reservoir in the Jianghan Basin. Compared with the mudstones, the argillaceous dolomites have relatively higher porosity values. Pores in the range of 25.5-253.0 nm contribute a larger fraction of the porosity of the argillaceous dolomite, while pores with smaller throat radii (below 22.4 nm) mainly contribute to porosity in the mudstones. Pores in the argillaceous dolomite have good connectivity, although there are some isolated and dispersed pores. (C) 2016 Elsevier Ltd. All rights reserved.