A preliminary study on the pore characterization of Lower Silurian black shales in the Chuandong Thrust Fold Belt, southwestern China using low pressure N2 adsorption and FE-SEM methods

• Both mineral matrix and organic matter pores are developed in overmature black shales.
• Organic pores contribute significantly to the total porosity of black shales.
• Organic matters contain more fine mesopores than matrix minerals.
• TOC is positively related with total pore volume and surface area of shales.

Although Lower Silurian black shales within and around the Sichuan Basin have received renewed attention during the past few years because of their potential prospectivity for shale gas, studies associated with shale gas reservoir evaluation are still rare. In this study, eight black shale core samples were collected from a well drilled recently in the Chuandong Thrust Fold Belt, southwestern China, and their geochemistry and pore structures were investigated using low pressure N2 adsorption analysis and field emission scanning electron microscopy (FE-SEM) observations. The results show that the black shales have total organic carbon (TOC) values ranging from 1.01% to 3.98% and their equivalent vitrinite reflectance values are in the range of 2.84%–3.05%. Both mineral matrix and organic matter pores are well developed with pore sizes ranging from several to several hundred nanometers. The total porosity for the eight samples ranges from 2.60% to 4.74% and the percentages of organic matter pores are estimated to be in the range of 31%–62%. The total surface area ranges from 5.06 m2/g to 19.32 m2/g and the micropore (<2 nm) surface area estimated by the t-plot method ranges from 3.13 m2/g to 9.27 m2/g. The TOC values have positive relationships with the total porosity, total surface area and the micropore (<2 nm) volume and surface area, indicating TOC may be an effective parameter for shale gas reservoir evaluation in the studied area.