Pore structure characterization for organic-rich Lower Silurian shale in the Upper Yangtze Platform, South China: A possible mechanism for pore development

Abstract

The Lower Silurian shale in the Upper Yangtze Platform has become one of the main targets of the exploration and development of marine shale gas in South China. This study is focused on the organic-rich Lower Silurian black shale from four wells in the Upper Yangtze Platform, and their organic geochemistry, petrology, porosity and pore characterization were investigated. A combination of field emission scanning electron microscopy (FE-SEM), low pressure N2/CO2 adsorption and mercury intrusion porosimetry (MIP) measurements was conducted to characterize the pore structure and the effects of organic matter and inorganic minerals on porosity were discussed. Then the contribution of individual mineral components to total porosity was analyzed quantitatively. The results indicate that (1) FE-SEM images show that there are mainly three types of pores in the Lower Silurian shale: OM pores, pores of framework minerals, and pores in clay minerals. (2) CO2 adsorption results suggest that the micropores of the shale samples are mainly contributed by the pores with width between 0.4 and 0.7 nm, while N2 adsorption results show that the mesopores of the shale samples are mainly contributed by the pores with width between 2 and 10 nm, which is consistent with MIP results. (3) The TOC content shows a weakly positive correlation with porosity, suggesting that organic matter has an important influence on shale porosity, but it is not the most essential factor. When the maturity is lower than 2.6%, the porosity increases with increasing maturity, and when the maturity value is greater than 2.6%, the porosity decreases gradually. (4) The presence of rigid grains led to the formation of rigid frameworks, which can prevent pores from collapsing and benefit the preservation of OM pores and pores in clay minerals. Pores of framework minerals are mainly generated by the corrosion effect of organic acids produced in the process of hydrocarbon generation. (5) Organic matter has the largest pore volume per unit mass, followed by clay minerals, and then framework minerals. This means that organic matter and clay minerals contribute significantly to shale pore space given the same mass percentage condition; the matrix pores are mainly composed of pores in clay minerals and OM pores, which account for approximately 90% of the matrix pores.