Effect of litho-facies on nano-pore structure of continental shale in shuinan formation of Jiaolai Basin

Abstract

Continental shale lithofacies has an important influence on nano-pore structure? However, nano-pore distribution heterogeneity (NDH) in continental shale lithofacies is not well documented. Continental shale was collected from the Shuinan Formation in the Jiaolai Basin, and the shale lithofacies of all samples are identified based on the mineralogical composition and TOC content. Nano-pore size distributions are characterized by LPN2 and CO2GA tests. Then, micro-pore (0.3∼2 nm) and meso-pore (2∼100 nm) distribution heterogeneity of different facies are discussed by single and multi-fractal model. Finally, the pore structure mechanism in shale lithofacies is clarified, and the results are as follows. The lithofacies of all shale samples are classified into three types, (A) calcareous shale lithofacies is characterized by the higher siliceous minerals and highest TOC content; (B) Calcareous/siliceous mixed shale lithofacies is characterized by middle TOC content; (C) Siliceous shale lithofacies is characterized by higher clay minerals and lowest TOC content. Micro- and meso-pore volume and A shale lithofacies NDH is much lower than that of B and C, which indicates that the lithofacies type has a great influence on the nano-pore structure. Felsic mineral and TOC content has a weak negative correlation with nano-pore structure, and there is a linear relationship between clay mineral content and nano-pore volume, indicating that clay mineral content is the main factor that can affect nano-pore structure. Moreover, fractal characteristics of the nitrogen desorption curve have been proved, and there are obvious differences in the fractal values of adsorption and desorption curves among those different lithofacies. Single fractal dimension D1 derived from LPN2 and CO2GA tests has a linear correlation with the multifractal parameter D-10-D0, which shows that a lower pore volume of pore diameter determines NDH variation. In conclusion, siliceous rich shale can provide a larger pore volume and specific surface area, and it acts as the dominant lithofacies for shale gas drainage in the study area.