Nano-scale dual-pore-shape structure and fractal characteristics of transitional facies shale matrix

Abstract

Nano-scale pore structure and fractal characteristics are significant information to understand shale gas reservoirs. Therefore, seventeen marine-continental transitional shale samples of Leping Formation in Qingjiang Basin, southeast China were investigated to study the potential relationship between pore structure and fractal characteristics. In this work, a dual-pore-shape method was firstly applied to construct pore structure in the shapes of cylinder and slit, which is closer to reality. Meanwhile, non-local density function theory method and Barrett-Joyner-Halenda method were employed for comparison as well. Fractal dimensions obtained from nitrogen isotherms via Frenkel-Halsey-Hill method were divided into two parts, one characterizes larger pores and the other characterizes smaller pores. Geochemical index, pore structure information, and fractal characteristics showed different correlated behaviors with each other. Total organic carbon content strongly affects pore structure of transitional shale, but clay and quartz content have no clear influence. Correlations between fractal dimensions and geochemical parameters were affected by the degree of isotropy. Pore parameters have conspicuous relationships with the fractal dimension of larger pores but not with that of smaller pores. It is found that slit pores play a significant role in pore space in shale matrix. Moreover, the connections between different shapes of pores and fractal dimensions are displayed. Pore volume of cylindrical pores positively correlates with fractal dimension of larger pores, whereas pore volume of slit pores negatively correlates with fractal dimension of smaller pores. The comparison of all pore parameters obtained by different methods shows that dual-pore-shape method could better construct real pore structure and obtain the pore volume distributions more accurately.