Insight into the nanoscale pore structure of organic-rich shales in the Bakken Formation, USA

Abstract

The Bakken Shale has attracted much attention because of its huge potential for shale oil. To characterize the nanoscale pore structure of the Bakken shale, ten samples were selected from the upper/lower Bakken Shale for N2 and CO2 adsorption, which can provide the pore structure parameters, such as pore size, specific surface area (SSA) and pore volume. The controlling factors of the pore structure were studied through the analysis of the mineral composition, total organic carbon (TOC) content, and thermal maturity (VRo-eq). Also, we investigated the fractal characteristics of samples based upon N2 adsorption. The results exhibited that the composition of the Bakken shale was mainly quartz and clay; the content of clay minerals varies between 24.07% and 53.92%; the content of quartz is between 38.04% and 63.59%. The average value of TOC content of the Bakken shale was 11.49 wt% and the vitrinite equivalent reflectance varied between 0.62 and 1.11%. Micropores (<2 nm) and mesopores (2–50 nm) are the dominant pores in the Bakken shale samples. Quartz, clay and TOC are the key factors controlling the pore structure of Bakken shale. The fractal dimensions D1 ranged from 2.606 to 2.896 and D2 ranged from 2.243 to 2.296. D1 shows a strong positive correlation with D2, with an R2 value of 0.8. Higher clay and TOC content lead to more complex pore systems in the Bakken shales. Smaller pore size leads to larger fractal dimensions; hence, the larger the SSA is, the larger fractal dimensions are.