Fractal-Based Pore Structure and Gas Adsorption of Shales Across a Maturation Gradient

Abstract

Vitrinite reflectance (Ro) is a generally used index to determine the thermal maturity of shales. To more accurately characterize pore structure and gas adsorption of shales across a maturation gradient, 15 marine shale samples (Longmaxi Formation, Sichuan Basin, southwest of China) with Ro ranging from 2.79% to 3.41% and 15 continental shale samples (Yanchang Formation, Ordos Basin, midwest of China) with Ro ranging from 0.80% to 1.06% was taken to employed the XRD (X-ray diffraction) analysis, TOC (total organic carbon) test, low-pressure N₂ adsorption and CH₄ isothermal adsorption experiments. Then micro pore evolution with thermal maturity, TOC and mineral component was studied, the thermal evolution law of shale pore fractal characteristics obtained by Frenkel-Halsey-Hill (FHH) method and its relationship with the pore structure parameters are also investigated.

Result show that the fractal dimensions of marine shale samples D_1m range from 2.71 to 2.82, continental shale samples D_1c range from 2.44 to 2.56, indicating that pores in shales express fractal characteristics in terms of shape and size. Fractal-based meso-pore volumes follow different evolutionary paths with thermal maturity between marine shale samples with average 3.16%Ro and continental shale samples with average 0.93%Ro, which is constricted by the transformation of kerogen during the early mature stage and secondary cracking of bitumen/oil at the over mature stage. Fractal dimension D_1m/D_1c increases with increasing BET surface area and decreasing pore diameter, and shows a weak negative correlation between D_1m and pore volume for over-mature shale samples, while a weak positive correlation between D_1c and pore volume for early-mature samples. Marine shale samples with higher Ro have larger D_1m, however, continental shale samples with lower maturity has lower D_1c, indicating complexity of maturity affecting on pore structure, higher maturity and enhanced fractal dimension leads to growing pore structure heterogeneity and increasing adsorption sites and gas adsorption capacity of shales. Therefore, precise study of influence of thermal maturity on shale pore structures and gas adsorption capacity deserves attention.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018