Comparison of Fractal-Based Pore Structure Characteristics Between Marine and Continental Shale

Abstract

Nanopore structure characteristics of marine and continental shale are quite different, which greatly affects shale CH₄ adsorption and seepage capacity. We investigated such structures in 15 marine shale samples from the Lower Silurian Longmaxi–Wufeng Formation in Sichuan Basin and 15 continental shale samples from the Upper Triassic Yanchang Formation in Ordos Basin using X-ray diffraction analysis, total organic carbon (TOC) tests, vitrinite reflectance (R_o) measurement, low-pressure N₂ adsorption, and CH₄ isothermal adsorption experiments. Isotherms of N₂ adsorption/desorption analyzes indicate that shales from both formations have a hysteresis loop at a relative pressure of 0.45–1. Marine shale belongs to type H₂, with inkbottle-shaped pores, whereas continental shale belongs to type H₃, with slit-shaped pores. On this basis, we obtained fractal dimensions D₁ and D₂ using the fractal Frenkel–Halsey–Hill (FHH) method, in which proposed fractal exponents “D=3+K” and “D=3+3K” were investigated. The results show that the fractal exponent, “3+K”, provides more realistic results.

We discussed relationships between fractal dimension D₁ and TOC, R_o, clay minerals, pore structure parameters, and gas absorption capacity. Results showed that the marine shale samples had fractal geometries with fractal dimensions (D_1m) ranging from 2.71 to 2.82, whereas continental shale samples had fractal geometries with fractal dimensions (D_1c) ranging from 2.44 to 2.56. Greater fractal dimensions indicate that marine shale pore structure is more complex. Organic matter is a controlling factor on fractal dimensions of marine shale, shown by a positive correlation between TOC and D_1m, whereas continental shale has a more complex correlation, as shown by a U-shaped curve between TOC and D_1c. The correlation between R_o and D_1m/D_1c is not obvious. Fractal dimensions D_1m/D_1c exhibit slightly negative correlations with clay mineral content, but show an obvious positive relationship with quartz content. Fractal dimension D_1m/D_1c increases with increasing specific surface area and decreasing pore diameter. Marine shale samples with larger D_1m have a higher CH₄ adsorption capacity, however, continental shale samples with larger D_1c have a lower CH₄ adsorption capacity because of their lower maturity. Fractal analysis leads to a better understanding and comparison of pore structure characteristics and adsorption capacity of shale gas reservoirs in marine and continental shale.

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