The Impacts of Matrix Compositions on Nanopore Structure and Fractal Characteristics of Lacustrine Shales from the Changling Fault Depression, Songliao Basin, China

Abstract

The Lower Cretaceous Shahezi shales are the targets for lacustrine shale gas exploration in Changling Fault Depression (CFD), Southern Songliao Basin. In this study, the Shahezi shales were investigated to further understand the impacts of rock compositions, including organic matters and minerals on pore structure and fractal characteristics. An integrated experiment procedure, including total organic carbon (TOC) content, X-ray diffraction (XRD), field emission-scanning electron microscope (FE-SEM), low pressure nitrogen physisorption (LPNP), and mercury intrusion capillary pressure (MICP), was conducted. Seven lithofacies can be identified according to on a mineralogy-based classification scheme for shales. Inorganic mineral hosted pores are the most abundant pore type, while relatively few organic matter (OM) pores are observed in FE-SEM images of the Shahezi shales. Multimodal pore size distribution characteristics were shown in pore width ranges of 0.5–0.9 nm, 3–6 nm, and 10–40 nm. The primary controlling factors for pore structure in Shahezi shales are clay minerals rather than OM. Organic-medium mixed shale (OMMS) has the highest total pore volumes (0.0353 mL/g), followed by organic-rich mixed shale (ORMS) (0.02369 mL/g), while the organic-poor shale (OPS) has the lowest pore volumes of 0.0122 mL/g. Fractal dimensions D1 and D2 (at relative pressures of 0–0.5 and 0.5–1 of LPNP isotherms) were obtained using the Frenkel–Halsey–Hill (FHH) method, with D1 ranging from 2.0336 to 2.5957, and D2 between 2.5779 and 2.8821. Fractal dimensions are associated with specific lithofacies, because each lithofacies has a distinctive composition. Organic-medium argillaceous shale (OMAS), rich in clay, have comparatively high fractal dimension D1. In addition, organic-medium argillaceous shale (ORAS), rich in TOC, have comparatively high fractal dimension D2. OPS shale contains more siliceous and less TOC, with the lowest D1 and D2. Factor analysis indicates that clay contents is the most significant factor controlling the fractal dimensions of the lacustrine Shahezi shale.

Pore Connectivity Characterization of Lacustrine Shales in Changling Fault Depression, Songliao Basin, China: Insights into the Effects of Mineral Compositions on Connected Pores

“…For the argillaceous shale sample (TS-6-7), 582 pores in total were extracted from the images (Figure 2c Overall, interP pores provide the highest proportion of pore numbers and a relatively larger mean pore size in the studied shale samples. The pores in the studied Shahezi shales are mostly irregular in shape as displayed by the fractal dimension data, which is consistent with previous image processing work [58]. Figure 3 illustrates the full-size PSD by combining the low pressure gas adsorption (CO 2 and N 2 ) and MICP data.…”

Tectonic and Thermal Controls on the Nano-Micro Structural Characteristic in a Cambrian Organic-Rich Shale

“…Shale gas exploration and development, initiated successfully in the North America and extended to China, will have application in several other countries in the coming years [1][2][3][4]. Organic-rich shale currently produces commercial unconventional hydrocarbons and exhibits a wide variation in nano-and microscale geological attributes and geochemical properties [1][2][3][4][5]. The lower Cambrian Lujiaping Formation marine, organic-rich shales in the Dabashan Thrust-fold Belt of South China are being studied for their potential as shale gas reservoirs [6][7][8][9][10].…”

Adsorption of Methane, Nitrogen, and Carbon Dioxide in Atomic-Scale Fractal Nanopores by Monte Carlo Simulation I: Single-Component Adsorption

“…(1) Atomic-scale geometrically heterogeneous surfaces: statistical self-similarity, a typical geometric feature of surface heterogeneity that can hardly be described by Euclidean geometry, is a ubiquitous characteristic of most materials in nature . Thanks to the fractal theory advanced by Mandelbrot, , the surface fractal dimension functions as an applicable measure of surface geometric heterogeneity. − The fractal pore morphology has been examined by mercury intrusion porosimetry (MIP), − the fractal Frenkel–Halsey–Hill (FHH) adsorption isotherm equation based on nitrogen adsorption, − ,− nuclear magnetic resonance, ,, small-angle and ultrasmall-angle neutron scattering, ,− ...…”