Full-Scale Pore Structure Characteristics and the Main Controlling Factors of Mesoproterozoic Xiamaling Shale in Zhangjiakou, Hebei, China

Abstract

Nanoscale pore structure characteristics and their main controlling factors are key elements affecting the gas storage capacity, permeability, and the accumulation mechanism of shale. A multidisciplinary analytical program was applied to quantify the pore structure of all sizes of Xiamaling shale from Zhangjiakou, Hebei. The result implies that Mercury injection porosimetry (MIP) and low-pressure N2 curves of the samples can be divided into three and four types, respectively, reflecting different connectivity performances. The maximum CO2 adsorbing capacity increases with increasing total organic carbon (TOC) content, pore volume (PV), and surface area (SA) of the micropores are distributed in a three-peak type. The full-scale pore structure distribution characteristics reveal the coexistence of multiple peaks with multiple dominant scales and bi-peak forms with mesopores and micropores. The porosity positively correlates with the TOC and quartz content, but negatively correlates with clay mineral content. Organic matter (OM) is the main contributor to micropore and mesopore development. Smectite and illite/smectite (I/S) assist the development of the PV and SA of pores with different size. Illite promotes the development of the nanoscale PV, but is detrimental to the development of the SA. Thermal maturity controls the evolution of pores with different size, and the evolution model for the TOC-normalized PVs of different diameter scales is established. Residual hydrocarbon is mainly accumulated in micropores sized 0.3 to 1.0 nm and mesopores sized 40 nm, 2 nm and less than 10 nm. Since the samples were extracted, the pore space occupied by residual hydrocarbon was released, resulting in a remarkable increase in PV and SA.

Microstructural Analysis of Organic-Rich Shales: Insights from an Electron Microscopic Study by Application of FIBSEM and TEM

“…Pore networks within shales range from simple to complex in morphology, abundance, and origin, with most pores being less than several microns. Several publications have provided quantitative data and excellent images of shale or mudstones pores acquired by LPGA, MIP, NMR, and SEM petrographic evidence [ 8 , 9 , 14 , 15 , 16 , 17 , 18 ]. On the basis of the classifications of the International Union of Pure and Applied Chemistry (IUPAC), the pores that were obtained fell into three classes: micropores (pore width < 2 nm), mesopores (pore width between 2 and 50 nm), and macropores (pore width > 50 nm).…”

“…For example, recent shale gas explorations in the Sichuan Basin, Xichang Basin, and Yanyuan Basin in Sichuan province from a total of 11 organic-rich shale formations in 2022 have technically recoverable reserves at 9 Tcf (trillion cubic feet), with economically recoverable-reserves of 5 Tcf. A great deal of research has been significantly reported in the literature about petrology, mineralogy, organic geochemistry, reservoir quality, generation potential, petrophysical properties, and microstructures in low-porosity and low-permeability shales [ 2 , 3 , 4 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ]. The pore system preserved in fine grain, organic-rich shale can be used to identify gas storage space and migration pathways and estimate the total gas resources, thus serving as a distinct signature of predicting how hydrocarbon molecule are adsorbed and how transport takes place in such porous medium because the morphology, size, continuity, and occurrence medium of the pores can provide insights [ 4 , 15 , 16 , 17 , 18 , 19 , 20 ].…”

“…SEM of FIB cross-sectioned sample surfaces is a major powerful technique, as secondary electron and backscattered electron SEM imaging (SE-BSE) combined with qualitative energy-dispersive spectroscopic (EDS) analysis is used to characterize the evolution of the mineralogical and organic structures and their related pore space in shales or mudstones, which is very successful in imaging micro- to nanometer-scaled fabric signatures down to ~100 nm [ 15 , 16 , 17 , 18 ]. However, many published data based on indirect petrophysical methods (i.e., HP, LPGA, MIP, and NMR) suggest there is a large pore size contribution from <100 nm features that are not accounted for in SEM methods [ 8 , 9 , 10 , 11 ]. By contrast, direct imaging by TEM is capable of resolving such fine structures because it can provide resolutions below 0.2 nm.…”

Fractal analysis of pore structures in transitional shale gas reservoirs in the Linxing area, Ordos Basin

“…At present, there are three main methods of pore splicing, as follows: 1) From experience, three splicing points-25, 50, and 100 nm-are widely applied (Zhang et al, 2017;Xu et al, 2021); 2) According to the PSD image, the intersection of two curves or the midpoint of their parallel segments is usually selected for the splicing location (Xu et al, 2018); 3) The splicing point can be determined from an NMR T 2 curve (Xiao et al, 2020;Zheng et al, 2019a). The problems with these methods are also obvious: For method 1, the splicing points are frequently chosen based on the splicing findings of similar lithologies or study areas, making the points chosen highly subjective.…”

Special Issue of Nanomaterials: Mechanics of Nanostructures and Nanomaterials

“…In the presented Special Issue, six research papers [ 1 , 2 , 3 , 4 , 5 , 6 ] are published. Topics of published papers cover analyses of different engineering problems of nanostructures.…”