A Comparative Study on the Microstructure and Petrophysical Properties of Undeformed and Naturally Deformed Shales

Abstract

Structural deformation is one of the main impacts responsible for understanding the hydrocarbon accumulation mechanism. Predicting its function in gas storage and migration in a microscopic scale is thus a significant issue in predicting reservoir quality. The purpose of this article is to make a comparative study on the microstructure and petrophysical properties of undeformed and naturally deformed shales developed in the Southeast Sichuan Basin. The results show that (1) organic pores, ranging from 10 to 200 nm, are more developed in undeformed shale, whereas carbonate solvopores, clay-hosted pores, and microfractures are dominant in deformed shale. (2) Undeformed shales have Dubinin–Radushkevich (DR) CO2 adsorbed contents ranging between 0.009 and 0.018 g, DR CO2 micropore surface areas of 3.90–15.59 m2/g, and Dubinin–Astakhov (DA) CO2 micropore volumes of 0.002 and 0.035 cc/g. Naturally deformed shales have larger DR CO2 adsorbed contents, DR CO2 micropore surface areas, and DA CO2 micropore volumes, typically in the ranges of 0.019–0.031 g, 13.15–20.97 m2/g, and 0.006–0.012 cc/g, respectively. (3) Undeformed shales have N2 adsorbed contents ranging between 1.94 and 6.03 cc/g, Brunauer–Emmett–Teller (BET) N2 mesopore and macropore surface areas of 0.55–7.35 m2/g, and Barrett–Joyner–Halenda (BJH) N2 mesopore and macropore volumes of 0.002 and 0.006 cc/g. Naturally deformed shales have larger N2 adsorbed contents, BET N2 mesopore and macropore surface areas, and BJH N2 mesopore and macropore volumes, typically in the ranges of 5.72–14.45 cc/g, 7.47–17.64 m2/g, and 0.005–0.015 cc/g, respectively. (4) All samples have a unimodal micropore size distribution with a distinct peak associated with a 1.5 nm micropore fraction and have a multimodal mesopore and macropore size distribution. Naturally deformed shales show more homogeneous pore size distribution curves with morphological consistency than undeformed shales, indicating that structural deformation can significantly influence pore structural heterogeneity. (5) Deformed shales exhibit higher porosity and permeability compared to the undeformed samples due to the open pore-fracture network associated with the inorganic pores and microfractures. Average porosity and permeability in deformed shales can be 1.8 and 15 times higher than those in the undeformed shales. We conclude that the microstructural heterogeneity from strong to weak and the changes of a relatively single organic pore-dominated pore system to complex pores and microfractures dominated the pore-fracture system within shales due to structural deformation being the key cause of the quantitatively and qualitatively studied microstructure and petrophysical property differences.

Microstructures and Pore Structures of Naturally Deformed Shales: Significance for Shale Gas Exploration in the Complex Tectonic Areas of the Margin of the Sichuan Basin, South China

“…SEM images showing the types of OM pores. Figure A was adapted with permission from ref . Copyright 2023 American Chemical Society.…”

“…Low-pressure gas adsorption and mercury intrusion porosimetry are commonly used to measure the pore size distribution for multipore structure materials. ,,,, Numerous studies have shown that the pore system in shale is dominated by nanopores (pore size <1000 nm) and a small number of fracture pores. − On the basis of the classification of the International Union of Pure and Applied Chemistry (IUPAC), pores are split into micropores (pore size <2 nm), mesopores (pore size between 2 and 50 nm), and macropores (pore size >50 nm). A plot of incremental pore volume versus pore diameter for low-pressure gas adsorption and mercury intrusion porosimetry is commonly used to illustrate the pore size distribution …”

“…Scholars have already conducted studies on pore characterization in different shales, in which it has been found that shale pore networks are dominated by organoporosity. − ,,, Consequently, we use intraparticle organic pores to illustrate why weakly deformed shales are mesopore-rich with low permeability and strongly deformed shales are macropore- and fracture pore-rich with high permeability, as shown in Figure . We assume that the micropores, mesopores, and macropores of the primary structural shale are all developed in a certain proportion and scale.…”

“…7−12 These pore networks have far-reaching implications for the gas storage and migration pathways and significantly control the amount of free gas and adsorbed gas in place. 5,13,14 Scholars have already conducted studies on the shale microstructures and pore structures in different strata and regions by various techniques, suggesting that their formation and evolution are affected by various geological conditions including the geochemistry, thermal maturity, lithology, and diagenesis. 1,5,7,8 In addition, typical deformation microstructures of naturally deformed shales have been observed and investigated in the literature.…”

“…China has experimented with marine shale gas exploration and made some achievements in the past decade. − Two sets of marine organic-rich shales (the Upper Ordovician–Lower Silurian and the Lower Cambrian) were deposited in the early Paleozoic in the upper Yangtze area of the Sichuan Basin, South China, and these marine shales have been studied as the main source rocks for many years. , Gas shale is fine-grained, organic-rich, low-porosity, and permeable. − Abundant organic pores and inorganic pores as well as very small numbers of microfractures or fracture pores can be observed in shale matrix with the size, shape, and number varying. − These pore networks have far-reaching implications for the gas storage and migration pathways and significantly control the amount of free gas and adsorbed gas in place. ,, …”