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

Abstract

Until recently, the characteristics of nano-microscale structures in the naturally deformed, overmature, marine shales were poorly known. Thermally overmature Lujiaping shales in the complex tectonic area of the northeast part of the upper Yangtze area, China have experienced strong tectonic deformation and are considered as potentially important strata for shale gas exploration. Naturally deformed samples from the main source rocks are selected from the Lower Cambrian Lujiaping Formation in the Dabashan Thrust-fold Belt to investigate nanometer- to micrometer-sized structures. A combination of scanning electron microscope (SEM), low-pressure nitrogen adsorption (LPNA), and low-field nuclear magnetic resonance (NMR) suggests that the pore types are dominantly fracture-related pores with a lesser abundance of mineral-hosted pores. These two pore types account for the 90% of total pore space. Organic matter (OM)-hosted pores are rare and make up a small part of the pore systems (less than 10%) due to high thermal maturity and intensive tectonic compression. Overall, the Lujiaping deformed, overmature samples have abundant nanometer- to micrometer-sized inorganic pores. High-resolution SEM images provide direct evidence of the formation of nano- and microsized structures such as OM–clay aggregates and silica nanograins. OM–clay aggregates are commonly observed in samples, which also exhibit abundant open microfractures and interparticle pores. Quartz can occur as silica nanograins and botryoids typically 20–100 nm in size, which may influence porosity through the creation or occupying interparticle pore space.

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

“…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 ].…”

Variation of Organic Pore Structure With Maceral Types in the Longmaxi Shale, Sichuan Basin

“…Microscopic OM pores have already been substantially studied (e.g., Loucks et al, 2009;Fishman et al, 2012;Mastalerz et al, 2013;Chen et al, 2015;Löhr et al, 2015;Ko et al, 2018). Many researchers point out that the OM contents, types, and distributions strongly affect the shale reservoir quality (Wang et al, 2016;Zhu et al, 2019).…”

Types and Quantitative Characterization of Microfractures in the Continental Shale of the Da’anzhai Member of the Ziliujing Formation in Northeast Sichuan, China

“…Finally, adsorbed gas escapes from the shale matrix. The desorption process of adsorbed gas is very slow, but when microfractures develop near the bottom of a well, the free gas in the microfractures quickly discharges, forming a low-pressure zone near the microfractures, in turn promoting the desorption of adsorbed gas in the area near the fractures and improving the gas production efficiency [35,36]. Microfractures in the shale can be used as penetration channels.…”