Comparative study of nanoscale pore structure of Lower Palaeozoic marine shales in the Middle‐Upper Yangtze area, China: Implications for gas production potential

Abstract

The Lower Cambrian Niutitang and Lower Silurian Longmaxi shales in the Middle‐Upper Yangtze area are considered the primary shale gas units targeted for development in China. To shed some light on the difference in nanopore structures between Niutitang and Longmaxi shales, systematic comparative investigations were conducted using various techniques, including geochemical analyses, field emission scanning electron microscopy (FE‐SEM), high‐pressure mercury intrusion porosimetry (MIP), and low‐pressure N2/CO2 adsorption techniques. The results show that both Niutitang and Longmaxi shales have high total organic carbon (TOC) content and complex mineral compositions. The porosity of Longmaxi shales is higher than that of Niutitang shales, with an average value of 3.26% and 2.04%, respectively. Interestingly, for both shale formations, the mesopores (2–50 nm) are the major contributors to pore volumes, whereas the specific surface area is dominated by micropores (<2 nm). For the mesopore size distributions (PSDs) calculated from the N2 adsorption, the Longmaxi shales have a dominant pore size ranging from 10 to 60 nm. In contrast, there are more fine mesopores (2–8 nm) in the Niutitang shales. Furthermore, we found that numerous nanoscale pores are well‐developed within graptolite‐derived organic matter (OM) in the Longmaxi shales. These interconnected graptolite periderm pore systems may not only provide a storage space for both adsorbed and free gas but also serve as pathways for gas transport. The Niutitang shales developed relatively fewer OM pores with smaller diameters, lower OM surface porosity, and lower connectivity compared to the Longmaxi shales. The differences in OM pore structure partly explain why there is a large production difference between these two formations.

Supercritical Methane Adsorption on Overmature Shale: Effect of Pore Structure and Fractal Characteristics

“…The Lower Cambrian Niutitang Formation is widely distributed in the Upper Yangtze area in China. Totally, 10 shale samples were collected from fresh outcrops located in northern Guizhou and northwestern Hunan region (Figure and Table ) by using short-hole drillings with the depth of 5–20 m. The Niutitang shale was formed in the deep-water shelf-slope sedimentary facies belonging to a kind of stranded reduction environment, with thicknesses of 50–200 m in the study area . The lithology of the Niutitang Formation is black siliceous shale, black carbonaceous shale, and sandy shale from the bottom to top.…”

“…The present study focuses on the supercritical methane adsorption on overmature shales from the Lower Cambrian Niutitang Formation in China. The Niutitang Formation is one of the most important reservoirs for shale gas exploration and development in China. ,− High-pressure (up to 20 MPa) methane adsorption experiments were performed to investigate the characteristics of supercritical methane adsorption on shales, especially at high pressures. FE-SEM, LP-N 2 -GA, and LP-CO 2 -GA experiments were carried out to comprehensively characterize the pore structure, and the fractal dimensions were calculated according to the Frenkel–Halsey–Hill (FHH) model and low-pressure N 2 adsorption isotherms.…”

Evaluation of Nanoscale Accessible Pore Structures for Improved Prediction of Gas Production Potential in Chinese Marine Shales

“…In addition, whereas all six samples examined in this study can be described as black carbonaceous shales, abundant and diverse graptolite fauna are observed in Longmaxi shales but not in Niutitang shales (Figure a and b). Numerous nanoscale pores are well developed within the graptolite-derived organic matter, , which could comprise a complex pore system which may improve the pore accessibility of the Longmaxi shales (Figure c). Despite all of these observed and field-related differences, however, the results of this study (Table ) show no obvious difference in pore accessibility between Longmaxi and Niutitang shales.…”

“…1 There are, however, large differences in shale gas production between these two formations. 3,7,70 Significant commercial gas The φ values were estimated on the basis of SANS ϕ total and helium ϕ. production has been developed from the Longmaxi shales, but no commercial gas has yet been obtained from the Niutitang shales. The main difference between the Niutitang and Longmaxi shales is paleo-burial depth.…”

Controlling Factors of Organic Nanopore Development: A Case Study on Marine Shale in the Middle and Upper Yangtze Region, South China

“…Previous studies have shown that the degree of pore development of shale varies with maturity (Jop Klaver et al, 2015;Maxwell Pommer and Kitty Milliken, 2015). In view of the highly mature marine shale in southern China, some research clarifies the developmental characteristics of the shale pores of the Qiongzhusi Formation and the Wufeng-Longmaxi Formation (Shan et al, 2015;Ran et al, 2016Chen et al, 2017;Hou et al, 2017;Hu et al, 2017;Tong et al, 2017;Wang et al, 2017;Yang et al, 2017;Zhou et al;, and preliminarily clarified that tectonic movement and compressive stress have a destructive effect on nanopores in shale , Liang et al, 2017Cui et al, 2019;Zhu et al, 2018). This work screened the organic-rich shale samples from various locations and different strata (Wufeng Formation-Longmaxi Formation and Qiongzhusi Formation) in the middle and upper Yangtze regions, and systematically analyzed the pore characteristics of the shale to explain the differences in productivity.…”