Pore Characteristics and Gas Preservation of the Lower Cambrian Shale in a Strongly Deformed Zone, Northern Chongqing, China

Abstract

The Lower Paleozoic marine shale in southern China has undergone several strong tectonic transformations in an extensive region outside the Sichuan Basin. Although some shale strata underwent strong deformation, they still contain a significant amount of shale gas. The gas preservation mechanism in the strongly deformed shale has become the focus of attention. In this paper, the Lower Cambrian gas-bearing shale samples with a strong deformation taken from an exploration well in northern Chongqing, China, were investigated on their pore types and structure, with the aim to reveal the reason for the gas preservation. The pore types of the Lower Cambrian shale are dominated by microfractures and interparticle (interP) pores occurring mainly between clay minerals and between organic matter (OM) and clay minerals, while pores within OM that can be observed by FE-SEM (field emission-scanning electron microscopy) are rare. The shale has a low porosity, with an average of 1.56%, which is mainly controlled by the clay mineral content. The adsorption experiments of low pressure N2 (LPNA) and CO2 (LPCA) indicate that the shale is rich in micropores and small mesopores (<2–3 nm) provided mainly by OM, but mesopores with a size range of 3–50 nm are underdeveloped. The shale, as revealed by LPNA data, has dominant slit-like or plate-like pores and an obvious low-pressure hysteresis (LPH), indicating a low gas diffusion. The deformed shale samples with a removal of OM by oxidation and their isolated kerogen further indicate that the LPH is completely related to OM, without any relationship with minerals, while an undeformed shale sample, taken from another well for a comparison, has no obvious LPH for both of its OM-removed sample and kerogen. Based on a comprehensive analysis of the relative data, it is suggested that the nanopores related to OM and clay minerals in the shale were significantly altered owing to the deformation, with a result of the pores being squeezed into the slit-like shape and converted into micropores. This extraordinary pore structure of the shale formed during the deformation process should be the main preservation mechanism of shale gas.

Fractal Characteristics of Nanopores and Their Controlling Factors of the Lower Cambrian Shale in the Western Hubei Area, South China

“…The D 1 and D 2 values of isolated kerogen samples are in the ranges of 2.410–2.498 (average = 2.456) and 2.488–2.767 (average = 2.576), respectively. The fractal dimensions of whole-rock samples are higher than those of the corresponding isolated kerogen samples, indicating that the IM increases the nanopore heterogeneity of shale reservoirs, which is consistent with nanopore characteristics of the Upper Carboniferous–Lower Permian MCT shale in the Qinshui Basin and the Lower Cambrian marine shale in the northeastern Chongqing area . Whole-rock and isolated kerogen samples have greater D 2 values relative to D 1 values, suggesting that these samples have a stronger heterogeneity of the pore structure relative to the heterogeneity of the pore surface .…”

“…44,70,86 The D 1 and D 2 values of the Lower Cambrian marine shales in the Hunan, Chongqing, Guizhou, and Yunnan areas were in the ranges of 2.24−2.82 (average = 2.63) and 2.49−2.96 (average = 2.83), respectively. 41,42,51,52,56 In contrast, the average D 1 value of the studied Lower Cambrian marine shale samples is 2.489, which is significantly lower than that of the above-mentioned shales, and the average D 2 value is as high as 2.896, indicating that the adsorption capacity of studied shale samples is weak and the permeability and connectivity of pores are poor. During the development of shale gas, the fracturing and reconstruction technologies should be used to form a pore-fracture network of the Lower Cambrian shale reservoirs, which could reduce the D 2 values, increase permeability, and accelerate the desorption and diffusion rate of shale gas, finally increasing the yields of shale gas.…”

“…The fractal dimensions of whole-rock samples are higher than those of the corresponding isolated kerogen samples, indicating that the IM increases the nanopore heterogeneity of shale reservoirs, which is consistent with nanopore characteristics of the Upper Carboniferous−Lower Permian MCT shale in the Qinshui Basin 30 and the Lower Cambrian marine shale in the northeastern Chongqing area. 56 Whole-rock and isolated kerogen samples have greater D 2 values relative to D 1 values, suggesting that these samples have a stronger heterogeneity of the pore structure relative to the heterogeneity of the pore surface. 70 The D 1 values of whole-rock and isolated kerogen samples are positively correlated with the D 2 values (Figure 8), revealing that the more irregular the pore surface, the more complex the pore structure, which should be related to the properties of shale and its OM.…”

“…The Poro PDP-200 laminating pressure pore permeator was used to determine the porosity and permeability of a shale sample. The working conditions of the instrument and calculating method were suggested by Meng et al 56 and Zhang et al 57 2.4. LPCA and LPNA Experiments.…”

Lower Cambrian Organic-Rich Shales in Southern China: A Review of Gas-Bearing Property, Pore Structure, and Their Controlling Factors

“…Han et al [191] found that well-developed micropores, high pore-specific surface area, and strong gas adsorption capacity might be responsible for the enrichment of the Lower Cambrian shale gas in northeastern Chongqing. Meng et al [192] thought that the Lower Cambrian shales are mainly characterized by micropores and small mesopores, with undeveloped mesopores, and their pores, such as OM-hosted pores and clay-hosted pores may be flattened by extrusion and/or compaction to have silt-like or layered shapes. This unique pore structure is obviously not conducive to gas loss and would play an important role in the preservation of shale gas.…”