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

Abstract

The Lower Cambrian shales are widely developed in southern China, with greater thicknesses and higher TOC contents. Although the shale gas resource potential has been suggested to be huge, the shale gas exploration and development is not satisfactory. At present, the gas-bearing property evaluation of the Lower Cambrian shale is still a hot spot of concern. According to previous works, this paper systematically summarizes the gas-bearing characteristics and controlling factors of the Lower Cambrian shales in southern China. The buried depth of Lower Cambrian shales mainly ranges from 3000 m to 6000 m, and the thickness of organic-rich shale intervals ( TOC > 2 % ) varies from 20 m to 300 m. The TOC content and EqVRo value are generally up to 2%-10% and 2.5%-6.0%, respectively. The gas content of the Lower Cambrian shales in the Weiyuan-Qianwei block of the Sichuan Basin and the western Hubei area generally exceeds 2 m3/t, and gas composition is dominated by CH4. In southeastern Chongqing, northwestern Hunan, and northern Guizhou areas, the gas content of the Lower Cambrian shales is generally <2 m3/t, and the N2 content is generally >60%. In the Lower Yangtze region, the Lower Cambrian shale reservoirs basically contain no gas. Higher maturity, lower porosity, and less-no organic pores are suggested to be responsible for low gas contents and/or the predominate of N2 in shale gas reservoirs. Strong tectonic deformation is an important factor leading to the massive gas loss from shale reservoirs, thus resulting in no gas or only a small amount of N2 in the Lower Cambrian shales. In a word, the Lower Cambrian shale gas plays with low maturity and relatively stable tectonic condition, especially deep-ultradeep zones, may be the favorable targets for shale gas exploration.

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

“…According to Li et al, the relationship between TOC and quartz contents of whole-rock samples was not obvious, suggesting that the biogenic quartz content of the Lower Cambrian shale samples was small. Thus, quartz content has little influence on the roughness of shale pore surfaces but exerts a positive influence on the heterogeneity of the shale pore structure, which is consistent with the nanopore characteristics of the Upper Ordovician–Lower Silurian marine shale reported by Li et al In general, quartz has high hardness and strong compaction resistance, which is conducive to preventing pore collapse during burial compaction, , and some quartz grains are susceptible to sliding or rotating due to structural deformation. As a result, intergranular pores or microcracks are formed in the regions of weak connection between grains, thus increasing the heterogeneity of pore structures.…”

“…The studied 12 shale samples were collected from the middle and bottom of the Lower Cambrian Shuijingtuo Formation in Well YD4, which was deposited in deep-water shelf facies. ,,− Geological background of studied well, as well as TOC content, mineral composition, and equivalent vitrinite reflectance (EqRo) of studied whole-rock samples were described by Li et al The TOC content of studied whole-rock samples was 0.91–10.70% (average = 5.38%), the EqRo value was 3.14–3.26% (average = 3.23%), and the mineral compositions mainly included quartz, clay, and carbonates, with their contents in the ranges of 17.50–57.80% (average = 31.95%), 18.10–54.70% (average = 33.71%), and 11.70–39.10% (average = 20.86%), respectively …”

“…Conventional pore structure parameters of shales mainly include pore size, pore size distribution (PSD), pore types, pore-specific surface area (SSA), and pore volume (PV). − A variety of techniques and methods has been applied to quantitatively characterize the shale pore structure, such as low-pressure CO 2 and N 2 adsorptions (LPCA and LPNA), high-pressure mercury injection (MICP), small and ultrasmall angle neutron scattering (SANS/USANS), and nuclear magnetic resonance (NMR). ,− In these methods, LPCA and LPNA experiments are usually applied to obtain PSD, SSA, and PV, which are effective in characterizing the shale pore structure. ,,− …”

Preparation and performance evaluation of intelligent plugging agent for thermo‐sensitive water‐based drilling fluid

“…4 As the basic condition for shale gas accumulation, a certain porosity and nanopore network have extremely high requirements for maintaining the wellbore stability of the drilling fluid in the process of production. 5 The stability of drilling fluid to wellbore is mainly through the following mechanisms:…”

Research on the differential tectonic-thermal evolution of Longmaxi shale in the southern Sichuan Basin

“…Since the successful production of Well Wei 201 in 2010 (Shi et al, 2022a), shale gas exploration in China has been pursued for more than ten years and seen several technical breakthroughs. Shale gas production grew rapidly from 0.25×10 8 m 3 in 2012 (Qiu et al, 2020) to 220×10 8 m 3 in 2021 (Li et al, 2022a). Undoubtedly, shale gas accounts for a significant proportion of natural gas reserves and is a major contributor to the increased output in China (Xu et al, 2019).…”