Reservoir Characteristics and Resource Potential of Marine Shale in South China: A Review

Abstract

Many sets of Paleozoic marine organic-rich shale strata have developed in South China. However, the exploration and development results of these shale formations are quite different. Based on the data of core experiment analysis, drilling, fracturing test of typical wells, the reservoir differences and controlling factors of four sets of typical marine organic-rich shale in southern China are investigated. The four sets of shale have obvious differences in reservoir characteristics. Ordovician–Silurian shale mainly develops siliceous shale, mixed shale and argillaceous shale, with large pore diameter, high porosity, moderate thermal maturity, large pore volume and specific surface area. Cambrian shale mainly develops siliceous shale and mixed shale, with small pore diameter, low porosity, high thermal maturity and smaller pore volume and specific surface area than Ordovician–Silurian shale. Devonian–Carboniferous shale has similar mineral composition to Ordovician–Silurian shale, with small pore diameter, low porosity, moderate thermal maturity and similar pore volume and specific surface area to that of Cambrian shale. Permian shale has very complex mineral composition, with large pore diameter, low to medium thermal maturity and small specific surface area. Mineral composition, thermal maturity and tectonic preservation conditions are the main factors controlling shale reservoir development. Siliceous minerals in Cambrian shale and Ordovician–Silurian shale are mainly of biological origin, which make the support capacity better than Devonian–Carboniferous shale and Permian shale (siliceous minerals are mainly of terrigenous origin and biological origin). Thermal maturity of Ordovician–Silurian shale and Devonian–Carboniferous shale is moderate, with a large number of organic pores developed. Thermal maturity of Cambrian shale and Permian shale is respectively too high and too low, the development of organic pores is significantly weaker than the two sets of shale above. There are obvious differences in tectonic preservation conditions inside and outside the Sichuan Basin. Shale reservoirs inside the Sichuan Basin are characterized by overpressure due to stable tectonic activities, while shale reservoirs outside the Sichuan Basin are generally normal–pressure. Four sets of marine shale in South China all have certain resource potentials, but the exploration and development of shale gas is still constrained by complicated geological conditions, single economic shale formation, high exploration and development costs and other aspects. It is necessary for further research on shale gas accumulation theory, exploration and development technology and related policies to promote the development of China’s shale gas industry.

Quartz Crystallinity Characteristics and Their Effects on Shale Gas Reservoir Performance: A Case Study of the Deep Longmaxi Formation Shale in the Sichuan Basin, China

“…These two types of quartz are typically surrounded by clay minerals, which are mechanically unstable, leading to lower brittleness in shale samples with high QCI values. Previous studies found that biogenic quartz has a smaller Poisson’s ratio and a lower compressive strength than other types of quartz, demonstrating better fracability. − It is worth noting that compared to middle-shallow shale gas reservoirs, deep-burial shale reservoirs experience high temperatures (>100 °C) and high pressures (>90 MPa), resulting in significant changes in the mechanical properties of shale reservoirs. Under these geological conditions, the plasticity of minerals within rocks increases, and even carbonate minerals (e.g., calcite) can undergo plastic transformation. , As the depth of burial increases, the mechanical response of deep shale becomes weaker than that of middle-shallow shale, primarily due to the influence of higher stress levels.…”

New insights into the evaluation criteria for high-quality deep marine shale gas reservoirs in the Longmaxi formation: Evidence from organic matter pore development characteristics

“…Furthermore, lithofacies division determines the high-quality reservoir of the Longmaxi Formation. The dominant lithofacies of the Longmaxi Formation is siliceous shale, which has the highest reservoir quality and the most developed OM pore network (Jia et al, 2021;Wang et al, 2022;Zhang et al, 2022).…”