Enrichment characteristics and exploration directions of deep shale gas of Ordovician–Silurian in the Sichuan Basin and its surrounding areas, China

Abstract

The enrichment characteristics of deep shale gas in the Ordovician Wufeng–Silurian Longmaxi formations in the Sichuan Basin and its surrounding areas are investigated through experiments under high temperature and high pressure, including petrophysical properties analyses, triaxial stress test and isothermal adsorption of methane experiment. (1) The deep shale reservoirs drop significantly in porosity and permeability compared with shallower shale reservoirs, and contain mainly free gas. (2) With higher deviatoric stress and axial strain, the deep shale reservoirs have higher difficulty fracturing. (3) Affected by structural location and morphology, fracture characteristics, geofluid activity stages and intensity, deep shale gas reservoirs have more complicated preservation conditions. (4) To achieve the commercial development of deep shale gas reservoirs, deepening geological understanding is the basis, and exploring reservoir simulation technology befitting the geological features is the key. (5) The siliceous shale and limestone-bearing siliceous shale in the Metabolograptus persculptus–Parakidograptus acuminatus zones (LM1–LM3 graptolite zones) are the high-production intervals for deep shale gas and the most favorable landing targets for horizontal drilling. Deeps water areas such as Jiaoshiba, Wulong, Luzhou and Changning with deep shale reservoirs over 10 m thickness are the most favorable areas for deep shale gas enrichment. It is recommended to carry out exploration and development practice in deep-water shale gas areas deposited deep with burial depth no more than 5000 m where the geological structure is simple and the shale thickness in the LM1–LM3 graptolite zone is greater than 10 m. It is better to increase the lateral length of horizontal wells, and apply techniques including high intensity of perforations, large volume of proppant, far-field and near-wellbore diversions to maximize the stimulated deep reservoir volume.