Reservoir characteristics and main controlling factors of the Leikoupo gas pools in the western Sichuan Basin

Abstract

Significant progress in natural gas exploration in the Middle Triassic Leikoupo Fm of the western Sichuan Basin have been achieved by Sinopec in recent years, and the western Sichuan gas field has become the key block for the increase of reserves and output of Sinopec in natural gas exploration and production in the 13th Five-Year Plan. For further understanding of the reservoir characteristics and the controlling factors of the Middle Triassic Leikoupo Fm reservoir in western Sichuan gas field, reservoir characteristics and the controlling factors of reservoir development in Shiyangchang–Jinma–Yazihe area were investigated based on detailed observation of rock slices and cores, utilizing porosity, permeability and intrusive mercury analysis in combination with well logging interpretation results. The research results show that: (1) the dolomite reservoirs of tidal flat facies were developed in the Leikoupo Fm of this area, which can be divided into an upper reservoir section, a lower reservoir section and an interlayer section; (2) the upper reservoir section is relatively simple in reservoir lithology, reservoir space type and pore–throat assemblage, and the main lithology is microcrystalline (crystal powder) dolomite with mainly pore-type reservoirs, and relatively high-quality micritic dolomite reservoirs with medium porosity and low permeability are locally developed; (3) the lithology, reservoir space type and pore structure of the lower reservoir section are complex and strongly anisotropic, and the reservoirs are mainly of fracture–pore type with (extremely) low porosity and permeability; (4) the reservoirs are vertically anisotropic, and different types of thin reservoirs are interbedded. The thickness of effective reservoirs is between 30 m and 56.6 m. The total thickness and overall physical properties of the lower reservoir section is superior to the upper section; (5) the distribution of dolomite is controlled by the flat and algae flat microfacies as well as dolomitization, which lays a lithological foundation for reservoir development; (6) multi-stage penecontemporaneous dissolution controlled by high-frequency intertidal cycles is the key factor for the development of high-quality reservoirs; (7) hydrocarbon charging during the burial stage inhibits the formation of scale cement, and makes the early-formed pores better preserved. Keywords: Sichuan basin, Western sichuan gas field, Middle triassic, Dolomite, Reservoir characteristics, Fracture–pore type reservoir, Pore-type reservoir, Controlling factors