Six characteristics and main controlling factors of shale reservoirs in the Wufeng-Longmaxi formations, southeastern Sichuan Basin

Shale gas exploration in the Wufeng-Longmaxi formations in the southeastern Sichuan Basin showed different results. Some wells were high yielding, some were low yielding, and some were dry. Well core description, geochemical analysis, SEM, X-ray diffraction, field gas content tests, and well logging analysis were applied to compare samples from over 10 shale gas wells. Six properties were studied, including lithology, geochemical features, brittleness, physical properties, gas-bearing capacity, and electrical resistivity. Lithology, geochemical features, and brittleness show good correlation. For example, radiolarian carbonaceous graptolite shale has high TOC content and brittleness index. These three properties were controlled by sedimentary environment and facies, and can be compared among wells in the study area. Physical properties are positively correlated with gas-bearing capacity. They were affected by tectonic deformations, and vary obviously among wells. Physical properties are positively correlated with TOC content. Shale attitude and fault development are the main factors which affect gas content because the horizontal permeability of shale is several times larger than vertical permeability. Gas content also affected shale porosity and permeability. Radiolarian carbonaceous graptolite shales with few faults are favorable for the preservation of pressure and pores, and can form industrial reservoirs featured by high GR, high resistivity, relatively high AC, high U content, low KTH, low NGR, and low density.