Geologic and geochemical characteristics of the Cambrian marine shale in the Tarim Basin of China and their implication for the prediction of shale-gas resources

Although the Cambrian marine shale in the Tarim Basin is usually considered to be the source rocks for overlying conventional reservoirs, its unconventional shale-gas potential has not yet been evaluated. To indicate the unconventional resource play potential and highlight more favorable zones, we conducted a comprehensive lithologic, petrophysical, and geochemical analysis of 228 drill core samples from 14 wells and 167 outcrop samples from eight geologic sections in the basin. The results indicate that the Cambrian marine shale was deposited in basinal and slope environments dominated by shale and siliceous shale. The average total organic carbon (TOC) content of the epsilon(1)xd (Xidashan) and epsilon(2)m (Moheershan) Formations ranges from 0.5% to 2.5%, and that of the epsilon(1)y (Yuertusi) Formation ranges from 1.0% to 7.0%. Phytoplankton is dominant in the epsilon(1)xd and epsilon(2)m Formations, whereas benthic algae are more prevalent in the epsilon(1)y Formation. The organic matter ranges in maturity from high to postmature. The shale consists of abundant meso- and macropores. The mineral matter is mostly composed of quartz (>50% in most samples) followed by carbonate minerals, with minor contributions made by clay minerals (<20%). The analysis of methane adsorption isotherm indicates a positive correlation between the gas and TOC content and a negative correlation between the gas content and burial depth implying the good adsorption capacity of the shale-gas play. Resource estimation of the Middle-Lower Cambrian indicates that the Yuli and Ruoqiang areas contain a potential 17,813.47 x 10(8) m(3) of shale gas, and they are the most favorable zones for further exploitation.