Mineral composition and brittleness characteristics of the inter-salt shale oil reservoirs in the Qianjiang Formation, Qianjiang Sag

Core samples of shales from the second (Eq₂), third (Eq₃) and fourth (Eq₄) members of the Qianjiang Formation in the Qianjiang Sag were analyzed using X-ray diffraction (XRD) analyses to determin their mineral composition, brittleness and geochemical characteristics. The results showed that the shales from different formations have different mineral compositions and geochemical characteristics. The shales in Eq₂ have sapropelic organic matter. The TOC content averages 1.38%. The R_o value ranges from 0.26%-0.5%, indicating an immature to low-maturity thermal evolution stage. Evaporite rocks are in high content. The shales in Eq₃ have sapropelic and humic-sapropelic organic matter. The TOC content averages 2.53%. The R_o value ranges 0.5%-0.88%, indicating a low to medium-maturity thermal evolution stage. Quartz and clay minerals are in high content. The shales in Eq₄ have sapropelic and humic-sapropelic organic matter. The TOC content averages 0.69%. The R_o value ranges 0.55%-1.3%, indicating a medium-maturity to mature thermal evolution stage. Carbonate minerals are in high content. The inter-salt shales in the Qianjiang Formation are characterized by "low clay content, low quartz content, high carbonate content, and low brittle mineral content" compared to those in the USA and other regions in China. The shales in Eq₃ have higher brittle mineral content (average 56%) and higher brittleness index (average 75.3%), which show excellent exploration potential.