Mineral composition andmicroscopic reservoir features of Longmaxi shales in southeastern Sichuan Basin

Mineral compositions and microscopic reservoir features of the Longmaxi shales in the southeastern Sichuan Basin were comprehensively studied using the analyses of petrological characteristics, mineral compositions, microscopic pore structures and rock mechanics.The results show that quartz and clay minerals are abundant in the Longmaxi shales with average contents of 36.07% and 41.55% respectively, while carbonate minerals are poor with an average content of 9.92%.Quartz is especially rich at the bottom of the Longmaxi shales. As burial depth decreases, the content of quartz is less while clay mineral content increases. Abundant bioclasts indicate that siliceous matter is mainly biogenetic. Many inorganic and organic pores are found in the Longmaxi shales. Granular margin micro-fractures develop well in inorganic pores,while intergranular and intragranular pores are less common. There is a positive correlation between quartz content and pore volume, which indicates that biogenic silica pores play an important role in the development of porosity.Biogenic silica contributes to a high content of brittle minerals, which is favorable for fracturing treatment.