Genesis of clay minerals and its insight for the formation of limestone marl alterations in Middle Permian of the Sichuan Basin

Limestone marl alternations (LMAs) usually contain abundant amounts of clay minerals. A set of Permian LMA source rock gas reservoirs controlled by clay transformations has been found in South China. Apart from the common detrital clay minerals that are rich in LMAs of other regions, Mg-phyllosilicates widely occur in South China. The dilution cycle of detrital clays, sedimentary and diagenetic conditions of authigenic clays, and paleoclimatologic indicators all provide references for understanding the diagenesis and periodic deposition of LMAs. To further understand the formation of the Middle Permian LMAs in South China, in addition to petrological and mineralogical methods, which included field investigations, core and thin section observations, scanning electron microscopy (SEM) and X-ray diffraction (XRD), elemental (major element, trace element and rare earth element) analyses were used to determine the genesis and proportions of clay minerals. Next, a discussion is provided regarding the formation of LMAs in view of the above findings. Based on wavelet matching the GR logs, the original sedimentary and diagenetic differences and rhythmic alterations of LMAs are clarified.