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 petro-logical 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 dis-cussion 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. The clay minerals in the study area consist mainly of detrital smectite, I/S and illite and authigenic talc and Mg-smectite that transformed from sepiolite, in which Mg originated from high-Mg2+ parent calcites and Si was due to seawater mixing with <-5% terrigenous and <-0.05% hydrothermal contributions. Compared with limestone beds, marl beds contain more quartz, detrital or authigenic clay minerals (with authigenic clays being present in higher proportions). Although the dominant detrital clay minerals consist of I/S and illite in both limestones and marls, smectite accounts for a higher proportion in marl layers. By referring to the geochemical analyses, it is certain that the higher smectite contents in the marls indicate a more humid climate. The coor-dinated variations in quartz or talc contents and paleoclimates match the periods of the wavelets, showing that short eccentricity (Milankovitch cycle) forced intensive weathering and increased rainfall and runoff during the deposition of marl beds, which formed a silica-rich environment and increased the introduction of high Mg2+ calcites, aragonite and detrital clay minerals (smectite) into marl layers. Due to the preferential dissolution of aragonite and stronger pressure dissolution during diagenesis, the greater amount of Mg2+ that was produced promoted the formation of Mg-phyllosilicates.