Mineralogical characterization and diagenetic history of Permian marine tuffaceous deposits in Guangyuan area, northern Sichuan basin, China

Abstract

Based on thin section identification, scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis of samples from cuttings and cores in drillings, tuffaceous deposits were identified to intercalate in marine carbonate rocks and mudstones in the upper part of Maokou Formation and the middle to lower part of the Wujiaping Formation in Guangyuan area, northern Sichuan basin, China. These tuffaceous deposits are of significance to understand the influence range of magmatic activity of the Emeishan Large Igneous Provinces (ELIP), and explore and exploit oil and gas reservoirs in Permian strata in the Sichuan basin. The research results indicated that tuffs formed 0.1–10 mm-thick laminae, and they formed lamellar interbedding with organic mudstones and carbonaceous mudstones, which were sandwiched among bioclast limestones and marls. Tuffaceous deposits in the study area include tuff, sedimentary tuff and tuffaceous sedimentary rock. Under the influence of sulfate reduction, acidic hydrotherm and alkaline fluid, tuffaceous deposits were altered by diagenesis such as dolomitization, argillization, pyritization and dissolution, and formed a variety of secondary rocks and pores which characterized by the micron-scale solution pores in tuff components and the nanometer-scale intercrystal pores among authigenic clay minerals. In addition, the primary directional sedimentary structure of tuffs and sedimentary tuffs and developed laminae in the rocks are conducive to the formation of fissures parallel to laminae, and effectively improve the reservoir property of rocks. The tuffaceous deposits in the study area are mainly the products of subaqueous pyroclastic flow that were dominated by acidic magmatism and indicated that they may not be directly related to the ELIP, which is located in the southwest Sichuan-Yunnan provinces and dominated by continental eruption of basalt.