Origin of dolomites in the Permian dolomitic reservoirs of Fengcheng Formation in Mahu Sag, Junggar Basin, NW China

The origin of authigenic dolomites in the dolomitic reservoir of the Permian Fengcheng Formation in the Mahu Sag, the Junggar Basin, is unclear. The occurrence and genetic evolution of such authigenic dolomites in the dolomitic rock reservoir of the Fengcheng Formation in Mahu Sag were analyzed by polarized and fluorescence thin sections, scanning electron microscope (SEM) analysis, electron microprobe (EMP) analysis, C, O and Sr isotopes analysis, and other techniques. (1) Dolomites were mainly precipitated in three stages: penecontemporaneous–shallow burial stage (early stage of Middle Permian), middle burial stage (middle stage of Middle Permian), and middle–deep burial stage, with the former two stages in dominance. (2) The dolomitization fluid was high-salinity brine in the alkaline-lake sedimentary setting. In the penecontemporaneous–shallow burial stage, Mg2+ was mainly supplied by alkaline-lake fluid and devitrification of volcanic glass. In the middle burial stage, Mg2+ mainly came from the transformation of clay minerals, devitrification of volcanic glass and dissolution of aluminosilicates such as feldspar. (3) The regular changes of Mg, Mn, Fe, Sr, Si and other elements during the growth of dolomite were mainly related to the alkaline-lake fluid, and the different influences of devitrification and diagenetic alteration of volcanic materials in the Fengcheng Formation during the burial. (4) Induced by alkaline-lake microorganisms, in the penecontemporaneous stage, the micritic–microcrystalline dolomites were formed by primary precipitation, replacement of aragonite and high-Mg calcite, and other processes; in the shallow burial stage, the silt-sized dolomite was formed by the continuous growth of micritic–microcrystalline dolomite and replacement of calcite, tuffs and other substances; in the middle burial stage, the dolomite, mainly silt- and fine-sized, was formed by the replacement of volcanic materials by dolomitization fluid. The research results are referential for investigating the formation mechanism and distribution patterns of tight dolomitic reservoirs in the Mahu Sag and other similar oil and gas bearing areas. © 2023 Science Press. All rights reserved.