Petroleum Exploration and Development (Feb 2023)
Origin of dolomites in the Permian dolomitic reservoirs of Fengcheng Formation in Mahu Sag, Junggar Basin, NW China
Abstract
Origin of authigenic dolomites in the dolomitic reservoir of the Permian Fengcheng Formation in the Mahu Sag of Junggar Basin is unclear. Occurrence and genetic evolution of the authigenic dolomites in dolomitic rock reservoir of the Fengcheng Formation in the Mahu Sag were analyzed by polarized and fluorescence thin sections, scanning electron microscope (SEM), electron microprobe (EMP), C, O and Sr isotopes analysis, and other techniques. (1) Dolomites were mainly precipitated in three stages: penecontemporaneous–shallow burial stage (early stage of the Middle Permian), middle burial stage (middle stage of the Middle Permian), and middle–deep burial stage, with the former two stages in dominance. (2) Dolomitization fluid was high-salinity brine originating from alkaline lake. 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) 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 to different influences of devitrification and diagenetic alteration of volcanic materials during the burial. (4) In the penecontemporaneous stage, induced by alkaline-lake microorganisms, 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 dolomites were formed by continuous growth of micritic–microcrystalline dolomite and replacement of calcites, tuffs and other substances; in the middle burial stage, the dolomites, mainly silt- and fine-sized, were formed by replacement of volcanic materials. 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.
