Autochthonous Dolomitization and Dissolution in the Microbial Carbonate Rocks of the Fengjiawan Formation in the Ordos Basin

Many euhedral dolomite crystals and related pores are found in the microbial siliceous stromatolite dolomite and siliceous oolitic dolomite in the Fengjiawan Formation of the Mesoproterozoic Jixian System in the southern Ordos Basin. With the application of the microscope, scanning electron microscope, cathodoluminescence and in-situ trace element imaging, it can be seen that different from the phase I dolomite that was damaged by silicification, the intact euhedral phase II dolomite occurred through dolomitization after silicification, concentrated mainly in the organic-rich dark laminae of the stromatolite and the dark spheres and cores of the ooids. A considerable number of phase II dolomite crystals were dissolved, giving rise to mold pores and vugs which constituted the matrix pores and also the major pore space of the Fengjiawan Formation. The formation and dissolution of the dolomite were controlled by the microenvironment favorable, respectively, for carbonate precipitation and dissolution under the influence of microbial biological activities and related biochemical reactions. The driving force and material supply of dolomitization and dissolution were confined to the fabrics enriched with microorganisms, which are highly autochthonous. This mechanism may be a key factor for the development of Precambrian dolomite and related reservoirs, in the context of the domination of microbial rocks.