Forming Mechanism of Ordovician Microbial Carbonate Reservoir in Northern Slope of Tazhong Uplift, Tarim Basin

The microbial carbonate reservoir is one of the hot research areas.The observation of microbialite's core and casting thin sections of the northern slope of middle Tarim basin in our study shows that fracture-pore type reservoir dominates in the microbialites of study area. Though the porosity and permeability analysis of full diameter core, we found that the microbialites were good gas reservoirs. On the analysis of the formation mechanism and characteristics of microbialites, combined with stress sensitive experiment and imaging logging, we summarized the advantage factors of the forming reservoir space of microbialites, firstly, the generation of native dolomite can promote the dissolution of calcite. The microbialites of the study area experienced eogenetic karstification, and parts superposition late hypogenic karstification, improving the reservoir physical properties. Secondly, rich organic matter content not only caused dissolution by organic acid generated by microbialites in the burial diagenetic stage, but also reduced the ultimate strength of microbialites which could easy to form cracks under the tectonic movement. Therefore the reservoir space of microbialites in the northern slope of middle Tarim basin is mainly secondary role. Though the determination of rock compressibility, it is concluded that microbialites' compaction resistance is common, but algae crumbs skeleton is advantageous to the protection of the algal hole. The strongest ability to resist compaction is silicified microbialites, followed by dolomitization.