Diagenesis and reservoir quality of carbonates rocks and mixed siliciclastic as response of the Late Carboniferous glacio-eustatic fluctuation: A case study of Xiaohaizi Formation in western Tarim Basin

Abstract

Late Carboniferous-Permian was a longest-lived and most widespread ice age. The depositional environment, diagenesis and reservoir quality of carbonate rocks as response to the glacio-eustatic changes was analyzed in this study. The Upper Carboniferous Xiaohaizi Formation in the western Tarim Basin is an important oil reservoir. The dolostones from the Xiaohaizi Formation have very good physical properties, with maximum porosity and permeability of 16.6% and 214mD, respectively. The sedimentary facies of the Xiaohaizi Formation consist of carbonate shoals and lagoon on a mixed siliciclastic-carbonate platform. There was a strong heterogeneity of carbonate rocks deposited on shoals. Porous dolostones always alternate with tight sparry allochemical limestones. Based on the petrographic observation and stable isotope measurement, the controlling factors for the heterogeneity and reservoir quality were revealed. The palaeosol, vadose silt, karst breccia, detrital kaolinite, darkening of dolostones and enrichment of 18O indicate the short-lasting subaerial emergence during the glacial period. During the glacial sea level fall, the mixed siliciclastic and carbonate allochems deposited on bioclastic and intraclastic shoals, while dolomitization of calcic grains occurred under a relatively low-temperature. Through the correlation analysis of physical property, lithology, rock components, it's suggested that the reservoir quality of the Xiaohaizi Formation was controlled by mixed sedimentation and dolomitization associated with glacio-eustatic fluctuation. At the burial diagenesis, the effect of hydrothermal fluid on physical properties of dolostones with relative high primary porosity was positive, whereas resulted in the poikilotopic calcite cementation. The assembly of fluorite, dickite and authigenic pyrite were indicators for hydrothermal fluid accumulation. The control of glacio-eustatic fluctuations on sedimentation and diagenesis in carbonates could be used to forecast the reservoir quality.