Microfacies And Diagenesis Research Of Cambro-Ordovician Carbonate Rocks In Tarim Basin

Based on the carbonate samples taken from Tazhong area and T₇⁴ unconformity interface in Bachu area, Tarim basin, this dissertation focuses on the research of lithology, geochemistry, diagenetic machanism of dolomite and reservoir characteristics by the methods of microfacies and diagenesis.Reservoir rocks of limestone in Tazhong area mainly distribute in Lianglitage formation in the Upper Ordovican and Yinshan formation in the Middle-Lower Ordovician especially in the grainstones bands in Lianglitage formation. The lime reservoir in Upper Ordovician characterised with lower porosity and lower permeability but some bands in the vertical orientation have compacted sequences and good quality for the reservoir. The sequences in the Lower Ordovician have the same characters as the Upper Ordovician. Fracture, pore and fracture-pore are the three significiant types for the reservoir spaces. Dolimite occurred in the whole Cambrian and some Lower Ordovician sequences. The key diagenesis to the dolomitic reservoir consists of burial compaction and mixing-zone dolomitization models. Three factors are important to the quality of intercrystalline pores in dolomite: with the content from 50% to 80%, fine or medium crystals, idiomorphic or subidiomorphic dolomite. The development of deep burial dolomite with tremendous thickness is benefit to the reservoir qualitiesOn the basis of the microfacies analysis, the lithology of T₇⁴ unconfirmaty interface mainly consists of packstone and wackstone. Echinoids especially in crinoids and ostrocods are rich in the bioclastic limestone. Detrital material of terrigenous origin includes quartz and clay minerals such as kaolinite, mica, chlorite, mixed-layer clay and vermiculite. According to the integrated facies analysis, marine diagenetic environment controlled the sedimentary of original sequences and then the sequences experienced deep burial, uplift and meteoric erosion. Diagenetic facies is in the shelf with low energy and relative warm temperature (paleotemperature range from 12.2℃to 16℃). The boundary sequences benefit to the fluid transports and control the development of karst.By the analysis of micrfacies and diagenesis, the grainstone bank and the lagoon in the restricted platform for limestone and deep burial dolomite have the ability to be the good reservoir. Facies, dolomitization and deep burial dissolution are the key factors to the development of reservoir. Therefore, the exploration aim should pay more attention to the reef flat and deep burial dolomite in Tazhong area.