Geochemical Characteristics And Reservoir Evaluation Of Cambrian-Ordovician Carbonate Rock In Tazhong Area

Tazhong area which locates on the central uplift of Tarim Basin is one of areaswhich have higher degree of exploration and maximal prospect potential in TarimBasin. So far, well exploration results and the finding of commercial oil flowsacquired by SINOPEC and CNPC show that there has a good resource potential andexploration foreground.On the basis of core observation and laboratory studies, in the combination of thesummary and research of regional tectonic setting, rock types, sedimentary facies,geochemistry and reservoir evaluation, we find that there is a close relationshipamong of sedimentary facies environment, rock types and geochemistry, which playsan important part in the carbonate reservoir researching process.The research of Carbon and Oxygen stable isotopes shows that Limestone ofCambrian-Ordovician in Tazhong area is mainly plotted in the marine calcite zoneand the vicinity of the meteoric water calcite zone, which suggests that majordepositional setting is marine and they were strongly affected by meteoric waterduring the process of diagenesis;Dolomite in the region of Tazhong can be classifiedinto four types, which are near surface evaporation seawater genenic dolomite, nearsurface mixing water genenic dolomite, burial genenic dolomite and deep mixingwater genenic dolomite, and establish corresponding dolomite model. Comparing theanalytic outcomes from trace elements and sedimentary facies, we find that similartrend exists in several group trace elements, which are related to the depositionalfaices. Strontium isotope test analysis indicates that relationship exists between⁸⁷Sr/⁸⁶Sr ratios and the variation of seawater, evolution of palaeo-geographicalconditions in the region of Tazhong is the direct factor which controls the change ofthe 87Sr / 86Sr ratios and dolomitization is also an important factor. REE researchingfinds that carbonate rocks of Cambrian-Ordovician in Tazhong area are belong toLREE enrichment type, δEu and δCe appear obviously negative anomaly, whichreflect that the diagenetic environment is anoxic condition. Dolomite degree of ordertested by X-ray diffraction proves that there is commonly high degree of order indolomite (sometimes complete ordering) with marine deposit setting in Tazhong area,which indicates that the joining of the fresh water during the process of the dolomiteformation leads to the low crystallizing rate, which confirms the conclusion got fromCarbon and Oxygen isotope and Trace elements analysis.Through analyzing the carbonate reservoir of Cambrian-Ordovician in Tazhongarea, we divide the reservoir into three types, which are pore, cavity and seam. Pore ismain type of reservoir. Diagenesis, contemporary corrosion, weathered crustkarstification and burial corrosion are main factor that affect the development ofreservoir. Major factors that have impact on the dolomite reservoir quality are originalsedimentary facies, dolomitization degree, dolomitization type, development degreeof tectonic fissure and property and intensity of deep burial corrosion. Thedevelopment of high quality reservoir has close relationship with dolomite types, atthe same time it appears different geochemical characteristics;In contrast,geochemical characteristics can verify the dolomitization type in turn, which offer theevidences for dolomite diagenic. Finally, we come up with the prediction of highquality reservoir: Upper Cambrian series-Lower Ordovician series restricted platformdolomite rock sections are high quality reservoir.