Research On Genetic Mechanism And Key Controlling Factors Of Ordovician Carbonate Reservoir In Tazhong Region

Various types of carbonate reservoir developed in Tazhong region,but many previous studies indicate that the reservoir properties are of the characteristics of strong heterogeneities,multiple influencing factors and petroleum exploration is characterized by large difficulty.Aiming at the shortage,this paper deals with the genetic mechanism of Ordovician carbonate reservoir in Tazhong region and its key controlling factors by integrating drilling and logging date analysis,cores observation,thin section identification,and so on.In addition,all these study are under the theoretical guidance of sedimentary petrology,reservoir geology,etc.Based on absorbing the studied results of former geologists, cores and thin section are observed detailedly by fully applying coring data in interesting area.As a result, this paper has generalized petrology characteristic and diagenesis of Ordovician carbonate rock in Tazhong region.And corresponding diagenesis sequence is also established,which could be used as a criterion for discussing the origin of reservoir.With the analysis of lithofacies,diagenesis,reservoir space and physical property of Lianglitage formation in interesting area,it is considered that the reservoir of Lianglitage formation is chiefly related to syndepositional karstification and epigenetic karstification. Diagenesis analysis approves that the syndepositional karstification is controlled obviously by sedimentary facies.And epigenetic karstification is closely related to unconformity,especially influenced by the second episode karstification of Middle Caledonian.According to all the study above,it show that slope break belts in the southern and northern of Tazhong region are the favorable position for the reservoir in Lianglitage formation.In conclusion,under the combined action of prevailing face belt,syndepositional karstification and epigenic karstification,the reservior in these zones become more favorable than others.Moreover,the reservior in the northern slope break belt(i.e. the zone close to the Tazhong-â… Fault) is better between these two slope break belts.The syndepositional karstification was not development in reservoir of Yingshan formation, it was mainly controlled by epigenetic karstification and burial karstification. Much unconformity developed on the top of Yingshan formation interprets it was influenced by epigenetic karst in the long history, so the karstification in Yingshan formation influenced a large area and has a significantly longitudinal zonation. Burial dissolution has important influence for the dolomite reservoir of lower Ordovician, especially in the period of Permian magmatism, hydrothermal fluid transformate dolomite and formate more solution pore, this result is better for discover the inner buried dolomite reservoir.There are several factors controlling the carbonate reservoir of Tazhong area, such as sedimentary facies, karstification and dolomitization. Sedimentary facies are the material basis for reservoir development. Differences in reservoir characteristics is caused by the different sedimentary facies, sediment composition and ancient geographical location. Variety of dissolution including syndepositional,epigenetic and buried karstification is key to the formation of carbonate reservoirs in the study area. Syndepositional karst with restricted distribution, mainly developed in the Lianglitage formation. Multi-period and large range epigenetic karstifications are distributed in different strata; Buried karstification which relate with hydrothermal fluids is mainly affected by the fault system. This karstification plays an important role on the reservoir reconstruction, however, its distribution is not controlled by strata. Dolomitization is vital to the formation and evolution of the dolomite reservoir in the Lower Ordovician. The structure type of dolomite and the degree of dolomitization control dolomite reservoir development jointly. The dolomite with best reservoir performance is distributed on the planar-e(s), finely crystalline dolomite formed in the shallow burial period and hydrothermal dolomite formed in the deep burial period.