Formation And Distribution Of Dovician Carbonate Reservoir In Tazhong Area, Tarim Basin

As controlled by palaeokarst and fracture, carbonate reservoirs are strongly heterogeneous and difficult to be predicted. Based on sequence stratigraphy, palaeokarst geology, thermodynamics of water-rock interaction, and reservoir geology etc., the paper takes the karstification as a masterstroke to systematically study the formation and distribution of carbonate reservoirs of Ordovician in Tazhong area, Tarim basin. It has been concluded that sedimentation is the basis, karstification is the condition, structure fracture is the key, and the burial preservation is the guarantee fore the formation of carbonate reservoirs with high quality.Based on the analysis of the development characteristics of karst system and the construction of sequence stratigraphic framework, the karstification within the sequence stratigraphic framework, especially under the different order of sequence boundaries has been studied in detail. . The karstification is controlled by the periodic emergence of the carbonate platform corresponding to high frequent and third-order of sea level fluctuations. Along with the fall of the base level, the scale of karstification segments developed at the top of progradational high frequent sequence set becomes larger. Therefore, unconformity corresponding to the first-order or second-order sequence boundary is the key which controls the karstified carbonate reservoirs in Tazhong area. It can be modeled that the onlap zone on the sequence boundary in the transgressive system tract and the toplap-truncation zone in the highstand system tract are the favorite area for karstification.Karstification regions and preservation trend under burial condition are tentatively studied with the thermodynamics of water-rock interaction. The preservation of palaeokarstification and the development of modern burial karstification are also evaluated by calculating Gibbs free energy AG value, which reflects calcite and dolomite dissolution-precipitation trend. It is the key to decide whether the fractures and caves are the efflective reservoir interspace and to provide proofs for the predication of the favorable karstification zones.Through the recognition of dissolution caves with FMI logging, statistics of fractures,correlation analysis of the fractures and karstification, It has been found that fractures are closely accreted with karstification and high angle fractures under unconformity surface promoted the development of weathering crust-type karst greatly, while lower angle fractures have perfect correspondence to burial karstification. This knowledge affords a new thought to evaluate and predict karst-fractures-type reservoirs by the characteristics of fractures in different occurrence!Based on the comprehensive analysis of characteristics of the reservoir, such as lithofacies, fractures, and karstification etc, the Ordovician carbonate reservoirs have been evaluated. The conclusion shows that in the lower Ordovician the areas where the karstification and fractures were well developed are near central fault zone and south of the line along Well Tazhong 48-Zhong 4-Tazhong 5-Tazhong 8-Tangbei 2, which are I -type reservoirs, most regions between Tazhong I Fault and central fault zone are II -type reservoirs;and in the upper Ordovician, the zones near Tazhong I Fault and south of Tazhong southern slope along Tangbei fault are reef- bank facies reservoirs, which belong to I -type reservoirs as a result of greatly improved capability by intensive lyngenesis karstification additionally. The onlap areas near central fault zone and two morphologies along buried hills in east of Tazhong, being favorable for development of karstification, and development of intensive fractures, belong to I - II -type reservoirs.