Research In Sedimentary Facies And Reef Reservoirs Of Upper Permian Changxing Formation In Wubaiti Structure, East Sichuan Area

Reef reservoir is a kind of special lithologic reservoirs with great exploration prospective. For the complicated geologic feature, however, there are difficulties in research in reef reservoirs. During the late Permian interval, the sedimentary paleotopography of Sichuan Basin showed a high-lying southwest area and north-eastern area was low. There was a carbonate ramp developing on the base of planation surface when transgression occurred, and trough developed in the platform. The distribution of the reefs in eastern Sichuan was constrained by Kaijiang– Liangping Trough. Carbonate platform margin reefs developed around the Kaijiang–Liangping Trough. The reefs in Wubaiti Area were the typical carbonate platform margin reefs developing during this period. In the west part of Wubaiti Area was the Kaijiang– Liangping trough, and open platform facies developed to the east.Open platform, platform margin and trough developed in Wubaiti Region in eastern Sichuan Basin in late Permian. The main depositional facies types contained with carbonate platform margin reef facies, open platform facies, and trough. The evolvement of the facies was that, during the deposition of the first member of Changing Formation, Kaijiang– Liangping Trough formed under the regional tension stress environment of fault blocks elevated and subsided differently which was constrained by an ordinary fault of NW- SE; during the deposition of the second member of Changing Formation, continuous platform edge shallow facies developed in the east of Sichuan, and in some parts patch reef evolved; when the lower part of the third member deposited, discontinuous carbonate platform margin reefs developed abundantly of which the distribution was constrained by Kaijiang– Liangping Trough; and when the upper part of the third member deposited, sea level dropped and the reefs developed before exposed repeatedly, while strong denudation and synsedimentary dolomitization happened.The Permian deposits in the study area had gone through diageneses of multistage of dolomitization, multistage of cementaction, anti-dolomitization, compaction and dissolution. Diageneses could be divided into constructive diageneses and destructive diageneses. Constructive diageneses contained with dolomitization and dissolution, while the destructive diageneses contained with cementaction, compaction and anti-dolomitization and so on. The formation of the reservoirs was the result of dolomitization and dissolution. The main reservoir spaces were intercrystalline pores and corroded pores.The development of reef reservoirs of Changing Formation in Wubaiti region was controlled by precipitation facies and diageneses as well. The main reservoir rocks formed from limestones of reef-bank facies which was dolomitizationed and then dissolved. Because of anisotropism of dolomitization and dissolution, reservoirs show strong anisotropism that whole porosity low while pores very developed in some regions. Based on analysis on Sediment feature, sedimentary facies and diageneses of reefs, this paper discussed the reef reservoir of Wubaiti Changxing Formation detailed.