The Research Of Reefs Growth Pattern And Reservoir Characteristics Of Changxing Formation In Yuanba Area

Based on the geological tectonic background,sedimentary structure and evolution characteristics of the previous studies,using the carbonate sequence stratigraphy,sedimentology,carbonate reservoir geology theory as the guide,taking the Changxing formation in Yuanba area as the research object,making full use of drilling,logging,rock cores,testing and other aspects of information,carry out the research of reefs growth pattern and reservoir characteristics of Changxing formation in the Yuanba area.Through the anaYsis of thin section and environmental scanning spectrum,for the first time,the albitization was identified in the Changxing formation of Yuanba area.There is a good corresponding relationship between the location of albitization and high gamma,high value of uranium of the logging curve.It may indicate the extensional faults and hydrothermal activity.The found of albite not on Y enrich the recognition of the hydrothermal activity sign,but also has a further understanding about the tectonic background during the sedimentary period of the Changxing formation.Moreover,it provides a new thinking direction about the controlled factors of the reefs growth and development.The distribution of reefs in the well section was been confirmed with the typical wells' rock cores and debris thin section observation.Thus the reefs of the Changxing formation not onY developed in the upper part of third-order sequence,but also in the lower third-order sequence.The period of development was general synchronization,and the reefs can be divided into five stages,respective Y is the reefs in the earY period of the lower third-order sequence highstand system tract,the reefs in the later period of the lower third-order sequence highstand system tract,the reefs of the upper third-order sequence transgressive system tract,the reefs in the ear Y period of the upper third-order sequence highstand system tract,the reefs in the later period of the upper third-order sequence highstand system tract.Combined with the previous knowledge of the tectonic activity in the Late Permian and the interpretation results about the seismic profile,the reefs growth pattern of Changxing formation in the study area was been established.And the reefs are mainY presented the aggradation growth pattern in the longitudinal direction.The existence of fault that paralleled to the direction of the platform margin(northwest strike and northeast dip direction)in the platform margin during the depositional period of Changxin formation,which controlled the growth and development of the reefs and leaded the reefs not to large-scale foreset to the sea side.Therefore,from the ear Y period to later period of the Changxing formation,reefs in the platform margin were general Y NW direction distribution,and ultimateY formed the distribution features of the narrow strip.The observation of thin sections shows that the destructive diagenesis types: cementation,compaction,silicification,special mineral(lapis lazuli,sodium feldspar)formation.The constructive diagenesis types: dolomitization,dissolution,tectonic disruption.Combined with the comprehensive ana Ysis method of cathode luminescence and geochemical data,an earY hydrothermal activity has been identified in the study area of the Changxing formation.It may be the product in the same hydrothermal period as the albtization,and developed in the ear Y diagenetic stage.That hydrothermal activity is main Y characterized by the transformation for dolomite of the syngenetic and shallow burial stage.It formed the hydrothermal powder-fine crystalline dolomite and coarse crystalline dolomite.The development scale is relative Y wide.Thus,the dolomite types in the study area: the powdermicrolite crystalline dolomite and powder-fine crystalline dolomite anhedral formed by the syngenetic evaporation pump and seepage reflux,which cathode rays are dark red light,powder-fine crystalline euhedral dolomite formed by the shallow buried environment,which cathode ray is rose shade,and the earY hydrothermal dolomite that cathode rays is shine red light.The distribution patterns of rare earth elements Eu,Sm has significant positive anomalies,and revealing this type of dolomite diagenesis fluid is magmatic hydrothermal.Cathode luminescence characteristics shows that there are four stages dissolution of the Changxing formation in the study area.During the first phase of syngenetic dissolution,the edge is often transformed cleanY and latter can filled by the asphalt.The second stage is the dissolution related to the ear Y hydrothermal activity.The dissolute pore edge shows the bright red transformation boundary through the cathode rays,and Cathode luminescence characteristic is distinguished aginst wall rock and filled by the asphalt.The third period is the dissolution related to the organic acid water solution.The fourth period is the dissolution related to the thermochemical sulfate reduction(TSR),and the dissolution holes have not any transformation boundary through the cathode rays.The research suggests that holes which formed by the latest dissolution is clean and no fillings,so it is the key to the reservoir formation.According to the influence of all kinds of diagenesis on reservoir,reservoir formation mechanism is anaYzed and the reservoir formation mode is established.Based on the reefs in the area growth background and diagenesis research to know distribution characteristics for the development of reservoir.There are tidal-flat facies reservoir,reef facies reservoir and beach facies reservoir in the Changxing formation.Vertical Y,the reservoir of the upper third-order sequence is better than the lower third-order sequence,its general distribution is isolated.The reservoir thickness increased upward and more concentrated.On the lateral,the platform edge reservoir is better than the platform reservoir,and has the poor continuity.The reservoir is controlled by the reef facies and tidal-flat facies distribution.The dolomite controls the high quality reservoir which is tidal-flat facies dolomite reservoir,beach facies dolomite reservoir and beach facies dolomite reservoir.Besides,tidal-flat facies reservoir and beach facies reservoir may form class-II reservoir,and reef facies reservoir form class II-III reservoir.