The Bank Facies Deposits and the Reservoir Characterisitics in Dengying Formation,Ziyang Sichuan

Key words: 
• Ziy ang Dengying /
• Formation /
• bank facies /
• facies assemblage /
• reservoir

Abstract: It is generally recognized that depositional bodies in high energy environment are favorable sedimentary facies for reservoir to form and evolve,but to carbonate deposits,depositional bodies in high energy environment are only one of fundamental factors which are suitable to the reservoir development,and influence of diagenesis in later stage may be more obvious,diagenesis can change depositional bodies in high energy environment which were dominated by primary porosity in early stage into dense one or into reservoir dominated by secondary porsity, so prediction and study on carbonate reservoir should integrate the study on sedimentary facies and diagenesis. According to macro and micro characteristics of cores from seven wells in Dengying Formation,Ziyang Sichuan,combining with data of physical property,the authors studied in detail the types of bank facies depositional bodies,features of the assemblages,main diagenetic events and the influence on reservoir property.The study indicates that nine micro lithofacies deposited in high to relatively high energy environment were developed within the the depositional bodies in Dengying Formation,which are sparry dolarenite,sparry spongiostromata dolarenite,grain bound dolostone,algal bound crumb dolostone and sparry oolitic dolostone etc.,vertically and laterally,the lithofacies overlap to form intraclast bank,spongiostromata clast bank,algal bound grain bank and oollitic shoal,the bank facies bodies superimposed other related ones to form five types of lithofacies assemblage which was related to bank facies deposition:the single bed of the bank facies bodies and related lithofacies assemblages are of small scale and thickness,vertically,the beds appeared as lens-like and laminate ones repeatedly occur in middle and upper parts of Dengying member Two and middle and lower parts of Dengying member Three,laterally,it is difficult to trace and correlate the single bed in a long distance within the wells,on plane,they distribute at all wells,but there is a great difference in the types of bank bodies and degree of development.Before and after diagenesis,there is an apparent change of reservoir property of the bank facies bodies;the change is tightly related to textural composition characteristics of bank facies bodies and cementation,recrystallization and burial dissolution happened during diagenesis.In the bank facies bodies,although most of primary intergrain porosity and algal bound bank are hardly preserved as pore space,depositional facies will help to form favorable environment for reservoir to form and evolution because of lack of undissolved residues such as algal and clay,which are suitable to recrystallization and burial dissoluteness to develop more secondary porosity;and such dark-colored banks as gray-dark algal bound grain bank contain more undissolved residues,the primary porosity in which will vanish after cementation by multi-phase sparry dolomite,and there is only a weak later stage diagenesis,the banks turn to non-reservoir facies.The light-colored bank facies bodies which are favorable for the formation and evolution of reservoir are mainly distributed in the area centralized by Zi Well 3,Zi Well 7,from the area,favorable facies bodies get thinner and the reservoir property poorer and poorer.