Carbonate Reservoir Genesis And Distribution Of Cretaceous Mishrif Formation In Halfaya Oil Field IRAQ

Halfaya oilfield of Iraq is giant oilfield mainly of bioclastic limestone,Mishrif formation is the main oil-producing horizon,large sedimentary thickness,which develop shoal facies and non-shoal facies two different types of reservoirs,two types of reservoir genesis are different,reservoir performance have obvious difference.Comprehensive utilization rock thin section observation,scanning electron microscope,cathodoluminescence,carbon and oxygen isotope,and reservoir physical property test technical data to research petrology characteristics,sedimentary characteristics,diagenesis and reservoir characteristics in order to define the control factors for the development of reservoir,at the same time respectively on the shoal facies and non-shoal facies reservoir genesis types of classification.On the basis of effective reservoir recognition according to the physical characteristics and difference of pore structure carry out different genesis types of the reservoir classification evaluation research,divided shoal facies reservoir into four class and divided non-shoal facies reservoir into three classes.Meanwhile,on the basis of single well reservoir distribution,comprehensive considering sedimentary microfacies types,reservoir genesis types and effective reservoir thickness geological factor to carry out the Mishrif formation high quality reservoir planar distribution evaluation research.Mishrif formation development within a second-order sequence,which can be further divided into two third-order sequence and four 4th order sequence,Mishrif formation sequence stratigraphy development mainly controlled by sea level changes,horizontal sequence comparative good and stable distribution,oil-bearing layer of this formation developed in transgressive systems tract and highstand system tract.Carbonate rocks in the study area age is relatively new and biological prosperous,according to the supporting type of Dunham classification divided into two categories: grain limestone and micrite,according to the differences of sedimentary hydrodynamic condition will be divided grain limestone into bioclastic/calcarenite limestone and foraminifera limestone,with bioclastic/calcarenite,foraminifera skeleton and micritic matrix as three terminal point,rock types in the study area can be divided into bioclastic/calcarenite limestone,foraminifera limestone and micrite three categories,which further according to particles content subdivided into six small classes,total of nine types of rocks,and studied the different characteristics of rock,at the same time,on the basis of lithology logging identification to carry out the distribution characteristics of different lithology,of which bioclastic/calcarenite limestone are widely distributed in the Mishrif formation,bioclastic/calcarenite limestone of the whole Mishrif formation mainly distributed in MB2 section,which thickness about 50m;micrite mainly distributed in MB1 section as a thin layer distribution between bioclastic/calcarenite limestone;foraminifera limestone is very rare.Mishrif formation development under the background of carbonate platform,through the study of sedimentary characteristics show that development many kinds of sedimentary facies,such as restricted platform,open platform and platform margin.Restricted platform development bioclastic shoal and intraplatform marsh land two kinds of microfacies,bioclastic shoal and plaster frequent mutual layered distribution,local is a small amount of hauling bioclastic after reformed.Intraplatform marsh land mainly micritic limestone deposition,reservoir undevelopment.Open platform development shoal back and intraplatform bioclastic shoal,which sedimentary hydrodynamic conditions stronger than restricted platform bioclastic shoal.Platform margin development many kinds of sedimentary microfacies combination,which has best physical property.Mishrif formation development shoal facies and non-shoal facies two types of reservoir,main shoal facies reservoir development on platform margin facies and open platform facies,reservoir rock class included shell-bioclast limestone,rudist bioclastic limestone,calcarenite and bioclastic packstone/wackestone.Reservoir space is mainly intergranular pore and mold pore,development of a variety of pore combination type;including biological framework pore only development in rudist bioclastic shoal reservoir.Shoal facies reservoir has higher porosity and permeability than non-shoal facies reservoir,which belong to high pore and medium-high permeability reservoir.Pore structure development large throat and small throat,large throat accounted less but control effect to the permeability,up to 765×10-3?m2.Non-shoal facies reservoir main development in restricted platform facies,reservoir rock particles is tiny,pore types combination mainly with visceral foramen pore,dissolved pore and mold pore,belong to medium-high pore and medium-low permeability reservoir,pore throat distribution mainly with medium throat,accounts for about 65% ratio.The carbonate reservoir of Mishrif formation mainly for interlayer heterogeneity and intraformational heterogeneity,besides,interlayer transverse distribution stable and thickness is larger than intraformational.In general,Mishrif formation reservoir showed weaker intraformational heterogeneity and interlayer slightly stronger heterogeneity characteristics.Through the study of Mishrif reservoir characteristics,it is believed that sedimentary facies is the fundamental factor controlling the carbonate reservoir in the study area,the diagenesis is mainly dissolution and a small amount of cementation in the same period,which is also controlled by sedimentary facies.On the basis of studying reservoir characteristics and main control factors,propose scheme for dividing the Mishrif formation of different types reservoir genesis classification: combining with the sedimentary microfacies,rock type and advantages of pore type combination difference,divided Shoal facies reservoir into platform-margin reservoir and intra-platform reservoir,platform-margin reservoir including calcarenite reservoir,pelletoid limestone reservoir,bio-fragmental limestone reservoir,rudist bioclastic limestone reservoir and bioclastic limestone reservoir.Intra-platform reservoir is mainly of bioclastic grain composition compound limestone reservoir,total two categories,four small classes.According to non-shoal facies reservoir characteristics and genesis analysis combine rock type and advantages of pore type combination divided non-shoal facies reservoir into two categories and three small classes,in which platform-margin slope and shoal back marsh land bioclastic foraminifer wackestone and intraplatform marsh land foraminifer limestone and foraminifer micrite are better reservoir.Based on the reservoir genesis types study,according to the characters of different genesis types of reservoir physical properties and pore structure difference,divided shoal facies reservoir into I II class,in which Class I reservoir is platform-margin shoal reservoir,mainly distributed in MB2 section,especially in shell-bioclast shoal,rudist bioclastic shoal,calcarenite shoal;class II shoal facies reservoir are pelletoid shoal and intraplatform bioclastic shoal,development in MA,MB2 and MC1 section,distribution of the most extensive;divided non-shoal facies reservoir into II III class,in which class II reservoir most development in the bottom of MB1 section and platform-margin slope and shoal back marsh land bioclastic foraminifer wackestone;Class III reservoir is most development in gentle slope and intraplatform marsh land foraminifer limestone and foraminifer micrite;Class IV is non-reservoir development in gentle slope and intraplatform marsh land,overall thickness is thin,the lithology is dominated by micrite,with a small amount of dense wackestone.Through single well reservoir distribution and study on reservoir horizontal distribution,it is reckoned that high quality reservoir are class I and class II shoal facies reservoir and class II non-shoal facies reservoir,which development in bioclastic shoal,grain shoal and the development of foraminifera non-shoal facies limestone region.