Characteristics And Genetic Mechanism Of Facies-controlled Eogenetic Karst Reservoir Of Cretaceous Mishrif Formation In The Halfaya Oilfield,Iraq

As one of the most important carbonate targets in the Middle East,Upper Cretaceous Mishrif Formation has been highlighted for a long time.Although consensus has been reached on the overall sedimentary background,disputes still exist in understanding the sedimentary environment changes among sub-regions due to relatively limited research,rare outcrop,and incomplete drilled core,which hinders the analysis on sedimentary environment and thus the horizontal and vertical correlation.Meanwhile,Penecontemporaneous dissolution has been considered as the dominate origin of reservoir formation while epigenetic dissolution has also played certain roles in the improvement of reservoir capacity in the Upper Cretaceous Mishrif Formation in the Middle East.In this study,we find that early selective dissolution is not only present in relative high-energy categories of rocks,but also in low-energy ones in the cored intervals,This contradicts previous result.For the formation of high-quality carbonate reservoirs,the microenvironment and karst effect are the most important factors.Therefore,taking the Halfaya oil field as an example,the contribution of the the sedimentary microfacies,karstification,and other diagenesis to form and transform the high quality reservoirs have been further research.The achievement are as follows.(1)A total of 11 sedimentary microfacies are identified through system identification of sedimentary microfacies and environmental analysis of Single Layers MC1-1 to MA2 of Mishrif Formation in Halfaya Oil Field.Two facies belts are determined according to microfacies assemblage analysis,namely restricted platform,open platform.The results are considered that restricted-platform possibly undeveloped,and open platform dominate,with sub-environments of high-energy shoal,low-energy shoal,inter-shoal sea,lagoon,open sea.(2)The rock-electricity model is established in the cored intervals,which has been demonstrated to have high prediction accuracy.Then vertical and horizontal correlation of sedimentary facies of Mishrif Formation is implemented based on the model.Finally the sedimentary evolution pattern is identified,which mainly consists of three stages and include two transgressive-regressive cycle.(3)Here we report a possible new reservoir origin based on a case study in the Halfaya oilfield of Iraq,i.e.,facies-controlled eogenetic karstification.The most representative evidence is that early selective dissolution is not only present in relative high-energy categories of rocks,but also in low-energy ones in the cored intervals.Meanwhile,the occurrence frequency of karstification varies among different environments and associated lithologies.The eogenetic karstification,can be attributed to a multiple superimposition of short-term exposure during the penecontemporaneous stage and the medium-term exposure after the shallow burial stage,three areas can be further divided,namely the tight bedrock area,the spongy-like dissolution pores area,and the karst channels and vugs-filled area.(4)The reservoir rocks of Mishrif Formation can be divided into micritic limestone,bioclastic wackestone,bioclastic packstone,intraclast bioclastic packstone,bioclastic grainstone,intraclast bioclastic grainstone,shell floatstone,karst rock.The occurrence frequency of bioclastic wackestone,bioclastic packstone and karst rock are relative high.In terms of physical property,the karst rock is the best.(5)The types of reservoir space include intragranular pores,intergranular(intragranular)dissolution pores,micropores among fillings,vugs and fissures.The micropores among fillings and intergranular(intragranular)dissolution pores are the best reservoir space types.(6)Favorable sedimentary microfacies are the basis of reservoir formation,eogenetic karstification is a decisive factor in the reconstruction of the reservoir.In all kinds of rocks,the karst channels and vugs-filled area property is superior to the spongy-like dissolution pores area,spongy-like dissolution pores area is better than that of the tight bedrock area.and the frequency of the karst channels and vugs-filled area and spongy-like dissolution pores area in grainstones and packstones are higher than that in the wackestones.Meanwhile,the under the influence of eogenetic karstification,pore-throat structures and seepage conditions of all the three kinds of karst areas in rocks are present in turn improve.(7)Eogenetic karstification has obvious control of oil bearing property of reservoirs,under the influence of facies-controlled eogenetic karstification,the pore-throat structure characteristics are improved in the interval of the karst channels and vugs-filled area and spongy—like dissolution pores development,often with oil-rich and oil immersion for characteristic;In the areas of limited karst transformation,and in,often with the tight bedrock area is given priority to,but the pore-throat structure characteristics are improved in the part of the karst channels and vugs-filled area and spongy-like dissolution pores development,the oil content relative to the bedrock area is more,but the distribution of oil-bearing area is uneven,which often form oil spot and oil stains.the karst channels and vugs-filled area is the most favourable for oil accumulation,with the average porosity of 22.64%,permeability of 24.85 mD,displacement pressure of 32.12 psia,median pressure of 194.88 psia,and median throat radius of 1.01 ?m.