| This paper conduct the comprehensive geological study by taking the upper Cretaceous Sarvak formation of Azadegan oilfield of the southwestern Iran,which located in the fringe zone of the Zagors fore-land Basin tectonically as the study object.The research content including the stratum distribution,structure characteristic,litho-facies,sedimentary sub-facies,depositional model,reservoir space type,physical property,reservoir main control factor,genesis and control factor of the rudist debris bearing reservoir and reservoir forming characteristic by make fully utilizing of the data of the field outcrop,coring,rock cutting,drilling,mud logging,well logging,3D seismic material,well production,well testing and geo-chemical analysis.Lastly,based on the findings above,we propose the oilfield development suggestions.Main findings details of this study are submitted as below:(1)Structurally,the Azadegan oilfield is a huge long-axis N-S trend anticline.The formation is continuous without any huge strike fault.Two domes are situated in the north and south respectively,and the southern one is higher than the northern one.The northern is an N-S trend small size structure with the steep limbs.The southern is in the N-S trend with relatively larger oilfield territory and it extends southwestward into Iraq.(2)Six significant sedimentary cycles were classified in the Sarvak formation in the Azadegan oilfield.The oil intervals primarily concentrated in the SQ-4,SQ-5 and SQ-6.The top boundary of these three Sequences are distinguished as the unconformity boundary,which named the M-T(top of the SQ-6,middle of the Turonian),C-T(top of the SQ-5,Cenonmainian-Turonian)and M-C(top of the SQ-4,middle of the Cenonmainian),and all these three are belong to the regression cycle.(3)37 types of the lithofacies are distinguished in the Sarvak reservoir,and five sub-sedimentary facies are divided which are the rudist mound,tidal flat,open platform,restrict platform and intracontinental sub-basin.The regional depositional model is established which highlight four key points:1.The distribution of the rudist-buildup is controlled by the pale-high;2.The build-up without into the wide colonize stage but reach the wave-base level in short time by the regression and formation uplift,and destroyed by the high energy current,forming the moundy allochthonous deposition after dispersed and re-deposited.3.The tidal flat exists widely in the upper Sar-3 and the duration stage depends on the paleo-structure relief;4.The exposure within the leaching effect by the meteoric water was appeared in the top of the SQ-4 and SQ-5.(4)The main lithology of the Sarvak is the limestone within the abundant of the rudist,foraminifer debris.The reservoir is influenced by the karstification and reservoir space dominate by the moldic pore,cavern,vug,and mud matrix dissolved pore.According to the MISC and por-per relationship,we divided the reservoir into four types which are the vug-pore,micro fracture-pore,matrix pore and non-reservoir.The reservoir main controlling factor are the 3rd sequence boundary(unconformity),palaeo-geomorphology and reservoir fracturing.The 3rd sequence boundary control the best reservoir vertical position,the palacogeomorphology control the reservoir quality distribution,and the reservoir fracturing control the oiliness property of the whole reservoir..(5)The reservoir diagenesis types mainly include micritization,compaction,cementation,dissolution,neomorphism,dolomitization and fiacturing.The main diagensis stage of the high quality reservoirs is the early diagenetic stage of the syngenetic-penesyngentic.The main diagenesis is dissolution effect.The favourable reservoir types are the reef-shoal reservoir with the abundant Rudist fragments.The diagenesis model is the typical syngenetic-penesyngentic succession karstification,and the reservoir forming is controlled by the 3rd sequence boundary.In the syngenetic stage,the reservoir influenced by the regression of the high frequency sequence boundary,and from the large of moldic pores.In the penesyngenetic stage,which caused by the 3rd sequence boundary,lead to the pores to redistribute in spatially,and make the most favourable reservoir concentrated close to 15-20 meters zone under the 3rd sequence boundary.(6)In the research zone,the Sarvak reservoir is massive and vertically heterogeneous,and impermeable interlayers are rare.The petrophysical properties dominated by the paleo-geomorphy and become poor from north to south laterally.It is sourced by the lower Cretaceous Kazhidumi formation of eastern Dezful sag,and sealed by marls and muddy condense limestone of the upper Cenozoic Gurpi,Pebdeh formation.Combined with the trap evolution,the accumulation evolution can be summarized as follows:The Sarvak play became a paleo-anticlinal trap(locate at the northern field)by the Alps tectonic activity(96mya)since late Cretaceous.The Kazhidumi bed entered in the oil window at the early Miocene(12-10mya),and charged the Sarvak bed after long-range lateral migration thus formed the paleo-reservoir.Under the impact of the Zagros Orogeny,the paleo-reservoir trap experienced a strong secondary deformation in the late Pliocene(4mya).The northern paleo-trap shrank dramatically and evolved into the current north structural high.The pre-low southern region was uplifted by the reactivation of the base fault and formed a secondary trap which evolved into the current south structural high.Ultimately,led to the current two highs structure with the south point higher than the north.The trap deformation broke the paleo-reservoir kinetic equilibrium and caused the secondary adjustment of reservoir.The upper seal prevented the vertical oil dissipation and migration,and the secondary migration mainly in interior Sarvak from north paleo-reservoir to south secondary trap.The strong reservoir heterogeneity and the degradation trend of reservoir properties along migration path(north to south)made the reservoir reformation process extremely slow,plus the short re-balance time,making the Sarvak form an "unsteady reservoir" which is still in the readjustment process.The current abnormal oil-water-contact of horizontal in north and tilted in south versus the trap evolutionary trend,and indicates the secondary readjustment is still in the early stage and has not affected the whole paleo-reservoir.Not all the reservoir is dominated by the current structure,and in the north the reservoir still stays in the paleo-reservoir form.(7)The comprehensive development suggestions are proposed:1,Vertically,the favorable reservoir location is controlled by the 3rd sequence of the inner Sarvak formation,hence,these boundary distinguishing is key factor for ensure the preferential vertical developing layer;2,Laterally,the favorable reservoir distribution is controlled by the paleo-geomorphy,due to the majority of the oilfields in the fringe area of the basin is through the secondary trap deformation,hence,the recovery of the paleo-high is key point for ensure the development area;3,the thick reservoir of the chalk limestone(Sar-4,5,6)which property and oilness is heavily rely on the fracturing effect.The fracturing effect is controlled by the tectonic activity with high uncertainty,and the distribution of the micro-fracture is extremely uneven,therefore,the reserve grading evaluation is very important for the project;4,In the northern structural high(paleo reservoir),the field development should be focused on the original paleo-reservoir zone.In the south structural high(Secondary reservoir),the lower sandstone reservoir(Burgan and Zubair delta)could be developed first,and the developing deployment of upper Sarvak optimized after obtaining the sufficient geologic data.Due to the similar reservoir characteristics and tectonic inheritance with Sarvak,the lower Cretaceous Fahliyan carbonate reservoir is also proved to be an unsteady reservoir with the tilted oil-water contact. |
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