Formation Mechanism And Dolomitization Of Pre-Salt Reservoir:the Ordovician Majiagou Fifth Formation,East-Central Ordos Basin

Many large oil and gas fields in the world have been found in the pre-salt strata.And dolomite reservior which have association with evaporites are often abundant in natural gas resources.During the sedimentary period of the Ordovician Majiagou Formation in the Lower Paleozoic of the Ordos Basin,several sets of thick layers of evaporites deposits are found in the central and eastern parts of the basin.The evaporites in M₅⁶ sub-member have the widest range and the largest deposition thickness of the Majiagou Formation.Therefore,the lower part of the M₅⁶ sub-member is called pre-salt strata in the specific exploration and development practice,and M₅⁶–M₅¹⁰ sub-member are claimed as the pre-salt strata.This set of carbonate dominated by dolomite is currently the main target of exploration in the Ordos Basin.By analyzing the genesis and characteristics of this dolomite reservoir,a cause of“dolomite problem”can be further discussed.And geological basis can also be provided for exploration and development of the basin.The petrological,mineralogical and geochemical characteristics of dolomite indicating that the pre-salt dolomite is mainly distributed in two sub-members,M₅⁷ and M₅⁹.The rock types are mainly composed of dolomicrite,very fine–fine crystalline dolomite and dolarenite.Part of the samples also contains thrombolite dolomite and coarse dolomite.Geochemical indicators show that the dolomicrite has a REE pattern similar to limestone.Its characteristics also contain higher?REE value and higher ⁸⁷Sr/⁸⁶Sr value.These results indicate that dolomicrite has a lower degree of dolomitization and its formation in the tidal environment during the synchronogenic to penecontemporaneous periods.Very fine–fine crystalline dolomite,which also has a REE pattern similar to that of limestone.Its geochemical characteristics contain a relatively lower?REE value and relatively negative?¹⁸O value.These data indicates that very fine–fine crystalline dolomite is mainly related to seepage–reflux dolomitezation during the penecontemporaneous to shallow burial periods,and it also has similar characteristics to deposits which experienced burial dolomitization.The original shape of particle is well preserved in the dolarenite.Variety of geochemical indicators tells us that the fluid of dolomitization originates from the contemporaneous seawater.In this paper,it is believed that intertidal located in the high land of paleotopography is the area where dolarenite mainly formed.The thrombolite dolomite has obvious characteristics of microbial mediation.The long-shaped and elliptical shape of carbonate particles which it attached indicates that after its formation,thrombolite dolomite was moved from evapotiets basin to tidal flat by the transportation of tidal action,and then it was deposited.Coarse dolomite is one of the pore fillings in the study area.Except for its relatively high?Eu value,coarse dolomite still has a similar REE pattern to the surrounding rock.The results indicate that the formation of coarse dolomite is essentially the result of hydrothermal reworked.The reservoir properties of the reservoir are clarified by analyzing the basic characteristics of the pre-salt reservoir,including the petrological characteristics of the reservoir,the characteristics of the reservoir space and the physical properties.The effects of diagenesis on the development of high-quality reservoirs are summarized through the analysis of the types of diagenesis and the establishment of diagenetic evolution sequences.According to the study of the vertical and horizontal evolution laws of dolomite,and the plane distribution law of porosity and permeability,two high-quality dolomite reservoir development zones were identified.The study shows that dolomite reservoirs are mainly composed of dolomicrite,very fine–fine crystalline dolomite and microbial dolomite.The types of reservoir space are mainly intercrystalline pore and intercrystalline dissolved pore,followed by interpartical dissolved pore,moldic pore and dissolved fractures.The physical properties data show that the pre-salt dolomite reservoir is a low-porosity and low-permeability reservoir,but high-porosity layers exist.The multi-stage dolomitization and dissolution are conducive to the development of secondary pores in subsalt reservoirs.recrystallization,fillings and compaction make significant destruction on reservoir properties of the reservoir.High-quality reservoirs of pre-salt dolomite in the study area are mainly developed in the M₅⁷ and M₅⁹ sub-members.And reservoirs in these two sub-members have certain inheritance in the distribution area of the reservoir.Area of JT-1 well to J-2 well and area of J-12 well to S-86 well are favorable areas for the development of high-quality reservoirs in the study area.The development of dolomite does not mean the development of dolomite reservoirs.Therefore,control factors of reservoir formation needs to be comprehensively discussed and the conditions for the development of high-quality reservoirs will be summarized based on the study of genesis and reservoir characteristics of dolomite.Study shows that formation of high-quality reservoirs in the study area is controlled by the factors like paleotopography,changes of sea level,diagenesis facies and evaporites types.Paleotopography not only laid foundation for the thickness of the development of dolomite,but also provided geological conditions for dolomitization.The sedimentation of shoal face in the central micro-uplifted area has good potential of being reservoirs.Under the background of the large regressive of the fifth member of the Majiagou Formation in the Ordos Basin,the development of carbonate and evaporites were formed due to multiple changes of sea sub-level.The decline of sea level caused ancient deposits at high sites of the terrain entering a short period of exposure.The selective dissolution of atmospheric fresh water formed partially dissolved pores.From the diagenesis facies,dolomite dissolution face is most conducive to the development of high-quality reservoirs,followed by autogenous mineral filling face.The reservoir performance in compaction face is the worst.The types of evporites are divided into gypsum and halite.Compared to carbonate developed under the halite,carbonate developed under the gypsum has higher degree of dolomitezation.The reason is that gypsum would absorb Ca²⁺and release high-salinity brine during the transformation into anhydrite.In this case,the ratio of Mg²⁺/Ca²⁺of the fluid in the formation will be increased.Therefore,the dolomite reservoir under gypsum has a better prospect for exploration than the reservoir under halite.With the combination of the tectonic evolution process,the formation progress of the pre-salt reservoir is simply divided into four stages:penecontemporaneous-shallow burial stage,the Late Caledonian-Early Hercynian tectonic uplifting stage,the Middle Hercynian-Early Yanshanian burial stage,and the Late Yanshanian.