Reservoir Types,Characteristics and Genesis in Restricted Environment in Mishrif Formation,M Oilfield in Middle East
[Objective]The restricted environment reservoir of Cretaceous Mishrif Formation in M oilfield in the Mid-dle East is complicated and the understanding of reservoir heterogeneity is insufficient,which restricts the effective development of this type of reservoir.Therefore,reservoir characteristics,distribution and main controlling factors of heterogeneity in restricted environment were studied.[Methods]Based on the data of core,cast thin section,physi-cal properties and mercury intrusion experiment.Through mathematical statistics and well correlation,the petrophysi-cal of different reservoirs are clarified and the spatial distribution is described.The main controlling factors of reser-voir characteristics in restricted environment are explained,and the origin models of different reservoirs are established.[Results]Three types of reservoir were found in the restricted environment,designated as reservoir types 1,2 and 3.Type 1 reservoirs were developed in high-energy depositional environments(tidal channels,intra-platform shoals etc.)and underwent quasi-contemporaneous dissolution.These reservoirs comprise mainly grainstone with medium-to-high porosity and medium-to-high permeability.The reserve space is mainly composed of intergranu-lar pores and dissolution pores.It is the preferred target type for reservoir development,as it is present on a huge scale with stable distribution.Type 2 reservoirs were mainly developed in high-energy depositional environments such as lagoons,and experienced dissolution in the quasi-contemporaneous period.These reservoirs include grain-bearing limestone,wackestone and packstone with mainly medium-to-high porosity and medium-to-low permeability.No pri-mary intergranular pores are developed,and the reserve space consists of matrix-host micropores,mddic pores and intercrystalline pores.This type of diagenetic reservoirs are present on the largest scale,but they are in the form of thin layers with highly frequent interlayers,and development would be difficult.Type 3 reservoirs were formed in intra-platform shoals and experienced quasi-contemporaneous dolomitization.They are mostly dolomite with medium-to-high porosity and medium-to-low permeability.Reservoir type 3 are small in scale and are only locally developed in the upper part of the Mishrif Formation.[Conclusions]The study concluded that the genesis of reser-voir type 1 was mainly controlled by sedimentation,which controlled the original structural components of the rock and in turn controlled the type and intensity of diagenesis in the quasi-contemporaneous period.Type 2 reservoirs exhibit weakly sedimentary hydrodynamic conditions and no primary intergranular pores are developed.Their reser-voir spaces are mainly the result of constructive diagenesis such as selective dissolution.Type 3 reservoirs have under-gone greater diagenetic re-formation and severe damage to the original structural components.The content of local residual biological debris indicates that type 3 reservoirs were formed in a medium-to-high energy sedimentary envi-ronment,with mixed dolomitization.
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