Sedimentary Microfacies And Reservoir Characteristic Of The Qom Formation In The Garmsar Block, Central Iran Basin

The Qom Formation, the major exploration target stratum in the Garmsar Block, is a Tertiary (Oligocene-Miocene) marine succession deposited in carbonate platform sedimentary environment in central Iran Basin, which is a typical Cenozoic back-arc basin, consisting of word's most unique diamict of carbonates with terrigeneous-pyroclastic material. Due to the complexities of its basin nature, reservoir characteristics, and reservoir-forming condition, oil and gas exploration in the study area has great challenges.Lithologically, the formation is composed mainly of bioclastic limestone, sandy bioclastic limestone, and micrite, forming a typical fractured-porous reservoir. In this paper, on the basis of comprehensive studies on a variety of data, in combination with the field survey, and taking full advantage of some field measured sections data and testing result of outcrop samples, the author firstly conducted the identification and type classification of sedimentary facies of the Qom Formation, then analyzed the sedimentary microfacies of Qom formation and its distribution in plane, and pointed out that the open platform facies -high-energy red algae microfacies and restricted platform-lagoon interbeded intraplatform microfacies are favorite position for reservoir development.Based on thin section analysis, reservoir physical property parameters testing, the fossil species, and reservoir space, pore throat relation, reservoir formation mechanism and distribution of reservoir properties of Qom Formation are confirmed. Thin section analysis shows that the pore types ??loped in Qom formation are diversity, including primary pore, secondary pore and fractures. Primary pores include biological antrum pore and residual intergranular pore, and the secondary pores include intragranular dissolution pore, intergranular dissolution pore, and intercrystalline pore etc. Fractures can be divided into dissolved ones and structural micro-fractures.Bottom-up qom group is divided into A, B, C, D, E, F, F six,Intragranular dissolution pores are well-developed within the F member of Qom Formation, however, dissolved fractures often occurred within C1 and C3 sub-members, structural micro-fractures are poorly developed throughout the whole Qom formation. In a word, the porosity and permeability conditions of the Qom reservoir are relatively poor, belonging to moderate-low porosity and low -ultra low permeability reservoir with severe inhomogeneity.Finally, the favorable region for oil and gas occurrence is pointed out, evaluation on block and drilling target are carried out, and the most favorable exploration target and optimum drilling locality are suggested.The oil and gas exploration in the Garmsar Block aiming at the Qom formation in the central Iran Basin may face some potential risks from the sedimentary facies and reservoir characteristics point of view.