مهران مرادپور

The Oligo-Miocene Asmari Formation is one of the most important hydrocarbon reservoirs in the Zagros sedimentary basin, especially in the Dezful depression. The Formation in most of the oil fields of the Dezful depression consists of a mixed carbonate-siliciclastic sequence. In this study, the relationship between sedimentary sequences and diagenetic processes with reservoir zones is considered. For this purpose, sedimentological data and petrophysical logs of a key well were used. Based on microscopic studies on the core thin sections, four detrital petrofacies and eight carbonate microfacies were identified that were deposited in four facies belts, including open marine, shoal, lagoon, and tidal flat environments, related to a homoclinal ramp. In addition, six depositional sequences (third order) were identified by facies analysis and well logs. Reservoir properties have been increased by dissolution and dolomitization with pore space expansion, while being decreased by cementation and compaction. The Asmari Formation in the investigated well is subdivided into eight reservoir zones based on sedimentary characteristics and petrophysical properties. The coincidence of the facies boundaries with the reservoir zone boundaries in most cases shows that the changes in the reservoir properties are mainly related to the changes in the sea level. This is well-known in the lower zones

Paleocene-Miocene sediments in the central part of the Gorgan Plain were studied to evaluate lithostratigraphy, biostratigraphy and reconstruction of the paleogeography of the South Caspian Basin (SCB). Based on nannofossils distribution in the studied interval, the following points were identified: Foram layers equivalent sediments have Thanetian –Eocene age and Upper Maykop- Diatom equivalent sediments have Middle-Late Miocene age. Evaluation of the nannofossils distribution in the studied interval has clarified a major unconformity between lower Paleocene-Eocene sediments and the upper Middle-late Miocene sediments. This unconformity is coincident with collisional event of the Afro-Arabia and Eurasia Plates and rising of the surrounding mountain ranges around the Caspian Basin including Alborz and Kopet-Dagh Mountains. Nannofossils distribution of the studied succession displays that the SCB was connected to the Black Sea and Mediterranean Basin in the lower Middle Miocene and was isolated in the Middle-Late Miocene. The interpreted east-west running seismic lines displays a clear increase in the sediment thickness and accommodation space from eastern to western part of the Gorgan Plain. The higher available accommodation space from east to west of the Gorgan Plain led to a delay in the subaerial exposure in the western part of the Gorgan Plain due to rising of the Kopet-Dagh and Alborz Ranges. This interpretation is consistent with the lack of the Paleocene-Eocene sediments and presence of the Cretaceous sediments below the major unconformity in the eastern part of the Gorgan Plain and presence of the Paleocene-Eocene sediment below the unconformity in the studied area.   

Carbonate and siliciclastic Aghchagil and Apsheron Formations has been studied in a subsurface section in the Gorgan Plain. The Aghchagil Fm. consists of lower sandstone and upper carbonate units in the studied area. Sandstones petrofacies include litharenite, sublitharenite and feldspatic litharenite. The grain-supported facies is dominated in this Fm. and displays abundant porosity as moldic, vuggy and fenestral forms specially in the carbonate sediments, which indicates considerable lithological differences in Iran and the neighboring countries like Azerbaijan and Turkmenistan. These features candidate the Aghchagil sediments as prone reservoir succession, hence emphasizes a comprehensive geological and exploration study in the studied area. The Apsheron sediments can be sub-divided into lower sandstone and upper mudstone units and consist of sandstone with litharenite and sublitharenite petrofacies and ooidal/bioclastic limestone and gray/brown mudstone/claystone. Based on the youngest nannofossil species the late Pliocene-lower Pleistocene and middle Pleistocene ages are defined for the in the Aghchagil and Apsheron Fm., respectively. Nannofossils distribution of the studied succession displays the SCB was connected to the Black Sea and Mediterranean Basin in the lower Pleistocene and middle Pleistocene and was isolated in the late Pliocene-earliest Pleistocene.

Based on lithostratigraphy analysis, Cheleken Formation in the studied section of the Gorgan plain, subdivided into lower sandstone and upper mudstone/marl units. Based on petrographic analysis, the sandstone sediments include low textural and compositional maturity litharenite and sublitharenite. Conglomerates are polymictite orthoconglomerate with variables carbonate and chert grains. High percentage of the porosity as vuggy, channels and fractures in the sandstone and conglomerates and even mudstone deposits displays high reservoir potentional for the studied sediments and hence necessitates the exploration studies in the Iranian part of the SCB. Based on the youngest nannofossil species, a late Miocene to middle Pliocene (?) age is defined for the Cheleken Fm. in the studied area. Nannofossils distribution of the studied succession displays the SCB was connected to the Black Sea and Mediterranean Basin in the late Miocene- early Pliocene and the Pleistocene and was isolated in the main part of the Pliocene.

Miocene siliciclastic sediments with 338m thick are studied in the southern Neka of Northern Alborz flank. Detailed sedimentology study allows recognition four facies associations consist of delta plain, distributary channel, delta front and pro-delta. Delta plain and tidal flat include heterolithic shale-silt-sand and limestone with fenestral fabric. Distributary channel and delta front associations consist of thin to thick channel shape sandstone, which display coarsening-thickening upward cycles with pro-delta/offshore association. Scares Ophiomorpha nodosa and Thalassinoides in these sandstones indicate a stressful condition such as sandy mobile substrate, turbidity and highly salinity fluctuation in the sedimentary environment. Shale-silty sediments of the pro-delta association with abundant framboidal and chamber filling pyrite and organic material display anoxic condition of the sedimentary substrate. Introducing Miocene hydrocarbon sediments in the neighboring region like Turkmenistan and Russia and sedimentary characteristics of the pro-delta/offshore shale of the studied succession allow us to introduce those as high potential candidate for hydrocarbon source.

The siliciclastic sediments of the Aitamir Formation (Albian–Cenomanian) at the Oghchi locality of the Gorgan Plain consist of a thick succession of the black-grey shales and thin to very thick-bedded sandstone, deposited in a shoreface to offshore depositional system. Four facies associations including offshore, offshore transition, lower and mid-upper shoreface associations are identified in this succession. Offshore facies association consisting of a thick package of the fissile dark shales with the abundance of the framboidal pyrites displays deposition below the SWWB and dysoxic condition. The offshore transition facies association includes the heterolithic sediments of the shale and very thin-bedded sandstones/siltstones. The lower and mid-upper associations are characterized by an increase in abundance and the thickness of the sandstone layers and display a generally thickening and coarsening up-ward trend of a classical shoreface depositional system. The lateral correlation of the studied section and central sections of the Aitamir Formation of the Kopet-Dagh Basin indicates an increase in subsidence effect in the creation of the accommodation space and a decrease in the sediment supply of the N-NW parts of the basin. The presence of a thick package of dark shales with abundant framboidal and chamber filling pyrites of the Aitamir Formation suggests these sediments as a prone candidate for hydrocarbon exploration.

The Palaeocene siliciclastic sediments with 318m thick in the southern Neka of Northern Alborz (Suchelma section) consist of thick marl and thin-thick bedded sandstone. Integrated petrography and field studies allow recognition three facies associations consisting of shallow mobile channels (FA), fan fringe-basin floor transition (FB) and basin floor (FC) of a deep sea fan depositional system. Generally, sediment sequence displays coarsening-thickening upward cycles of the FB-FA and FB-FC associations. Zoophycus, Palaeodiction and Halopa in association with coccolith, planktonic foraminifers and abundant framboidal and chamber filling pyrite, phosphate and organic material in these sediments display a deep see environment with low sedimentation rate and anoxic condition in the sedimentary substrate. Paleocene succession in the studied section consists of four 3rd order sequences, which include LST, TST and HST. The Late Cretaceous-Palaeocene uplift in a compressional tectonic regime tends to formation of the Alborz range. This element along with Eustatic sea level changes are the main controlling factors for depositional condition (including deep sea fan in this study) and relative sea level changes in the South Caspian Basin.

The Early Cretaceous Eastern Haft-Savaran Zn-Pb (Ba) deposit situated in the Arak mining district and occurred within the extentional back-arc of Malayer-Esfahan basin. Mineralization occurred in two horizons in the Estern Haft-Savaran deposit. First horizon is major horizon that formed within the most upper portion of the massive limestone and second horizon occurred in the thin limestone which is alternate with shale and marl. Textures of the mineralization in this deposit are replacement, massive and vein- veinlets, and main alterations are silisification, dolomitization and calcitization. Sphalerite, galena, chalcopyrite, tetrahedryte, pyrite, bornite, chalcosite and covelite are the sulfide minerals and quartz, calcite, dolomite and sericite are gangue minerals in the ore deposit. Cathodoluminescence study in this deposit indicates three dolomites and one calcite generations. First generation of dolomite associated with weak mineralization and second generation of dolomite is Fe-rich and associated with main mineralization. Third generation of dolomite is late dolomitization and formed with calcite and cross-cutted the mineralization. Presence of framboidal pyrite with galena and sphalerite in its matrix and fine-grain euhedral barite suggest mineralization began during sedimentation and early diagenesis stage near seafloor. In continuum, main ore is identified as replacement occurrence of first generation sulfides by second generation coarse-grained sulfides under seafloor. With regarding to the formation of the Eastern Haft-Savaran deposit in the extentional tectonic setting, massive limestone as the host rock, and mineralization formation as replacment during diagenesis of the host rock under the seafloor, this deposit would be classified as Irish type deposit.

The Kangan Formation is a carbonate-evaporate sequence that is considered as a part of the largest carbonate reservoir in the South Pars Field at Persian Gulf. Petrography analysis led to the recognition of 5 different dolomite types: such as dolomicrite, dolomicrospar, dolospar, dolomite cement and saddle dolomite. Elemental studies on dolomicrite and dolomicrospar samples show a relative increase of Sr and Na and relative decrease of Fe and Mn in comparison with the dolomicrosparite samples. It is notable that the oxygen-carbon isotope trend in dolomite samples are due to slight influence of meteoric diagenesis (about dolomites type I, II and III) and burial diagenesis (about dolomites type IV and V). The dolomicrites have been undergone slight meteoric diagenesis, although they have formed in sabkha environment. The source of Mg for dolomicrites is the magnesium of seawater and interstitial waters equilibrium with seawater and on the other hand probably for the coarser grain dolomites in the Kangan Formation is connate waters and basinal brines. Based on the heaviest oxygen isotope in dolomicrite samples, paleotemperature of depositional environment of the Kangan Formation was around 44.5°C.