نشریه رسوب شناسی کاربردی
پیاپی 13 (بهار و تابستان 1398)

The clastics Paleozoic Era deposits especially the Zakeen and Faraghan formations in some locations of the Arabian Plate host hydrocarbon reservoirs, but reservoir potential of these formations has never studied in north of this plate and the central part of Persian Gulf. The Zakeen Formation (Givetian to Famennian) completely studied for the first time in one exploration well in the Persian Gulf  Basin. Studied section is located on the north-east of the Arabian Plate and is the thickest reported section of the Zakeen Formation. It rest unconformably on Sarchahan Formation (Silurian) and it is coverd by clastics of the Faraghan Formation (Lower Permian). This study has investigated petrofacies and microfacies, diagenetic properties and depositional environments of  the Zakeen Formation by studying thin sections prepared from cuttings in one well in the central part of Persian Gulf. Results show that the Zakeen Formation is composed of quartz arenite, sub-arkose, arkose, siltstone, claystone, mudstone petrofacies and carbonate mudstone microfacies. Based on sedimentary propertiesand Palynofacies results, the Zakeen Formation was deposited on clastic shallow marine environment consists of beach plain, shoreface and offshore sub-environments. Main diagenetic processes include compaction, cementation (silisic, carbonate, pyrite and iron oxide), feldspar alteration, sericitization, neomorphism and dolomitization. Heterogeneity of the Zakeen lithology and variety of environmental conditions and diagenetic features created favorable conditions for reservoir feasibility studies.

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.

Sedimentological aspects of the Iranian side of the Persian Gulf were investigated in present study. The research objective was to identify the sediment components and compositions of the Persian Gulf continental shelf and incoming rivers. In total, 200 surface samples were collected from bed materials of the study area ranging 10 to 100 m water depths and incoming rivers (between 1 to 4 m water depths) using Van Veen Grab. Sample preparation was implemented to granolumetric test to identify biota and detrital content and to determine mineralogy (x-ray diffraction analysis) of sediments. Results showed low contribution of detrital components (i.e. quartz, feldspar, mica and rock-fragment) and high contribution organic-biogenic components (i.e. gastropoda, ostracoda, echinoderm, bryozoan and benthonic-planktonic foraminifera) in sediment composition. Accordingly, more than 50% of seabed components composed of mud (silt and clay)-size particles. Four types of sediment textures, slightly gravelly mud, sandy mud, gravelly mud and slightly gravelly sandy mud have formd more than 75% of these sediments. Detrital components of the Persian Gulf foreland basin are drived almost the Anatolia-Zagros mountain belt.  The similarity of fluvial and shallow-deep marine sediments (with the exception of shell-fragments in marine deposits) indicates similarity in their origins. In addition, effects of various sediment transport mechanisms such as fluvial, tidal and wave-induced currents on distribution of sediments in the shallow zone have been proved agine in present study. Incoming rivers to the northern part of the Persian Gulf are the most important factors in supplying and transporting detrital components. Detrital and organic-biogenic components are scattered by tidal and marine currents across the Persian Gulf. The most clay minerals (kaolinite, illite, smectite and chlorite) with the exception of palygorskite (autogenic origin) are detrital in the northern part of the Persian Gulf.

The Oligo-Miocene Asmari Formation is one of the main petroleum reservoir in the Middle East. To determine the biostratigraphy characteristics, facies types and sedimentary environments two stratigraphic sections in the internal Fars (Firouzabad section) with 270 meters thick and in the costal Fars (Bastak section) with 286 meters are collected. In both sections, Asmari Formation is largely composed of limestone and marly limestone. Based on distribution of benthic foraminifera four biozones with ages Oligocene (Rupelian to Chattian) to Miocene (Aquitanian) are distinguished. Facies analysis let to identification 12 microfacies belonging to inner (tidal flat and lagoon), middle (shoal) and outer ramp (open marine) facies groups that have been deposited in a homoclinal ramp with gentle slope during Rupelian to Aquitanian age. Frequency evaluation of facies indicates that outer ramp facies group shows most frequency in Firuzabad section and inner ramp facies group (specially lagoon subenvironment) is most aboundant facies group of Bastak section. Results shows that during Asmari Formation deposition, the Basin depth of internal Fars (Firuzabad section) has been deeper than costal Fars (Bastak section).

The carbonate succession of the Mobarak Formation (Mississippian) located in Central and Eastern Alborz, have a diverse trace fossils in Toyeh Rodbar and Siaheh mountain sections. In this research, different lithofacies cycles of the Mobarak Formation are examined by Markov method as well as field observations, and depositional environment has interpreted by proposed ideal sequence. To do this, the deposits of this Formation have divided into five lithofacies associations including argilaceous limestones intercalated with dark shales bearing Zoophycos ichnofacies, thin- to medium-bedded limestones bearing Cruziana ichnofacies, limestones bearing Skolithos ichnofacies, medium- to thick-bedded limestones bearing mixed Skolithos and Cruziana ichnofacies, and limestones intercalated shales without any trace fossils. Markov chain analysis shows that Mobarak Formation deposits are composed of coarsening upward cycles. A complete cycle from base to top consists of limestones intercalated shales without any trace fossils, argilaceous limestones intercalated with dark shales bearing Zoophycos ichnofacies, thin- to medium-bedded limestones bearing Cruziana ichnofacies, limestones bearing Skolithos ichnofacies, and medium- to thick-bedded limestones bearing mixed Skolithos and Cruziana ichnofacies. The development of sedimentary cycles bearing Ichnofacies is primarily controlled by environmental factors, e.g. sediment type, food accessibility, oxygenation and energy level. In this Formation, four ichnofacies have been recognized including Cruziana, Skolithos, Zoophycos and mixed Skolithos–Cruziana ichnofacies. The inner ramp sediments are characterized by low-diversity Skolithos elements.

In this research, the geochemical characteristics of the Garau Formation in two sections of the ground surface (west of Khorramabad and North-west of Kermanshah) were done by Rock-eval method. Based on the available data analysis, Most samples of the Garau Formation in the study areas have Type III kerogen. However , samples of the Garau Formation in North-West Kermanshah also contain lower amounts of type I and II kerogen. The TOC of samples from West Khorramabad and North-west of Kermanshah are about 0.24-1.28 and 0.05-1.96%wt. The Tmax values of the samples vary between 416-444 and 441-480°C, respectively. The kerogen of the studied samples in West of Khorramabad are in immature to maximum maturity stage, However, The kerogen of samples of the Garau Formation in the North-west of Kermanshah are in maximum maturity (oil window) to wet and dry gas production stage.

In this research, the biostratigraphy, microfacies, sedimentary environment and paleoecology of Asmari formation were investigated in Vazag, Eshgar and Gorgdan outcrops in Izeh zone of Zagros basin of southwest of Iran. Based on their depositional textures, petrographic analysis, and fauna 8 facies and 18 subfacies, were distinguished. They indicate different depositional settings, inner ramp, middle ramp and outer ramp. Also, based on the distribution of larger benthic foraminifera, 6 assemblage zones were identified, including :assemblege zone I to the age of the Rupelian, the assemblege zone II in the age of the Rupelian-Chattian ,The assemblege zone III to the age of Chattian, the assemblage zone IV to the age of the Aquitanian, the assemblage zone of V in the age of the Aquitanian and assemblage zone VI Burdigalian age. In addition, in the studied areas it was identified two subfamilies of red algae non-geniculate Melobesioideae and Lithophylloideae ,Corallines genus of the red algae geniculate from subfamily Corallinoideae and Subterraniphyllum thomasii Elliott may be transitional species and evolutionary link between geniculate and nongeniculate coralline algae .paleoecology of Asmari Formation based on the main components of carbonate sedimentation (Benthonic foraminifera, coralline red algae and corals) in the studied sections salinity varied normal to highsalinity, Considering the nutritional status, the eutrophic and oligotrophic conditions, with conditions predominantly in oligotrophic to mesotrophic conditions, Light conditions aphotic to euphotic, and in terms of depth, it has been deposited from relatively deep to shallow areas.

In Iran, the Permian-Triassic sediments are located in the northeastern part of Hambast Mountain, 60 km south-east of Abadeh city. One of the unique features of these sediments is the existence of complete outcrops from the middle Permian to the Upper Permian, which makes it possible to fully investigate the incidents at the end of middle Permian. The study area includes two stratigraphic sections in the Hambast valley, including Unit 3 of Surmag Formation (Capitanian) ,Abadeh Formation (Capitanian) , including Unit 4a, Unit 4b and Unit 5, and the Primary part of the Hambast Formation (Wuchiapingian) . Based on studies done on 380 microscopic thin sections, 10 sedimentary microfacies include Bioclastic Sponng Spicule Wackestone , Bioclastic Staffelid Packstone , Hemigordiopside Wackestone , Bioclastic Wackestone / Packstone , Algal Wackestone , Ostracod Wackestone , Fenestral Bioclastic Wackeston , Stromatolite Wackestone , Dolostone , Mudstone , were identified. These microfacies are related to unlimited lagoon and tidal zone that have been deposited in a platform of homoclinal ramp type.

In this study, for determination of dolomitization model of the Taleh Zang Formation, 2 suitable outcrops have been used in the South and Southwest of Kermanshah city. The Barikeh section in southwest of Kermanshah city, with a thickness of 282 m conformably overlies the Amiran Formation and is overlain by the Kashkan Formation. The thickness of the dolomites in this section reaches to about 40 m which are present in the lower part of the TalehZang Formation. The Kaboutar Bala section with a thickness of 65 m,like the Barikeh section conformably overlies the Amiran Formation and is overlain by the Kashkan Formation with a disconformity surface. The existing dolomites in this section are also occur in the upper part of the Taleh Zang Formation which has 12 m thickness. Based on the fieldwork evidences, petrographic studies (polarizan microscope, SEM and CL) and geochemical analysis (AAS), four different types of dolomite have been distinguished in the Taleh Zang Formation which include dolomicrite, dolomicrosparaite, dolosparaite and pore-filling dolomite cement. In these dolomites, there is no evidence of the saddle dolomite and dolomicrosparaite have the highest amount of porosity among all types of dolomites. Based on Energy Dispersive X-ray (EDX) analysis, the smectite clay mineral was identified among the upper dolomites sequence in the Kaboutar Bala section. Dolomitization model of the Taleh Zang Formation in the upper part of the section (Kaboutar Bala section) is sabkha type evaporation model and in the lower part of the section (Barikeh section) is shallow burial model.

Quantitative and qualitative classification of hydrocarbon reservoir rocks is one of the most important issues in geological reservoir studies. Nowadays, classifications of reservoir rock type are carried out by applying mathematical relationships to porosity and permeability parameters. Since the reservoir properties of the succession are directly related to their depositional environment and sedimentary conditions, therefore the influence of sedimentation and diagenesis should be considered in these classifications. In this study, based on core analysis results from Kangan Formation in six wells in one of the Persian Gulf hydrocarbon fields, hydraulic flow units were investigated by using FZI/RQI and DRT methods. The Unit 4 (HFU-4) has the best reservoir properties index that including the grainstone facies which is deposited in carbonate shoal environment. In this research, based on Carbonate shoal environment setting and diagenesis effects on that, the HFU4-B class that deposited in the center part of carbonate shoal environment, introduce for the best reservoir quality facies. Connected Moldic and interparticle porosities have the main diagenesis parameters that effect to increase the quality of reservoir properties. This facies with 13% average porosity and 199MD permeability has the best hydraulic reservoir unit.