فصلنامه پژوهش های چینه نگاری و رسوب شناسی
سال سی و سوم شماره 2 (پیاپی 67، تابستان 1396)

In order to determine stratigraphy and the introduction of biozonationes in the Pabdeh Formation based on the new planktonic foraminifera biozonationes in the Thethys strict, one stratigraphic sections in the Izeh zone at Gurpi anticline are selected and sampled. The thickness of the Pabdeh Formation in this section is 672 m., and 4 lithological units (Purple shale (106.25 m. thickness), Lower limestone and marl (50/75 m. thickness), Limestone (295 m. thickness) and Upper limestone and marl (220 m. thickness)) in Chahardeh stratigraphic section at Izeh zone. Biostratigraphic studies led to recognition of 30 genera and 141 species of planktonic foraminifera and then 21 biozones in realm (Wade et al. (2011) were identified. These identified biozones (P3-P5 (Paleocene),E1-E16 (Eocene), O1(Oligocene)) in the studied section in Izeh zone are in well correlated with biozones of Tethys region. Based on the indicated biozones and fossil contents, the age of the Pabdeh Formation is Middle Paleocene (Selandian) - Late Oligocene (Chattian).

A mixed siliciclastic-carbonate sequences of the Triassic Nakhlak Group in Central Iran has a unique lithology, but appears some similarities with the Agh-Darband Group in northeastern Iran. Nakhlak Group is composed of three formations: the Alam Formation which is a marine sequence of mixed siliciclastic-carbonate rocks with turbiditic affinity; the Baqoroq Formation is conglomerate sequence with sandstone and shale intercalations composed of volcaniclastic, metamorphic, and carbonate grains; and the Ashin Formation which is a thin bedded shale and sandstone sequence accompanied with limestone intercalations with open marine trace fossils and turbidite sedimentary structures. Petrographic evidences of the Baqoroq Formation diagnosed three main lithologies including carbonate pebbles which are most-likely originated from the Alam Formation, low grade metamorphic rocks that are probably derived from the Anarak Metamorphic Complex(?) and volcaniclastic materials from syn-sedimentary volcanic activities. Petrofacies study revealed three detrital modes of sandstone including litharenite, feldspathic litharenite and lithic arkose. Chemical index of alteration calculated from major oxide elements showed medium grade chemical alteration. Geochemical analysis shows felsic igneous parent rocks and palaeo-tectonic situation of magmatic arc in an active continental margin, which could be in connection with magmatic activities in southern Eurasian active margin. In general, all data and information confirmed that depositional basin of the Agh-Darband back-arc basin at the southern Eurasian active margin as depositional environment for the Nakhlak Group. Later the Nakhlak Group detached from the Turan plate and attached to the Cimmerian block and moved to the present day position by rotation of Central Iranian Microcontinent.

The Dalichai Formation at the Guydagh stratigraphic section in the southeast of Maragheh, (Western Alborz), with measured thicknesses of 198 meters (consisting of limestone, Calcareous shale, shale limestone, shale and marl) contain terrestrial (spores and pollen grains) and marine (dinoflagellate cysts, acritarchs, foraminiferal test linings and scolecodont) palynomorphs. This assemblage is formed from diverse and moderately-preserved palynomorphs. The recognized palynofloras includes 16 species of spores (assigned to 14 genera) and 19 species of pollen grains (belong to 11 genera). Based on presence of Klukisporites variegatus, Klukisporites Zone and Klukisporites variegatus Subzone are identified and assigned to the Middle Jurassic (late Bajocian - Callovian) in age. Comparing obtained data from this study and data gained from Dalichai equivalent sediments in Iran (Central and Eastern Alborz, Central Iran and Kopet Dagh geological zones) and other parts of the world (Australia, Afghanistan, Egypt, China, India and etc.), resulted similar climates (mild to warm and humid) during Middle Jurassic (Dogger) in these areas (due to similar vegetation). Based on recognized miospores and their frequency, Conifers, Pteridospermophyta, Pterophyta, Ginkgophyta, Lycophyta and Cycadophyta formed Dalichai vegetation at the studied section. The presence of warm water dinoflagellates such as Gonyaulacysta jurassica, Ctenidodinium continuum and Nannoceratopsis pellucida with fungal and algal spores confirmed the domination of warm and humid climate.

In order to understand the impact of salt plug on facies, depositional environment and diagenesis of the Sarvak Formation in the Kourdeh Anticline, three sections close to the salt plug measured and studied. The thickness of Sarvak Formation in three sections are 25 (close the salt plug), 90 and 185m (away from the salt plug) and is composed of limestone. Based on facies analysis and petrography, 11 microfacies identified. Microfacies analysis, high dip of sediments, differences in thickness, reducing of interbedded shale and shallowing environment toward the salt plug show that the Sarvak Formation deposited in three facies belts including outer, middle and inter shelf. Approaching to the salt plug, facies are shallower so that tidal flat facies have been in the margin of salt plug and deeper facies formed away from the salt plug. Digenetic processes affected this formation include micritization, neomorphism, dissolution, compaction, fracture, replacement (silicification, dolomitization, hematitization and pyritization) and cementation (equant, darusy, blocky, overgrowth and isopachous). The salt plug caused changes such as fracture and dolomitization of the bioclasts, reduction of dissolution, increases compaction and silicification that are operated in three Eogenetic, mesogenesis and telogenetic stages

Kangan and Dalan formations are the main reservoir rocks of the central Persian Gulf hydrocarbon fields. The aim of this study is determining the approprate sequence stratigraphic model to separate the reservoir zones of the carbonate-evaporate Dalan Formation. In this regard, a total of 9 facies and 4 depositional environments determined using petrographical studies. The sequence stratigraphic boundaries determined in two transgressive-regressive and EXXON models using all available data. One complete third order sequence as well as two other incomplete sequences determined in the first method that continue in Nar anhydrite and Kangan Formation respectivly. In the second method, the studied interval was divided into two sequences and 6 systems tracts. Regarding the chosen methods, the LST of each EXXON sequence is part of the HST of the previous sequence. In LST, precipitation of evaporites cause low reservoir properties. TSTs are the same in both models and have good reservoir properties due to the low anhydrite and frequent marine cements. HST is also a good reservoir zone in EXXON model. This systems tract has dual reservoir properties in T-R model because the LST of the EXXON model is also included. The results show that the EXXON model with separating the LST and HST with different reservoir properties has higher ability for differentiation of the reservoir zones in Dalan Formation. These systems tracts have different reservoir quality parameters (porosity and permeability) that is ignored in transgressive-regressive model.

The Nayband Formation (Norian_Rhaetian) in Bolboluieh section includes 105m of limestone, quartzarenite, lime shale and shale. Orthochemical, allochemical carbonate components (skeletal grains and non_skeletal grains) and also allochemical and orthochemical non_carbonate components form deposits of the Naiband Formation. Orthochemical components consist of cements, micrite, microspar and pseudospar. Skeletal grains include foraminifer, sponge, gastropod, brachiopod, echinoderm and bivalve. Non_skeletal grains include ooid, peloid and cortoid. Allochemical non_carbonate components include clastic quartz. Field and petrographic studies led to identification of carbonate microfacies of sandy peloidal oolitic packstone in sand shoals, bioclastic wackestone in outer ramp/sandy bioclastic wackestone with diverse fossils in middle ramp, bioclastic grainstone with echinoderm in sand shoals and sandy mudstone in lagoon and clastic petrofacies included quartzarenite in offshore transition environment, shale in offshore transition environment and lime shale in offshore environment . Based on observed facies pattern and field studies, we can suggest a homoclinal carbonate ramp as sedimentary environment of the Nayband Formatin in the studied area. Studied deposits have been affected by marine diagenesis (micritization and calcitic cementation of sandstones), meteoric diagenesis (neomorphism, pendent, meniscus and syntaxial cements) and burial diagenesis (fracture in grains and stylolitization).