فصلنامه پژوهش های چینه نگاری و رسوب شناسی
سال سی و دوم شماره 3 (پیاپی 64، پاییز 1395)

The post extinction facies are known as unusual deposits in sedimentary record. This study focuses on the role of these facies in reservoir behavior of the carbonate-evaporite series of Kangan and Dalan formations in central Persian Gulf. Sedimentological studies in three wells in the studied field show that these facies could be recognized with the unusual bloom of thrombolites through the field. Increase in seawater dissolved carbonates, caused by the demise of most carbonate-secreting organisms, produces frequent and huge cements immediately after the Permian time. This was coupled with agitation in sedimentary environment caused by sea-level fall at the end-Permian time. The nature of the thrombolites as well as the precipitation of these cements reduces both porosity and permeability of these strata. In spite of this fact, analysis of initial reservoir pressures and gas volume percentages as well as gas carbon isotopes reveals strong similarities between two reservoirs and so this unit could not be considered as a static reservoir compartmentalizer. The linear trend of reservoir initial pressures with increasing depth and also cluster analysis of gas volume and carbon isotope data confirms this interpretation. Regarding low porosity and permeability and also sedimentological characteristics of this unit, the post extinction anachronistic facies of the base of Kangan Formation will act as a dynamic reservoir compartmentalizer at the time of production. This situation separates the Kangan and Dalan formations into two distinct reservoir units from reservoir pressure point of view. Regarding such variations, the initial dynamic reservoir predictions will change and this will have a strong effect on future economic situations of the field.

In order to study the echinoderms of the Kalat and Chehel Kaman formations in the western part of the Kopet-Dagh Basin, Maraveh Tappeh stratigraphic section has been selected and sampled. In this section, Kalat Formation with 9 m in thickness lithologically composed of sandy bioclastic limestone. The lower boundary of the Kalat Formation with marl and argillaceous limestones of the Abtalkh Formation is continuous. The upper boundary of the Kalat Formation with the Chehel Kaman Formation which is predominatly composed of limestone, argillaceous limestone and marl is also concordant. In this research along with Kalat Formation, 46 m of the basal sediments of the Chehel Kaman Formation for echinoderm analysis has been investigated. 4 genera and 5 species from the echinoderm collected specimens has been identified which are belong to Hemiaster, Cyclaster, Echinocorys and Pseudogibbaster. This is the first report of the Cyclaster, Pseudogibbaster and Hemiaster stella from Iran. These genera with respect to their test morphology categorized in three epiphonal, semiphonal and in epifaunal, semifaunal and infaunal group.

In this research, gray, green, red to cream limestone, shale and marly deposits of the Julfa, Alibashi and lower part of Elikah Formation at the Zal stratigraphic section, NW Iran, with 67.20 meter thickness have been studied based on conodonts. This investigation is led to identification of 27 species and 4 subspecies belong to 5 different genera of conodonts while a proposed new subspecies Clarkina leveni zalensis n. sub sp. belonging to the species Clarkina leveni have been described. Also, 18 conodont zones with the age of Wuchiapingian – Griesbachian have been determined. Amongst them, 5 biozones belong to Wuchiapingian, 10 to Changhsingian and 3 to Griesbachian whereas the Late Permian extinction horizon is located on the upper surface of the Paratirolites Limeston last bed like other sections in the Alibashi Mountains re gion. In addition, Wuchiapingian – Changhsingian boundary is located at the Julfa to Alibashi formations boundary and the Permian – Triassic boundary placed at the gray calcareous parts of the lower Elikah Formation, .90 m above the extinction horizon. Unlike some recent entries, the boundary between Alibashi and Elikah formations are continuous due to the presence of two conodont zones Clarkina meishanensis – Hindeodus praeparvus As. Zone and Merrillina ultima – Stepanovites ?mostleri As. Zone in the marine shaly Araxes member and lower carbonate part of the Elikah Formation, occurrence of small orthotetid brachiopods, bellerophontid gastropods and numerous Bairdiidae ostracods in this shaly member even several preceding geochemical and isotopic studies.

The studied section located in the Djam-Semnan area 55 kms south west Damghan, at the vicinity of the Amrovan rail way station, in the northern flank of Shorab unassymetric anticline. The section is 440 meters thick, including limestone, dolomitic limestone and sandstone with rich fossils contents (e.g.: trilobite, coral, brachiopod, crinoid segments and vertebrate remains). The lower boundary is covered with the recent alluvial and the upper boundary disconformably overlain by the red marls and sandstones of the Permian Dorud formation. In order to determine biostratigraphy and bifacies of the deposits of Bahram formation in Shorab section, twenty five samples (4-6 kgs) were collected and processed with conventional actic/formic acid. Nineteen samples were prolific and totally yielded 340 conodont elements. The extracted conodont elements mainly separated out of the carbonate units. Thirty-one species belong to four genera (Polygnathus, Icriodus, Pelecysgnathus and Ancyrodella) were discriminated as: Icriodus excavatus, I. expansus, I. cedarensis, I. subterminus, I. iowaensis, I. alternatus, I. tafilaltensis, I. brevis, I. cf. expansus, I. sp., Polygnathus brevilaminus, Poly. angustidiscus, Poly. pollocki, Poly. cf. webbi, Poly. aspelundi, Poly. politus, Poly. alatus, Poly. webbi, Poly. cf. olgae, Poly. dubius, Poly. xylus, Poly. zinaidae, Poly. sp., Pelekeygnathus inclinathus, Pele. serradentatus, Pele. sp., Ancyrodella pristina, Acny. cf. pristina, Ancy. aff. binodosa, Ancy. sp. Based on the frequency and distribution of the collected elements Late Givetian to Early Famennian age were suggested to the studied interval. Conodonts biofacies (icriodid-polygnathid) confirm the deposition of the Bahram formation of the Shorab section in the shallow carbonate condition.

Abderaz Formation is studied along Ghareh-Soo stratigraphic section, in eastern part of Kopet-dagh Sedimentary Basin which measures 958 meters in thickness. Nannofossil bioevents have been utilized to biostratigraphically classify the exposed sedimentary succession based on schemes of Sissingh (1977 modified by Perch-Nielsen, 1985), Roth (1978 modified by Bralower et al., 1995) and Burnett et al. (1998) and accordingly the rocks are attributed to Late Cenomanian to Early Campanian time interval. The results of palaeoecological studies along the succession show a decrease in high fertility marker taxa like Biscutum spp., and point to warm and oligotrophic surface water conditions in the late Cenomanian sediments i.e. upper parts of Aitamir Formation. A relative increase in high fertility taxa at the base of Abderaz Formation spanning the Cenomanian-Turonian boundary interval is also distinguishable. The Turonian part of Abderaz Formation is characterized by an increase in diversity and abundance of forms belonging to family Polycylolithaceae. Appearance of cool water taxa such as Gartnerago segmentatum, Ahmuellerella octoradiata and Kamptnerius magnificus in middle part of Abderaz Formation may probably be an indication of the Late Turonian to Campanian gradual and continuous cooling.

The Lower Cretaceous Fahliyan Formation is one of important hydrocarbon reservoirs in the Persian Gulf and neighboring Arabian countries. Lithologically, this formation is mainly composed of limestone, argillaceous limestone and dolomite. Three major units have been recognized in the Fahliyan Formation (i.e. Lower, Middle and Upper Fahliyan) which are respectively equivalent to the Sulaiy, Yamama and Ratawi formations in the Arabian countries of southern Persian Gulf. To the northwest of Persian Gulf the Fahliyan Formation interfingers with the Garau Formation. The Fahliyan Formation is considered as time equivalent to some part of the Garau Formation, and is recognized as a source rocks in northwestern part of the Persian Gulf. Analysis of available geology and petrophysical data of the key wells penetrated through the Fahliyan and its equivalent Garau formations led to the recognition of 21 facies that were deposited in three sub-environments (belts): inner, middle and outer carbonate platform and also an intrashelf settings. The observed lithofacies pattern in the study area indicate a homoclinal carbonate ramp as the depositional environment for the Fahliyan Formation. Based on the recognized facies within the Fahliyan Formation the spatial distribution pattern of reservoir, source and seal rocks are presented in all the stratigraphic units in order to use a more accurate facies map for the petroleum system modelling.