فصلنامه پژوهش های چینه نگاری و رسوب شناسی
سال سی و هفتم شماره 2 (پیاپی 83، تابستان 1400)

Upper Dalan and Kangan carbonate formations with Late Permian–Early Triassic ages are considered the main gas reservoir rocks in some Persian Gulf fields. In this study, to investigate the factors affecting the distribution of reservoir rock types, microfacies and diagenetic processes in the upper part of Dalan and Kangan formations in one of the Persian Gulf fields have been investigated. A total of 1110 thin sections and 840 porosity and permeability data (RCAL data) were used. In the petrographic studies, 15 microfacies were classified into four facies belts of the tidal zone, lagoon, shoal, and open marine in an epeiric carbonate platform environment. Studies showed that diagenetic imprints such as calcite cementation, dolomitization, anhydrite nodule formation, dissolution, and compaction had been occurred in three marine, meteoric, and burial diagenetic environments. To investigate and group the reservoir rock types on the Lucia petrophysical diagram, the porosity and permeability data were plotted on this diagram. Depending on the type of microfacies and diagenetic processes, porosity and permeability data are included in all parts of the diagram. This study shows that the reservoir rock of the studied field is very heterogeneous in terms of reservoir properties and is affected by sedimentary and diagenetic processes. Accordingly, grain supported samples have been moved from classes one and two to the lower parts of the diagram, in the non-reservoir section, due to calcite and anhydrite cement. In contrast, in some cases, dissolution and dolomitization in mud-supported microfacies led to data placement from class three of Lucia to classes one and two. Dissolution and dolomitization in grain-supported microfacies have resulted in the placement of these samples in the upper parts of classes one and two.

In order to study biostratigraphy and the determination of biozonationes in the Pabdeh Formation based on the new planktonic foraminifera biozonationes in the Thethys region, one subsurface section in well 1 of Parsi Oil Field was selected in the southern Dezful Embayment. Based on this study, biostratigraphic study of 320 thin section from well 1 of Parsi Oil Field has been carried out. The Pabdeh Formation with a thickness of 404 m consists mainly of shale, marl, argilaceous limestone and cherty limestone.The lower boundary of the Pabdeh Formation with the Gurpi Formation is unconformable and the upper boundary with the Asmari Formation is conformable. Biostratigraphic studies led to the recognition of 25 genera and 75 species of planktonic foraminifera and then 19 biozones equivalent to new biozone of Wade et al. (2011) in the Thethys region. These identified biozones include 3 biozones in the Paleocene, 11 biozones in the Eocene and 5 biozones in the Oligocene in the studied section in well 1 of Parsi Oil Field and are well correlated with some biozones in the Tethys region. Based on the identified biozones and fossil contents, the age of the Pabdeh Formation is Middle Paleocene (Selandian) – Late Oligocene (Chattian). Defined biozones are compared with the other sections presented in the Izeh Zone (Tange Hatti, Tange Pabdeh and Chahardeh)

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 Asmari Formation is a thick carbonate-terrigenous succession in the Zagros Foreland Basin  deposited during the Oligocene–Early Miocene ages. The Stratigraphic forward modeling is n used in this study to determine the sediment supply, transportation parameters, and production rate of the Asmari Formation during precipitation in the Abadan Plain. The Dionisos Flow software is used for modeling studied depositional sequences. Argillaceous limestones, sandy dolostones, sandy limestones, sandy shales, limestones, sandstones, and shales are the main lithology of the Asmari Formation in studied wells. Also, the studied formation consists of four to five depositional sequences in the studied wells. The sedimentary environment of the Asmari Formation in the selected wells includes the inner to outer ramp (hemipelagic and pelagic deposits). The input parameters to the software for modeling include gamma log data, global sea-level curve, thickness and bathymetry maps, as well as sedimentary facies. Examination of the calculated data shows that the wave energy and water flow rate are effective factors in the sediment transportation and change in the sedimentation process of the region. The highest rate of carbonate production is estimated in the transgressive system tract of the second depositional sequence of studied wells in the inner ramp environment.

In this study, the paleoecology, biostratigrapgy, and biofacies of the Khoshyeilagh Formation has been evaluated based on the found conodont species. The Khoshyeilagh Formation includes terrigenous-carbonate sequences which contains conodonts of shallow environments (Icriodus Polygnathu). The identification of 7 conodont species and sub-species led to the identification of 2 conodont biozones which are Upper rhenana-linguiformis, Lower triangularis-Lower crepida, respectively and also a biofacies named as Icriodus-Polygnathus. The identified biozones prove Late Devonian (Frasnian-Fammenian) age for the studied parts of the Khoshyeilagh Formation. The determined biofacies and the recognized conodonts in the studied sections represent shallow marine environment and warm tropical climate during the deposition time. Considering to the obtained fossil assemblages and comparing with the other sedimentary basins, the studied sections belong to the northern parts of Gondwana land. The analysis of biofacies and lithofacies show a decrease of depth at the end of Frasnian which is followed by an intense sea level drop similar to global examples. After a period of time and during Fammenian, the environment became deeper and open marine sub-environments (bioclast specule wakstone-packstone) were deposited.

The Shurijeh Formation is a Late Jurassic–Early Cretaceous (Late Kimmeridgian–Hauterivian) siliciclastic rock unit of the Kopet-Dagh Basin, up to 392 m thick. In the Estarkhi section, it rests disconformably on the Mozduran Formation (Oxfordian), and is disconformably overlain by carbonate rocks of the Tirgan Formation (Barremian–Aptian). The sandstones are predominantly subarkose, sublitharenite, feldspathic litharenite, and litharenite with minor quartzarenite, rich in quartz and feldspars, sedimentary and igneous rock fragments (plutonic), and rarely metamorphic fragments. Based on petrological and geochemical studies, minor diagenetic events in the early diagenetic stage include cementation by calcite and iron oxide. Deep burial diagenetic events were dominated by compaction, cementation (silica, dolomite, chlorite), fracturing, dissolution, pressure solution, and albitization. Minor late diagenetic events include dissolution and cementation (calcite, iron oxide). Based on diagenetic events, a temperature of >80°C during burial diagenesis is suggested for Shurijeh Formation. The existing porosity is secondary, resulting largely from the dissolution of feldspar and carbonate cement, with some fracture porosity. The porosity and permeability of 11 core plugs average 7.78% and 4.84 md, and the point count of 38 thin sections shows an average porosity of 10.05%. The good porosity and permeability of the middle part of the section predict an acceptable reservoir potential for this formation in the study area.