فصلنامه پژوهش های چینه نگاری و رسوب شناسی
سال سی و هفتم شماره 3 (پیاپی 84، پاییز 1400)

The Oligo-Miocene Asmari Formation is the most known oil reservoir in the southwest of Iran and the Middle East. In the present study, in order to understand the reservoir quality controlling factors of the Asmari Formation, detailed petrographic studies were carried out in the Khesht Oil Field to identify microfacies, diagenetic processes affecting these microfacies and their paragenetic sequence. Six carbonate microfacies are indicated based on about two hundred microscopic thin section analyses from core samples in this field deposited in tidal flat and lagoon sub-environments on a ramp type platform. The results  show that important diagenetic processes that have affected the Asmari Formation carbonates  includ micritization, cementation (marine, meteoric, and burial cements), dissolution (meteoric and burial dissolution), neomorphism, mechanical and chemical compactions, fracturing, dolomitization (dolomicrite, dolomicrosparite, dolosparite and dolomite cement) and anhydritization (in different types). Dissolution and fracturing have enhanced porosity and permeability values, while cementation and compaction have decreased the reservoir potential of the Asmari Formation by occluding the pore spaces. Dolomitization has had a dual impact on the reservoir quality of the Asmari Formation.

The Late Cambrian was associated with tremendous changes in world oceans and lands including the deposition of specific facies in different parts of the world known as “ribbon rocks” due to their thinly bedded undulatory banded nature. In this study, the Upper Cambrian succession of the central Alborz (members 3 and 4 of the Mila Formation) is sedimentologically investigated with an emphasis on its upper part’s ribbon rocks. The study tries to give an understanding of facies analysis and depositional environment of this succession. In this regard, the strata are divided into nine lithostratigraphic units and 12 microfacies representing three subenvironments of supra- inter-and sub-tidal. The results show that the microfacies were deposited in an epeiric platform with marked changes in the facies distribution and platform physiography in time so that the evolution of this platform can be divided into three stages. During the first stage, dominantly tidal flat facies were deposited in broad belts without significant effects from disturbing agents such as storms. In the second stage, the platform was drowned due to a rapid transgression where deep-marine glauconiterich green shale was deposited on former platform facies. In the third stage, the environment was strongly affected by disturbing factors such as storms and tides where severe facies changes occurred, represented by the deposition of ribbon rocks. The ribbon rocks of the Deh-Molla Formation show a spectrum of subtidal to intertidal-supratidal depositional settings.

The Late Jurassic Arab Formation is one of the main oil reservoirs in the Middle East. To investigate factors reservoir quality controlling, the integration of the results from geological and petrophysical data was utilized. Ten main facies were recognized and grouped in five facies belts of tidal flat, intertidal, lagoon, shoal, and shallow open marine, indicating deposition of the formation on a homoclinal carbonate ramp platform. The diagenetic processes occurred in marine, meteoric, and burial diagenetic realms. According to the frequency within the reservoir, five main pore types comprising intercrystalline, interparticle, vuggy, moldic, and microporosity were determined. Four third-order depositional sequences which are correlatable with four reservoir zones (A–D) of the Arab Formation were identified. The development of grainstone facies played an important role in high reservoir potential intervals. In view of their controls on reservoir quality, diagenetic features can be categorized into two classes: (1) diagenetic processes enhancing reservoir quality that include dissolution, dolomitization, and fracturing; (2) diagenetic processes reducing reservoir quality including cementation, evaporate mineralization, overdolomitization, and compaction. Considering its layer cake nature, it is possible to use petrophysical logs for the correlation of different reservoir zones, and depositional sequences between the studied wells.

The study of Faunal biodiversity distribution in the strata close to the Middle and Upper Permian boundary in the Hambast Valley section was carried out. Considering obvious environmental and faunal changes in the topmost horizon of the subunit 4a of the Abadeh Formation, the end-Guadalupian extinction has happened before Guadalupian–Lopingian boundary. The significant changes near the Guadalupian-Lopingian boundary in this section are the presence of dolomite and stromatolitic limestone at the topmost part of the subunit 4a, implying the shallowness of the depositional environment as well as the noticeable decline of faunal biodiversity. The cause of end-Guadalupian bio-crisis in the Hambast Valley section is more likely due to the falling sea level. The examination of the end-Guadalupian extinction horizon in South China and Japan shows its position at the end of Capitanian. In South China (Penglaitan section) and Abadeh (Hambast Valley section), the end-Guadalupian eustatic regression has only left shallowing effects with environmental and faunal changes implying the end-Guadalupian extinction evidence. In the Hambast Valley section, the latest Capitanian Hemigordius irregulariformis Zone with some Codonofusiella species in the basal part of the subunit 4b are representative of the re-occurrence of the foraminiferal fauna after the end-Guadalupian extinction, while fusulinid Zone of early Wuchiapingian Codonofusiella-Reichelina is indicative of post-extinction foraminiferal recovery.

Lower Cretaceous deposits are outcropped in the Markesh Mountain, 15 km north of Ravar (Kerman Province), with a thickness of 143 m, and this section includes intercalation of gypsiferous marls, thin-bedded limestones, thin- to medium-bedded limestones, and medium- to thick-bedded limestones. For the shale and gypsiferous marls in the base of stratigraphic column due to the absence of fossils and also based on previous geological studies, the Upper Jurassic age has been considered. In the studied succession, 14 genera and 17 species of foraminifera, three genera and four species of calcareous algae, as well as 16 genera and 24 species of ostracods were identified, and the Barremian to Albian age was determined. The upward section, the Lower Cretaceous succession at the Markesh Mountain are covered by Neogene deposits. In general, the studied deposits include evaporitic facies and two marl and calcareous facies. The calcareous facies based on the microfacies characteristic consist of three sub-facies, (A) corresponding to the coastal sub-environment above the tidal zone, (B) the lagoonand (C) the bar sub-environment. Based on studies, Lower Cretaceous succession at the Markesh Mountain section was deposited in a shallow carbonate platform.

The sandstones of Gadvan Formation (Barremian–Aptian) in the Abadan Plain show a wide range of porosity and permeability due to their complex diagenetic history. Therefore, their petrography was investigated by polarizing and scanning electron microscopy. The porosity-permeability was estimated by routine core analysis. Based on the results, Gadvan sandstones were divided into five diagenetic facies. Limitedvolume of chlorite has greatly reduced the effect of compaction by creating a coat on quartz grains and limiting the overgrowth of silica cement. Carbonate cements up to 9% of the total rock have improved the reservoir quality whereas mesogenetic carbonate cemented sandstones (25 and 32% cementation) have often acted as a barrier to the continued movement of hydrocarbons. After determining the porosity and permeability, in order to find the relationship between these two parameters with diagenetic facies, the flow units were determined by the flow zone indicator (FZI) method. It seems that HFU 1 and 2, which are equivalent to diagenetic facies 3 and 5, have the best reservoir quality in the studied wells.