Microfacies, depositional environment and paleoecology of the Asmari Formation in the Karanj oil field, norhtern Dezful embayment, SW Iran

The Asmari Formation (Oligo-Miocene) is spread in the Zagros region. The Formation was first identified in Tang-Gel-Torsh, located on Mount Asmari. In a sample type section, the thickness of this formation is 314 meters. Kranj oil field is one of the most important oil fields in the North of the restricted embayment of Dezful. The Karanj anticline structure, one of the most important oil fields in Iran, is located in the southwest of the Zagros sedimentary basin and in the southern part of the north of restricted Dezful embayment adjacent to the Persian fields.

The study section is located 115 km east of Ahvaz. In this study, 105 thin sections of core drilling, belonging to Asmari Formation wells, were prepared in Karanj oil field. They were prepared with the help of a microscope equipped with a camera and carefully examined in lithology and fossiles studies. In the study of microfacies, the naming of limestones was done by Dunham and Amberi and Calvan methods and the identification of microfacies was based on the Flogel model. Asmari Formation is 132 meters thick in this well.

In this study, microfacies, depositional environment and paleoecology of Asmari Formation have been performed based on the study of 105 thin sections (including Core drilling) prepared from 132 m of subsurface cutting sequences from one of the wells of Karanj oil field.12 microfacies related to the environments, tidal flat, lagoon, shoal and open marine were identified and interpreted, including: 1) Anhydrite, 2) Mudstone-Dolomudstone, 3) Stromatolitic boundstone, 4) Quartz mudstone, 5) Bioclastic benthic foraminifera (perforate and imperforate) packstone-grainstone, 6) Benthic foraminifera (perforate) wackestone-packstone-grainstone, 7) Bioclastic grainstone, 8) Ooid grainstone, 9) Corallinacean coral floatstone, 10) Lepidocyclina corallinacean Neorotalia packstone-grainstone, 11) Lepidocyclina Nummulitids wackestone-packstone, 12) planktonic foraminifera wackestone, which can be grouped into five depositional environments: tidal flat, restricted lagoon, open lagoon, shoal and open marine.Investigating the important factors in the distribution of large benthic Foraminifera in this well The following factors play a role in the distribution of large benthic Foraminifera in the study period: 1- Salinity, 2- Distribution of food, 3- Temperature, 4- Water mobility, 5- Water depth, 5- Light.

In deposits of Asmari Formation in this well in Karanj oil field, aging from chattian to Burdigalin, with 132 meters in thickness, 12 microfacies were identified. The microfacies identified in this section belong to four ancient environments, including the tidal flat, lagoon, shoal, and open marine. Based on these microfacies and how to expand them, a hemoclinical ramp is recommended for sedimentation of this Formation. .Based on the presence of benthic and other components of sediments and how they are distributed in the carbonate platform, these sediments were formed with normal salinity in aphoic, oligophic and oligophic to mesophotic zones and under low nutritional (oligotrophy) and medium (mesotrophy) food conditions in chatian age. In Aquitanian-Burdigalian age, in the study section, high salinity conditions in the uphotic zone with high (uphotic) feeding conditions were present. Within the studied profile, the Asmari Formation was deposited during the Chattian period in the open marine with normal salinity (Psu 40-34) in the aphotic, oligophic to mesophotic light zone, and under the oligotrophic to mesotrophic nutrient.  At the time of Aquitanian to Burdigalian, it was deposited in a restricted lagoon environment with a psu 50-45 salinity and in an open lagoon with a salinity of more than 50 psu, in the uphotic light zone, under a uphoty nutrient.