Biostratigraphy, microfacies and depositional environment of the Gadvan and Dariyan formations, in the Gachsaran oilfield, southwest Iran

The Cretaceous system includes carbonate sediments and covers a large part of Zagros. Gadwan and Darian formations are located in the upper part of Khami group. These formations have been investigated by many researchers in several terms. The purpose of this study is to investigate the biostratigraphy of Gadwan and Darian formations, as well as to identify sedimentary facies to reconstruct their paleo-environment.

In this study, thin sections were prepared from cores and cutting and examined with a microscope. A microscopic study was done to identify microfossils (Loeblich and Tappan 1988). Also, skeletal and non-skeletal elements and microscopic characteristics were determined (Flügel, 2010). These were named according to Dunham's method (1962).

Biostratigraphy in Well A: 1) Choffatella decipiens - Pseudocyclammina littus -Trocholina elogatus assemblage Zone. It is introduced from the depth of 4740 m (the lower limit of Gadvan Formation) to 4623 m.
2) Mesorbitolina texana - Choffatella decipiens assemblage Zone. This zone is introduced from the depth of 4623 m to 4552 m.
Biostratigraphy in Well B: 1) Choffatella decipiens - Pseudocyclammina littus -Trocholina elogatus assemblage Zone. Zone No. 1 is introduced from the depth of 4860 m (the lower part of Gadvan Formation) to 4744 m.
2) Mesorbitolina texana - Choffatella decipiens assemblage Zone. This zone is introduced from the depth of 4744 m to 4677 m.
Facies of Darian Formation (Well A): MF1) Mudstone. This facies is associated with deep basin microfacies.
MF2) Planktonic Foraminifora Wackestone. This microfacies represents the low-energy environment.
MF3) Bioclastic Planktone Foraminifora Packstone. The presence of micrite indicates a low energy zone.
Facies of Gadvan Formation: MF4) Bentic Foraminifera Bioclast Packestone. It indicates a high salinity and indicates a lagoon.
MF5) Bioclastic Wackestone. It indicates low-energy environment of the middle ramp.
MF6) Mudstone. This microfacies represent a lagoon environment.
Facies of Darian and Gadvan formations (Well B)
Facies of Darian Formation: MF1) Mudstone. The facies formed in the outer ramp.
MF2) Planktonic Foraminifora Wackestone. The facies deposited in a low-energy in the external ramp.
MF3) Bioclastic Plankton Foraminifora Wackestone. The facies deposited in the outer ramp.
MF4) Orbitolina Peloid Packstone. The facies indicate an environment with high salinity in shallow waters and in the lagoon environment.
MF5) Bioclastic Wackestone. The facies deposited in the middle ramp.
MF6) Mudstone. The facies composed of micrite and has less than 10% bivalves and bryozoans.

Sedimentary environment: 

These facies were deposited in the marine environment and in the outer and middle ramp and a small part of the inner ramp.

In order to study the depositional environment of Gadvan and Dariyan formations in Gachsaran oil field, thin sections have been studied. Based on the biostratigraphy data, the age of Barremian to Aptian for Gadvan and late Aptian for Dariyan was considered. Microfacies study of Dariyan and Gadvan formations led to recognition of 6 microfacies in 3 facies belt, Inner ramp, Mid-ramp, Outer ramp in marine environment. Also, based on variation of microfacies, the studied area predicted as hemoclinical carbonate ramp.