Facies analysis and sequence stratigraphy of the Fahliyan Formation in “A” oil field, Zagros structural zone, SW Iran

The Fahliyan Formation is an important part of the Khami Group reservoir in the Zagros structural zone, particularly in the Dezful Embayment. The main objectives of this study are to investigate the microfacies, depositional environment and sequence stratigraphy based on petrographic data of six hundred and seventy thin sections and gamma ray log. Accordingly, 21 carbonate microfacies and two clastic lithofacies were recognized within the Fahliyan Formation, which were deposited in four distinct facies belts including open marine, shoal, lagoon and tidal flat. Based on facies characteristics and sedimentary features, a carbonate rimmed shelf setting was purposed. The predominant abundance of lagoon facies indicates that the study area is located in the shallow part and margin of Zagros structural zone during sedimentation of the Fahliyan Formation. The lower part of the Fahliyan Formation was deposited in a rimmed carbonate platform, and the upper part gradually passes to a mixed carbonate-clastic environment (Khalij Member of the Gadvan Formation). This transition is well documented in the upper parts of well H3. The Fahliyan Formation comprises two third-order depositional sequences in this oil field and could be subdivided into two parts including lower Fahliyan (Berriasian-Valanginian) and upper Fahliyan (Hauterivian-Barremian). Also based on foraminifers, two biozones including Pseudocyclammina lituus-Trocholina sp. zone (Valanginian-Barremian) and Choffatella-Cyclammina zone (Hauterivian-Barremian) were recognized.