Study of Microfacies, Sedimentary Environment and Diagenesis of Sarvak Formation in well No. 6, Ahwaz Oilfield

Intoduction: In the geological record of the Zagros fold-thrust belt and Dezful Embayment, Cretaceous successions consist of a thick sedimentary packages, which host numerous economically important hydrocarbon reservoirs such as Sarvak Formation (Ziegler, 2001; Ghabeishavi et al, 2010; Rahimpour-Bonab et al. , 2012, Mehrabi et al, 2014). Sarvak Formation (Albian-Turonian) is one of the main reservoirs in Iran and is the second major hydrocarbon reservoir in southwest Iran. (James and Wynd, 1965; Alavi, 2007). Ahwaz Oilfield is one of the largest oilfield in Iran that is located in northern Dezful Embayment (Motiei, 1995). In this research, petrographic studies and facies identification, for interpretation of sedimentary environment, have been carried out in order to understand the reservoir rocks of the Sarvak Formation. Furthermore, the influence of diagenetic processes on carbonate rocks of this formation in well No. 6 in the Ahwaz oil field, has been investigated which could be important in understanding of this formation for further development in this field.

Materials and Method: 205 thin sections, prepared from drilled cores, from Sarvak Formation in well No. 6 Ahwaz oil field have been studied in order to identify facies and interpret depositional environment as well as to investigate the diagenetic processes affected the Sarvak Formation. The frequency of the components which constitute the studied carbonates have been estimated by Flügel chart (Flügel, 2010). Different researchers study, for example Boudagher-Fadel (2008), Ghabeishavi et al. (2010) and Rahimpour-Bonab et al. (2012), have been taken into consideration for identification of foraminifera which are important component in the studied carbonates. For classification of carbonate facies followed the nomenclature of Dunham (1962) and Embry and Klovan (1971). The thickness of the Sarvak Formation in well No. 6 in the Ahwaz oilfield is 345 m that consists mainly of limestone. Since Sarvak Formation in the studied oilfield has not been completely drilled and with regard to the available data, the depth of 3310 to 3545 of this well which contains core has only been studied.

Discussion and conclusion: Petrographic studies of 205 thin sections from Sarvak Formation (235m thickness) in the studied well has led to identification of 7 carbonate facies including miliolid mudstone, packstone with highly diverse benthic foraminifera, bioclastic peloidal packstone, foraminiferal peloidal grainstone, rudist boundstone, rudist floatstone-rudstone, and planktonic foraminiferal wackestone. Based on identified of facies as well as vertical facies distribution and lack of slumping and turbidity deposits (Tucker and wright, 1990; Watts and Blome, 1990), gradual changes of facies, lack of aggregate grains, pisoids and oncoids (Burchette and Wright, 1992), the carbonate deposits of studied formation have been deposited in three environments including inner, middle and outer ramp, where sedimentation took place in a homoclinal ramp. Carbonates of the Sarvak Formation in the studied well have been affected by various diagenetic processes such as micritization, compaction, cementation, neomorphism, dissolution, fracturing and replacement (dolomitization, hematitization and pyritization) that operated in diagenetic environments such as marine, meteoric, burial and uplifting. According to Petrographic studies, in carbonates of the Sarvak Formation, dissolution and cementation processes are more important than the other. In these carbonates, the dissolution process increased and the cementation decreased the reservoir quality