(Based on Subsurface Studies) Ashkan Vafadar1, Massih Afghah2 Sequence Stratigraphy and Depositional Environment of Dariyan Formation in The Kish Gas Field (Based on Subsurface Studies) Ashkan Vafadar1, Massih Afghah2 1 Islamic Azad University Tehran North Branch vafadar@geologist.com 2 Islamic Azad University Shiraz Branch massihafg2002@yahoo.com

Outline: Introduction of study area - Geographical setting (Kish Gas Field) - Geological setting (Dariyan Formation) Research significance Methodology Microfacies analysis - Microfacies Types - Depositional Environment (Facies Belts) - Sedimentary Model Sequence stratigraphy investigations - System Tracts - Sequence stratigraphic surfaces Conclusion

Kish Gas Field Study Area

General Field Data (Geographical setting) Kish island is located in the Persian Gulf 19 km from mainland Iran. 91 km2 area with an outer boundary of 40 km and a nearly elliptical shape. Along Kish's coast are coral reefs and many other small islands. The island's surface is flat, lacking mountains or even high hills. The Kish Gas Field is situated under the Kish island and around it. The field as geophisical structure of the Kish anticline is dome-shaped with a low deep and W-E trend. The field is surrounded by several other fields such as: Charak, Chiru, Hendurabi, Thundar, QSC-1, Farur A&B.

Dariyan Formation (starigraphy) (James and Wynd, 1965)

Basic Formation Data (Geological setting) Dariyan formation is the youngest among the fife members of Khami Group including Surmeh, Hith, Fahliyan, Gadvan and Dariyan. This formation is a major hydrocarbon reservoir rock of Khami Group. Previously, the Dariyan has been called the Orbitolina limestone. The type section (at Gadvan mountain just north of the village of Dariyan) consists of 286.5 m (940 f) of grey to brown thick-bedded, feature-forming and Orbitolina-rich limestones. In coastal Fars and Khuzestan there is generally evidence of a disconformity in the beds just between Dariyan and kazhdumi. The Dariyan is considered to be correlative with the Shuaiba of Saudi Arabia, Kuwait and Iraq.

Research significance High percentage of gas reservoirs of the world are gathered in the coastal Fars and Persian Gulf of the Iran. Dariyan formation is one of the important reservoir rocks and also, source rocks in some fields of Zagros basin at south and south-west of Iran. Sedimentology and sequence stratigraphy investigations of hydrocarbon reservoirs have high significance in petroleum geology. It’s the first sequence stratigraphy and sedimentological study of Dariyan formation in the Kish Gas Field.

Research method Avalable data sources of the Kish-2 well were included drilling cutting and petrophysical logs. Preparing 82 thin sections for microscopic study. Microfacies characteristics including sedimentary texture, microfossils and components were described. Microfacies types were recognized according to the scheme porposed by Dunham (1962). Depositional environment and sedimentary model were described by MF vertical analysis and comparing with the models proposed by Wilson (1975), Flugel (2004), Read (1985) and Schlager (2002). Sequence stratigraphy studied based on the Emery and Myers (1996) and Catuneanu (2006) methods. +25 m +25 m

Microfacies Types: MF-D1: Bioclastic Mudstone-Shale MF-D2: Benthic Foraminifera Bioclastic Mudstone MF-D3: Bioclastic Shaly Wackestone MF-D4: Intraclastic Bioclastic Wackestone MF-D5: Benthic Foraminifera Bioclastic Wackestone MF-D6: Orbitolina-Rich Bioclastic Wackestone

Depositional Environment Facies Belts: Tidal Flat: - comparable with the facies belt 8 of Wilson belonging to inner ramp - Including MF-D2 and MF-D4 comparable with SMF24 and SMF19 & 23 Lagoon - comparable with the facies belt 7 of Wilson (Open Lagoon) belonging to inner ramp - Many benthic foraminifera such as Orbitolina, Miliolid and Textularia - Including MF-D5 and MF-D6 (with Micritization) Shallow Open Marine - comparable with the facies belt 2 & 3 of Wilson belonging to middle ramp - Including MF-D1 and MF-D3 generally deposited during the TST system tract

Sedimentary Model Homoclinal Ramp Comparing with other sedimentary models such as the models proposed by Wilson (1975), Flugel (2004), Read (1985) and Schlager (2002). Lacking of Reef facies Lacking of Calciturbidite facies Gradual changes of microfacies in vertical trend

Sequence Stratigraphy Definition of depositional sequence as a unit bounded by unconformities or correlative conformities (Vail et al, 1977; Posamentier and Allen, 1999; Galloway, 1989) Investigation of depositional sequences: - Facies analysis - Recognition and description of depositional environment - Vertical changes because of relative sea level changes - Petrophysical logs interpretation specially Gamma Ray Log Sequence subdivisions: - System tracts - Sequence stratigraphic surfaces

Dariyan formation in the studied section: Lower Dariyan: Including HST system tract belonging to the lower sequence (36m) Middle Dariyan: Including TST system tract belonging to the upper main sequence (35m) Upper Dariyan: Including HST system tract belonging to the upper main sequence (68m)

Conclusion: Dariyan formation in the studied section consists of 139m carbonate sediments mainly comprising bioclastic wackstone and mudstone microfacies with considerable shale particles in some depths. Investigations have led to recognition of three facies belts including tidal flat, lagoon and shallow open marine. Dariyan in understudy section were deposited in a homoclinal carbonate ramp. Sequence stratigraphy study devided this formation to three sections as the lower Dariyan (36m HST) belonging to lower sequence. Also, the upper sequence includes the middle Dariyan (35m TST) and the upper Dariyan (68m HST). Middle and upper Dariyan together comprise a 3rd order sedimentary sequence deposited in the Aptian stage (Lower Cretaceous). The lower contact of Dariyan formation with the Gadvan in understudy section is gradational and the upper contact with the kazhdumi is likely unconformable.

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