1 Depositional sequences of the Lower Triassic Kangan Formation, SW Iran M.A. Kavoosi 1, ∗ , R. Jamali 1 , M.R. Naiji 1 , R. Nematollahi 1 1 ,∗ National Iranian Oil Company Exploration Directorate, 1 st Dead-end, Seoul Street, NE Sheikh Bahaei Square, PO Box 19395-6669, Tehran, Iran ∗Corresponding auther. Tel. +982182703810; Fax:+982188604533 E-mail address: m.kavoosi@niocexp.ir (Mohammad Ali Kavoosi) Summary The Lower Triassic Kangan Formation is a major gas reservoir in the southwest Iran. It is composed mainly of limestone and dolostone with evaporite interbeds. The lower and upper contact with the Upper Permian of the Dalan Formation and the Lower-Middle Triassic of Dashtak Formation, repectively are discontinuous but conformable. To study facies, depositional environments and depositional of the Lower Triassic Kangan Formation, eight drilled wells and the Surmeh outcrop section were studied. Field and petrographic studies combined with wireline well logs analyses resulted in recognition of various facies related to tidal flat, lagoon, shelf margin, basinal facies tracts, which deposited on a carbonate ramp. Considering indicative surfaces such as hardgrounds, flooding surfaces and discontinuity surfaces, vertical and lateral facies tracts changes combined with the diagenetic overprints such as dissolution features, led to the recognition of three third-order depositional sequences. This study shows that relative sea-level changes had a main role in creation of depositional sequences of Lower Triassic deposits. The overall facies and thickness variation and sequence boundaries are in phase with the climatic changes. Relative sea-level changes of the Lower Triassic deposits are compatible with global trend. Key words: Kangan Formation, Lower Triassic, depositional sequences, reservoir, Iran Introduction The Dehram Group (Upper Permian-Lower Triassic) hosted giant gas reservoir especially in the Fars region and the Persian Gulf. The Upper Permian-Lower Triassic carbonates of Dalan and Kangan formations have reservoir continuity, which are capped by Lower-Middle Tirassic evaporites of the Dashtak Formation. The Kangan Formation comprises of carbonates and evaporites and at type section (in well of Kuh-e Siah 1) has a total thickness of 178 meters; meanwhile, at Surmeh outcrop/reference section, the formation thickness decreases to 140 meters (Szabo and Kheradpir, 1978). The Lower Triassic Kangan Formation can be subdivided into three lithostratigraphic units. Presence of Claria spp. from the basal argillaceous and shaly beds of the Kangan Formation, suggest an Early Triassic (Scythian) age for the lowermost of formation (Szabo and Kheradpir, 1978). After gas exploration from Permo-Triassic sediments (the Khuff Formation equivalent) in late 1975, no comprehensive studies have been carried out yet regarding depositional sequences of mentioned interval. As sequence stratigraphy integrates time and relative sea-level changes to track facies migration (Handford and Loucks, 1993), so it helps us to follow reservoir facies. It has long time been known that the base of the Kangan Formation is regarded at the base of shaly unit, which is along with high gamma-ray readings. A key aspect of this study is to re- evaluation of the contact of Dalan and Kangan formations. Detailed field survey carried out at at Surmeh outcrop section, presence of strong iron-staining at the top of Dalan Formation, which is overlain by thrombolite boundstone facies, suggests that the base of the Kangan Formation is started with thrombolite boundstone facies. Iron-stained surface that is overlain by thrombolite boundstone provides evidence for a relative sea-level fall, subaerial exposure and then sudden sea-level rise.