Facies analysis, sedimentary environments and correlative sequence stratigraphy of Gachsaran formation in SW Iran

Abstract

Lower Miocene in SW Iran consists of Gachsaran formation. This formation forms one of the main caprocks of reservoirs of oil fields in the SW of Iran that comprises a thick succession of evaporitic rocks in the Zagros basin. This study presents a facies analysis, sedimentary environments and correlative sequence stratigraphy in the SW of Iran. To study petrography, facies, sedimentary environments and sequence stratigraphy of Gachsaran formation, 10 subsurface sections from Aghajari and Rag-e-Sefid oil fields in the SW of Iran and one surface stratigraphic section in the Shiraz area have been chosen and studied (Hossini-Al-Hashemi section). Facies analysis led to the recognition of 13 facies which are deposited in 2 major sub sedimentary environments, tidal flat (sabkha) and lagoon in homoclinal ramp sedimentary environment. Petrography, microfacies analysis and sedimentary environment of Gachsaran formation show this formation in the study area consists of 4 depositional sequences. These depositional sequences have been separated by both type-1 and type-2 sequence boundaries. Based on gamma-ray, sonic log and thin section from cutting sample permit the identification of six 5th, two 4th and a small part of 3rd order depositional sequences in caprock member 1 and lower part of member 2 Gachsaran formation. Lower sequence boundary with Asmari formation is erosional SB1. Fifth order sequences are observed in a different thickness between 4 and 10 m. In caprock two 4th order sequence with the thickness of about 20 m was recognized, which is only a part of the depositional sequence (3rd order). Lower boundary with Asmari formation is SB1. TST systems tract consists of sabkha and lagoon facies parasequences. MFS in this sequence are designated by bitominous shale and early HST systems tract consists of interbedded sabkha and lagoon facies. Gachsaran Formation in the surface section consists of nine 4th order depositional sequences, which are located in four 3rd order depositional sequences. These parasequences are composed of evaparite carbonate succession of green marl, limestone, sandstone, gypsum and red marl, respectively. In Oligocene–Miocene sea-level transgressive and carbonate Asmari formation has deposited in the south-west of Iran and in the Miocene sea-level regressive, and Gachsaran formation has deposited in the study area under a succession of sabkha/tidal flat conditions.