Origin of the natural gas seep of Çirali (Chimera), Turkey: Site of the first Olympic fire

Abstract

The Olympos Mountain, located to the west of the city of Antalya (SW Turkey) in the Çiralı (Chimera) area, is well known since 2500 years ago for the occurrence of gas seeps from ophiolites. The area comprises a large allochthonous slab known as the Tekirova ophiolite and is represented by intensely serpentinized ultramafic rocks. The Chimera gas escapes through fracture zones in the ophiolite. The ophiolitic slab is structurally bounded by a carbonate platform, termed the Beydagları autochthon. Emplacement of the ophiolite nappes onto the carbonate platform took place in the Tertiary (Eocene). This study is aimed at determining the origin and possible source rocks of this gas. Several samples of both gas and the ophiolite were collected at different sites to determine the molecular and isotopic composition of the gases and mineralogical composition of the ophiolites. The seep gases contain hydrocarbons up to C 5, carbon dioxide (<1%) and a considerable amount of nitrogen (up to 20%). The gaseous hydrocarbons are dominated by methane (>91%). The stable carbon isotope ratios of methane, ethane and propane are defined with a δ 13C 1 of −12.5‰ to −11.6‰, δ 13C 2 of −23.5‰ to −22.0‰ and δ 13C 3 of −23.7‰ to −21.0‰, respectively. The δD 1 values of methane range from −129‰ to −96‰. XRD measurements on the ophiolite revealed the presence of brucite, hydromagnesite, aragonite, calcite and lizardite-chrysotile mineral assemblage. Two possible sources are determined for the Chimera gas. The first is an abiogenic process of serpentinization within the Tekirova ophiolite, which would result in heavy methane formation. The second are the Upper Paleozoic to Lower Mesozoic organic-rich shales in which methane to pentane hydrocarbons were thermally generated.