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Characteristics, Origin and Respective Petroleum Systems of Natural Gas Occurrences of Turkey

Abstract

Although Turkey has a relatively low gas potential, number and type of natural gas occurrences are remarkable. Most of the occurrences are related with shows in wells and seepages on surface. But some gas producing fields do also exist. This contribution aims a review of the existing knowledge on the characteristics, origin and respective petroleum systems of these gas occurrences. Hereby, results of our own investigations and research are utilized in addition to published data. In the Tertiary Thrace Basin several gas fields of thermogenic and biogenic hydrocarbon gases are in production. Fields are formed by a mixture of thermogenic gases from two different source rocks or of thermo- and biogenic gases. The Palaeozoic Zonguldak Basin is the only site of coal-bed methane (CBM) in Turkey. In addition to thermogenic gas from mature coals, secondary biogenic gas is also reported at certain localities. The offshore Akcakoca field to the north of Zonguldak Basin produces biogenic gas from Eocene sandstones. Gas shows observed in many wells in the Adana and Iskenderun basins have to be of biogenic origin, as the Miocene and younger strata are still immature for gas at present. In South-eastern Anatolia, the main oil province of Turkey, some minor gas fields are also in production. Except the Dodan field they are thermogenic hydrocarbon gases, sourced probably by different source rocks. In the Dodan field mainly carbon dioxide, formed by acidolysis and thermochemical sulphate reduction (TSR), will be produced. Some small amount of hydrogen sulphur of TSR origin does also exist here. Carbon dioxide will also be produced in Central Anatolia. This gas is formed by thermal decarbonation of limestones, which is enabled by the relatively recent volcanic activity. The Cirali seep by Antalya is the only abiogenic methane occurrence in Turkey. Methane is formed together with unusually high amounts of hydrogen (app. 10 %) by serpentinization process. The relation of hydrocarbon gases with respective petroleum systems in these regions will be addressed with the help of basic petroleum geological considerations, organic geochemical data on source rocks, the isotopic composition of gases and in certain areas with the help of basin modelling.