Gas-source rock correlation in Thrace basin, Turkey

Abstract

The Tertiary Thrace basin is the most productive gas province of Turkey. Since, exploration in the basin started almost 40 years before, many elements of the petroleum system are now known. However, a gas-source rock correlation is still not available, as modern methods of correlation have still not been utilized yet. A recent study on molecular and isotopic composition of gases could only show that gas accumulations have different origins. Accordingly, a mature source rock with a mixed type of organic matter, an early mature source rock with a mainly marine organic matter and a biogenic source have contributed in varying combinations to the formations of gas fields in the basin. All of the three potential source rocks, namely the Lower Eocene Hamitabat formation, Upper Eocene–Lower Oligocene Ceylan formation and the Oligocene Mezardere formation can fulfill the required maturity and organic matter type requirements in different parts of the basin. Furthermore, the basin wide variation of these parameters is not known due to insufficient subsurface data and non-representative distribution of data points. Therefore, a new approach, which considers the temporal development of maturation and gas generation in different source rocks within the drainage area of the gas fields and the presence of traps during this critical period was necessary. In this study, maturation and gas generation history of these three potential source rocks in the drainage areas of gas fields were determined by basin modelling. Comparing the presence of traps for a given gas field at these critical time periods of maturation and gas generation, the most probable source rock could be determined. The evaluation of the respective petroleum system in the drainage areas of the gas fields indicated that the Hamitabat formation was the source for the pure thermogenic mature gases in the Kuzey Marmara, Hamitabat and Karacaoglan fields. It also contributed to the gas accumulations in several reservoir horizons in the Degirmenköy field. The Mezardere formation was the source rock for mature thermogenic gas in the Karaçalı, Hayrabolu and Tekirdağ fields. In these fields, mixing with a biogenic gas also took place. The Mezardere formation was also the source of early mature thermogenic gas in the Degirmenköy field.