Molecular and isotopic composition of gas occurrences in the Thrace basin (Turkey): origin of the gases and characteristics of possible source rocks

Abstract

Molecular and isotopic compositions of nine gas samples from seven fields in the Thrace basin (northwest Turkey) are determined in order to define the characteristics of the respective possible source rocks. According to previous organic geochemical studies, the Hamitabat, Ceylan and Mezardere formations are considered as potential source rocks. A first evaluation of the δ 13C values of methane, ethane, propane and the δD isotope value of methane indicated in general a thermogenic origin. However, a further evaluation, which considers the isotopic composition of δ 13C from methane, ethane and propane vs. 1/ n, where n is the respective number of carbon atoms in each gas molecule, showed mixing of gases of different origin in most of the analysed gas samples. Consequently, three gas groups are distinguished: group A—a thermogenic gas from a single source rock, group B—a mixture of thermogenic and microbial gases, group C—a mixture of thermogenic gases at least from two different source rocks. A modelling approach, which predicts the isotopic composition of a thermogenic gas from a marine or terrestrial organic matter as a function of isotopic composition and maturity of the respective source rock enabled definition of source rock properties. A mature (1.3–1.6 vitrinite reflectance) source rock with a mixed (terrestrial+marine) type of organic matter and an early mature (0.7–0.8 vitrinite reflectance) source rock with marine organic matter were identified as precursors of thermogenic gases in the basin. Furthermore, microbial gas also contributed to the formation of some gas fields. In all natural gas samples investigated, the mature source rock of mixed type organic matter contributed the major part of thermally generated gas. In this way, it has to be considered as the main gas producing source in the Thrace basin. However, a final gas-source rock correlation was not performed, since all the three possible source rocks have gained the predicted maturity ranges in the kitchen areas of the gas fields, and the spatial distribution of organo-facies of the potential source rocks is unknown. This study in Thrace basin has shown that determination of source rock properties by isotopic modelling contributes to gas-source rock correlation, which is problematic in basins with more than one possible source rock.