THE THERMAL GRADIENT HISTORY IN THE THRACE BASIN, NW TURKEY: CORRELATION WITH BASIN EVOLUTION PROCESSES

O. Huvaz¹* N. Karahanoglu² and V. Ediger³

¹Shell International E & P., Rijswijk, Netherlands.

² Geological Engineering Department, Middle East Technical University, 06531, Ankara, Turkey.

³ Cumhurbaşkanlığı, Çankaya, TR-06689 Ankara, Turkey.

*corresponding author, email: huvaz@tpao.gov.tr

Thermal maturity modelling is widely used in basin modelling to help assess the exploration risk. Of the calibration algorithms available, the Easy%Ro model has gained wide acceptance. In this study, thermal gradients at 70 wells in the Thrace Basin, NW Turkey, were calibrated against vitrinite reflectance (%Ro) using the Easy%Ro model combined with an inverse scheme. The mean squared residual (MSR) was used as a quantitative measure of mismatch between the modelled and measured %Ro. A 90% confidence interval was constructed on the mean of squared residuals to assess uncertainty. The best thermal gradient (i.e. minimum MSR) was obtained from the MSR curve for each well, and an average palaeo-thermal gradient map of the Thrace Basin was therefore created. Calculated thermal gradients were compared to the results of previous studies. A comparison of modelled palaeo-thermal gradients with those measured at the present day showed that the thermal regime of the Thrace Basin has not changed significantly during the basin’s history. 

The geological and thermal characteristics of the Thrace Basin were compared and the thermal anomalies were evaluated as a function of basin evolution processes. The basin’s thermal regime was controlled by: (1) basement edge effects; (2) crustal thickness and basement heat flows; (3) thermal conductivity variations within the stratigraphic column; (4) transient heat flow effects; and (5) the influence of tectonic features. The impact of these factors on variations in the thermal gradients is discussed in detail.

Basement edge effects are most marked on the steep northern margin of the basin where heat is preferentially retained in highly conductive basement rocks rather than being transferred into less conductive sedimentary rocks. Thus, heat is significantly focused onto the northern edge of the basement, resulting in a thermal anomaly along the northern basin margin.

The margins of the basin, with relatively thick upper crust, have relatively higher thermal gradients compared to the central areas. This is due to radiogenic heat production in the upper crust. Thus, thermal gradients increase above highs and at the margins where thicker upper crust is present. A heat flow map of the Thrace Basin, constructed using a basin-scale crustal thickness map and a basement heat-flow algorithm, is presented and demonstrates the heat generation potential of the upper crust.

The Eocene Ceylan Formation, which has relatively low thermal conductivity, significantly reduces the thermal gradients by blocking heat transferred from the basement. Areas of high sedimentation rate are associated with low thermal gradients due to the transient heat flow effects of young, thick and “thermally immature” sediments as a function of the heat capacities of these deposits.

A direct relationship between thermal gradients and major structural trends could not be established because of a number of factors including the inactivity of buried Miocene fault systems, which did not allow the flow of high temperature fluids through to shallow depths; also, the steady burial and sedimentation rates since the Early Eocene have kept the pressure system in equilibrium.

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