Evolution of the Thermal Maturity in the Thrace Basin: Revisited

Authors: Sedat İNAN, M. Namık YALÇIN

Abstract: It is known that thermal maturity (vitrinite reflectance versus depth) profiles display a jump at a depth of about 3000 m in most of the wells drilled in the Thrace Basin. This jump is observed either in the Upper Eocene Ceylan or the Upper-Eocene-Lower-Oligocene Mezardere formations. Previous researchers have attributed this discontinuity to the higher-than-normal temperature gradients brought about either by higher paleo-heat flow in the early stages of basin formation or by the additional heat source of circulating hot water during the recent past. In this study, we attempted to model the observed jump in vitrinite-reflectance profiles using one-dimensional basin-modeling software. The results suggest that, in order to model these discontinuities in measured vitrinite-reflectance profiles, both a relatively high paleo-heat flow during the early stages of basin development and an unconformity involving as much as 3000 m of erosional removal have to be assumed. Neither of these assumptions are based on geological evidence. For instance, an erosional unconformity between the mentioned units (except an angular unconformity between Middle-Upper Eocene and Lower Oligocene units mapped along the basin margins) was not recognized either in seismic sections or at borehole locations in the basin. Because model-prediction of measured maturities was not possible using geologically reasonable models for burial and thermal histories, it is concluded that the observed jump in thermal maturity profiles may be due to non-geological (e.g., borehole cave-ins) factors. Information gathered from several wells seems to support this conclusion. This re-evaluation of the evolution of thermal maturity has important implications for the timing and type of hydrocarbon generation from the organic-rich Mezardere formation. The hydrocarbon generation histories obtained from two geologic scenarios matching the maturity trends above and below the jump are quite different. This re-evaluation, made in the light of the modeling results and what is known about the geology in the region, suggests that the geological-evolution scenario that produces a match with the thermal-maturity trend below the jump is more realistic. Accordingly, the Mezardere formation, which is rich in oil-generating Type II kerogen, generated most of its oil in the center of the basin at 22 mybp, during the Early Miocene. This episode of peak oil generation took place prior to the formation of major structural traps due to Late Miocene wrench tectonism. Consequently, most of the oil generated from the Mezardere formation did not accumulate in structural traps. This must be one of the reasons for the low success ratio in the basin.