ABSTRACT: Effects of Heating Rates on Hydrocarbon Generation and Occurrence in the Thrace Area, Turkey

M. Arif Yukler, Tufan L. Erdogan

The Thrace area is a complex of several subbasins formed during the middle Eocene. The metamorphic and granitic series of the Istranca Massif underlie the area. The sedimentary sequence from the middle Eocene to Pliocene is mainly composed of clastic rock units that are transgressive over the Istranca Massif. Because of the existence of several subbasins and the complex tectonic setting of the area, major lateral lithofacies changes occur at short distances. The dramatic changes in sedimentation rates further complicate the geological picture.

A deterministic basin analysis model has been applied in the area to quantify the geologic evolution of the subbasins in terms of compaction, pressure, and temperature histories and hydrocarbon generation, migration, and accumulation. Owing to the complex nature of the tectonic evolution and sedimentation patterns, the area was subjected to varying heating rates. The conceptual model and input data were optimized using all available petrophysical and maturity data. The results were then checked against all available pressure, temperature, and geochemical data.

Heating rate maps were prepared from the temperature variations during the active hydrocarbon generation periods. The heating rate was high (> 3°C/m.y.) along a northeast-southwest axis in the central and southwestern parts of the area, normal heating rates (1-3°C/m.y.) took place in the northwest and in a limited zone in the central part, and low heating rates (< 1°C/m.y.) occurred along the boundaries of the area. Peak oil generation occurred at vitrinite reflectance values less than 0.7%, 0.7-0.8%, and >0.8% R_o (for type II kerogen) in the high, normal, and low heating rate zones, respectively. High heating rates led to high expulsion rates for oil, whereas low expulsion rates occurred in the low heating rate zones. High expulsion rates resulted i oil accumulations where effective seals existed at the time of migration. In the zones of low heating rates, liquid hydrocarbons that were not expelled later cracked into gas, leading to gas accumulations.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990