ABSTRACT: Effects of Burial Rate and Shale Compaction on Expulsion Efficiency of Hydrocarbons from the Mezardere and Ceylan Formations, Thrace Basin, Northwestern Turkey

INAN, SEDAT, Penn State University, University Park, PA, and RALPH L. KUGLER, Geological Survey of Alabama, Tuscaloosa, AL

The Thrace basin is a Middle Eocene-Pliocene intermontane basin that accumulated more than 9000 m of sedimentary rocks. The basin began to form through subsidence and sedimentation controlled mainly by downfaulting. During the middle Miocene, the basin was deformed by wrench tectonics that resulted in uplift and erosion in some areas and deposition in others.

The late Eocene Ceylan Formation is composed of interbedded shales, siltstones, fine-grained sandstones, and siliceous tuffs. The Ceylan shales contain an average of 0.6 wt.% TOC. Their kerogen is predominantly type III, and rarely type II. They generate gas upon maturation.

The early Oligocene Mezardere Formation is composed of interbedded shales, siltstones, and sandstones. Shales of this formation contain an average of 1 wt.% TOC. Their kerogen type is predominantly type I and II, and they are the only oil source materials within the basin.

Both formations have been subjected to high heating rates caused by rapid burial in the center of the basin. The Ceylan and Mezardere were buried at 300 m/m.y. and 200 m/m.y., respectively. As a result, such a high deposition rate resulted in undercompaction (arrested porosity reduction) due to lack of water escape. The undercompacted nature of these shales is evident from their high shale interval transit times (equivalent of 10 to 20% porosity) and kinky vitrinite reflectance-depth well profile encountered for these formations. However, along the basin margin, these shales were buried at a slower rate and attained normal compaction.

The thermal maturity of the shales was modeled by reconstructing their burial and temperature histories and applying the kinetics of hydrocarbon generation. Then, the model parameters were adjusted until a satisfactory match between predicted and observed values was obtained. The models predict that both shales entered peak oil generation stage at 0.7 to 0.8% Ro during the middle Oligocene, and matured further, in central basin, prior to the onset of uplift and erosion during the middle Miocene. However, toward the marginal areas, the shales entered the peak oil generation stage just prior to the uplift of the basin.

Saturation of liquid hydrocarbons in the high-porosity Mezardere shales is calculated to be at peak oil generation at the basin center. Despite the assumption of 30% of the pore water as stationary, oil saturation threshold was not reached to account for pressure-driven hydrocarbon phase migration. Thus, the generated oil could not be expelled by but instead cracked into gas within the Mezardere shales. However, toward the margins, the lower porosity Mezardere shales reached the oil saturation threshold and expelled oil that charged two small oil fields (Devecatagi and K. Osmancik).

The oil-source correlations of other workers confirm this prediction of Mezardere-sourced oils for these fields. Some gas accumulations in the central basin have also been attributed to a Mezardere shale source.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)