OZKANLI, MEHMET, and KUBILAY KUMSAL, T. P. A. O. Petroleum Company, Ankara, Turkey

ABSTRACT: Petrophysical Evaluation of Volcanic Gas Reservoirs of Upper Eocene Formations in Thrace Basin, Turkey

This paper covers a petrophysical study of silicified tuffs developed at the lower part of the Ceylan formation which is upper Eocene in age. It was accumulated only in deeper part of the Thrace Basin. These tufs named as Karacaoglan tuffs are in acidic volcanic character and productive in Karacaoglan gas field.

Water saturation (Sw) and porosity (phi) calculated from well logs do not represent productive zones. The main factor in identifying the productive zone on well logs is the type of tuff. Rhyolitic tuffs comprise the gas productive zones in the field. Therefore, differentiation of rhyolitic tuffs from dasidic tuffs is of great importance. Conventional log interpretation techniques are not applicable to determine the potential reservoirs; therefore, new interpretation techniques are needed to locate the pay zones on well logs of Karacaoglan field.

Density, neutron, sonic porosities, M-N crossplots, and density-neutron crossplots are used in the separation of the tuff types. In Dasidic tuffs densite, neutron, sonic porosities are approximately same; however, in Rhyolitic tuffs density porosities are higher and with increasing glass content sonic porosities get higher. The washouts and rugosities along the wellbore generally concentrated in rhyolitic tuffs. This indicates that the rhyolitic tuffs are brittle and they are most affected from Miocene tectonism. Under the influence of Miocene tectonism, secondary porosity developed within these tuffs, creating a fractured reservoir.

This study show the importance of well log characteristics in locating the potential reservoirs. Furthermore, the techniques used in this study give better results than conventional log evaluation techniques in volcanic rocks.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.