Clay, secondary dissolution, and tectonic controls on the reservoir properties in Upper Eocene tuffs, West Thrace Basin, Turkey

The diagenetic history of the Upper Eocene tuffs in the West Thrace Basin is based on petrological analysis of samples from six boreholes. Diagenesis in the tuffs principally involves the progressive development of various types of cements in the following order: quartz overgrowth, zeolite authigenesis, chlorite and illite authigenesis. After the formation of the cementation phases, there was a dissolution phase creating secondary porosity via the dissolution of volcanic class and feldspars, accompanied by generation of analcime and a late mordenite cementing phase. The tuffs also have been affected by the following diagenetic processes; fracturing and calcite, quartz, and zeolite cementation.
   Tuffs in the West Thrace Basin may contain significant amounts of secondary porosity owing to unstable grain and volcanic glass dissolution caused by relatively rapid rates of pore-fluid flow. Also tectonic stress appears to have controlled development of secondary porosity formation in the West Thrace basin which in turn might have been responsible for high porosity of the deep reservoirs. At depths greater than 2000 m, the porosity increases with depth due to secondary solution activities and fracturing in the West Thrace basin. Secondary Porosity is very important for hydrocarbon explorations in the Upper Eocene tuffs in the West Thrace basin. The dominant porosity type produced by dissolution processes (intragranular, intergranular). Fracture porosity also significantly increases reservoir quality. Authigenic clays may affect reservoir quality depending on type of clay and its distribution. However, dissolution and fracturing are generally a more effective diagenetic process than cementation in the tuffs of West Thrace basin.