S. I. Ozkaya1*, T. Dölek2, K. Yapan3 and B. Alper Durukan2

1Independent Consultant, Manama, Bahrain.

2Turkish Petroleum Corporation, Ankara, Turkey.

3Soccar Exploration and Production Company, Ankara, Turkey.

*Corresponding author, email:

Identifying controls on the permeability of fluid-conductive fractures is critical in tight reservoirs, but this is challenging in tectonically complex regions such as foothills belts where there may have been multiple stages of deformation and fracturing. Fracture permeability depends on fracture aperture and connectivity, both of which are affected by tectonism and cementation. Among the many factors that control the cementation history, oil charging may play an important role. Important challenges in studies of fractured reservoirs in tectonically complex regions include determining the timing (and intensity) of fracturing events relative to that of the oil charge, verifying the presence of matrix storage, and establishing the fracture cementation history.
This paper reports on a comparative fracture study of four small-scale oilfields in the west Adıyaman Basin, located within the foothills belt of the Tauride suture zone in SE Turkey. Here the tight reservoir carbonates of the Sayındere Formation (Campanian) were subjected to repeated phases of structural deformation. Major deformation phases took place in Campanian and Maastrichtian times, before oil charging into the reservoir began in the Eocene; and in the Late Eocene – Oligocene and Late Miocene, after the oil charge. Fractures that were generated before oil emplacement appear to have been cemented or partially cemented by calcite as indicated by cross-cutting cemented fractures on borehole images. Partially-cemented fractures in cores are oil-stained with cement-lined walls, suggesting cementation began before oil emplacement but was not completed. Image logs and cores also show the presence of clean, open fractures with no cement present on the walls. These open fractures cut across the cemented or partially-cemented fractures, and are in general related to Late Miocene compressional folding. Open fracture density is correlated to Late Miocene fold curvature and asymmetry in the four oilfields studied. Of these fields, the Şambayat structure is the tightest and most asymmetric anticline and hence has the maximum open fracture density; this field also has the highest oil potential.
Although the available data is not sufficient to evaluate the effects of oil charging on fracture cementation definitively, the observations are consistent with a model that oil charge into the fractured Sayındere Formation carbonates inhibited or slowed calcite cementation. Hence fracturing of a carbonate reservoir after oil emplacement may significantly enhance the fracture permeability, and may even render a tight reservoir prospective.

Key words:SE Turkey, Tauride suture zone, west Adıyaman Basin, carbonate reservoir, Sayındere Formation, Campanian, fractured reservoir, oil charge, fracture cementation.

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