Complex Sandbody Architecture of Basin Margin Shallow-Marine Delta System: The Eocene Cemalettin Formation, Boyabat Basin, North Turkey

Beate L.S. Leren¹, John Howell¹, Ediz Kirman², Allard W. Martinius³, and W. Nemec^1
1 University of Bergen, Bergen, Norway
² Ankara University, Ankara, Turkey
³ Statoil R&D, Trondheim, Norway

Shallow marine sandstones deposited at the basin margins of tectonically active basins form important but potentially complex reservoirs. This study documents an outcrop analogue that illustrates key issues related to heterogeneity at the reservoir scale. Geocellular modeling of the outcrop data has been used to highlight the effects of macroscale facies heterogeneities on simulated fluid flow.

The Cemalettin Formation which crops-out in the Eocene Boyabat foreland basin of northern Turkey represents a shallow-marine system that includes delta, shoreface and mouth bar systems. Locally the shoreface deposits are cut by incised valleys. This study focuses on the confluence of two incised valleys, each filled with Gilbert-type delta systems that partly merge, and onlap each other. Beneath the Gilbert-type deltas, the sequence boundary is overlain by fluvial deposits which indicate a fluvial origin for the valley systems. The top of the valley infill is a transgressive surface with hummocky cross-stratified sandstones overlying the Gilbert-type topset or foreset facies.

The studied outcrop illustrates the complexity that can occur within shallow-marine systems that appear superficially simple. Complex facies architecture record multiple phases of intra-valley delta progradation, inter-fingering and cannibalisation resulting from the interplay between sedimentation, relative sea level changes and tectonics. The analogue potential of the system is illustrated by geocellular modeling and comparison to the analogous Oseberg Formation from the North Sea. These studies illustrate that even within high net:gross settings, stratigraphically controlled facies architecture is the first order control on flow within both the reservoir and the analogue.