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Tectono-Stratigraphic Controls from Gas-Bearing Palaeogene Turbidite System Architecture and Sandbody Continuity, Eastern Bulgaria

Watkinson, Matthew P.1; Enfield, Mark A.2; Plummer, Colin 2
1 School of Earth, Ocean and Environmental Sciences, University of Plymouth, Plymouth, United Kingdom.
2 PDF Ltd., Ewelme, United Kingdom.

The Kamchia Depression of western Bulgaria has been interpreted as the foredeep of the Balkan Mountains which formed during contractional deformation spanning the late Cretaceous to Neogene. During the Eocene, a series of gas bearing turbidite depositional systems were deposited here during the progressive development of Balkan thrusting. Although gas shows are abundant in this area, there has been limited commercial production due to the stratigraphic and structural complexity of the plays. Integrated structural, stratigraphic and petrophysical analyses are being applied with the aim of realising the potential of these gas bearing sands.

Key exploration objectives in this region have been to establish the controls on turbidite sandbody geometry and reservoir quality, and to account for the complex distribution of gas bearing horizons prior to drilling modern exploration wells. This has been achieved through the integration of sequence stratigraphic, facies and petrophysical analyses of old well-log and core. These analyses have been integrated with field and petrographic analyses, and seismic interpretation of existing 2D and newly acquired 3D data.

The southern Kamchia Depression is composed of three tectonic elements: southern autochthonous thrust sheets, central para-autochthonous imbricate stack of Mesozoic and early Palaeogene rocks, and a northern autochthon (late Eocene to Neogene). The autochthon was deposited against a progressive unconformity which developed coincidentally with thrusting.

During initial thrust emplacement in the latest Palaeocene to early Eocene, axially flowing laterally extensive turbidite systems developed showing a high degree of sand-body continuity. During the middle to late Eocene, a c.800m thick wedge of turbidite systems was deposited from turbidite systems flowing parallel to the unconformity. Initially, basin-floor topography caused turbidite sands to rapidly thin away from the unconformity. As onlap proceeded, turbidite lobes became less restricted and expanded across the basin floor. Gas charge appears to have been strongly controlled by sand body continuity, stacking patterns relative to the onlapped unconformity surface.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009