The geology of the Gryphon Oil Field

Abstract

The Gryphon Oil Field is located 320 km northeast of Aberdeen in UK Ninth Round Block 9/18b. The field was discovered by Well 9/18b-7 in 1987. This well proved the prognosed Eocene/Paleocene submarine fan system. To date, Kerr-McGee has drilled 13 wells and 10 sidetracks on the field. In 1990 a 3D seismic survey was acquired over the whole trend, in Blocks 9/18a, 9/18b, 9/23a and 9/23b. The field is a complex combination structural/stratigraphic trap formed on the northerly plunging nose of the Crawford Ridge. The principal reservoir sands are of the Balder Formation (latest Paleocene-earliest Eocene) and consist of thick, massive, well-sorted, high porosity sands deposited from high-density gravity flows. Water-escape and soft sediment deformation features are common and no primary sedimentary structures are preserved. These sands formed a 'Lowstand Fan' complex within a lowstand systems tract. Areas of sand deposition were determined by the sea floor topography, which was controlled by basement structure, soft sediment deformation and the disposition of older sandbodies. In order to establish accurate time lines, it is important to identify the relative ages of these sands correctly. This was achieved using detailed biostratigraphy combined with the innovative techniques of 'desanding' and petrophysical analysis of the Balder Tuff. Overlying the Balder sequence is the earliest Eocene interbedded claystone, silt and sand sequence of the Frigg Formation. These sediments were deposited within the 'Lowstand Wedge' channel/levee complex which built out across the previously deposited Balder Fan. Oil and gas migrated into the structure via the ridge fault system from the mature Kimmeridge Clay source in the deep graben to the north and east.