Abstract: Tertiary and Cretaceous Submarine Canyon and Slope Plays, Atlantic Margin and Northern North Sea - Models for Exploration

Timbrell, G. and G. Kovacs - Norske; Conoco

Tertiary and Cretaceous deepwater clastic reservoirs form the primary exploration targets along much of the NE Atlantic Margin and some areas of the North Sea. Canyons provide a pre-determined pathway for prospective reservoir sands and control or significantly influence the location of several play types. Several large oil discoveries have been made in the U.K. and Norwegian sectors which have a "slope-feeder" association.

A challenge is to devise a screening method to locate favorable sites for sand input / canyon development over large areas of the Atlantic Frontier and follow up with seismic using an appropriate exploration model. It has been appreciated for sometime that one of the most important factors controlling the geometry or initial location of canyons is that of fault and fracture patterns that cut basement rocks. Such lineament patterns are easily recognizable on LANDSAT data from Norway and NW Scotland. Using potential fields data major linears have been extrapolated offshore and good correlation is seen in several areas between the proven and interpreted lineaments and the occurrence of canyon-like features on seismic data.

Two Norwegian case histories are presented where detailed mapping of 3D seismic data has enabled the canyon morphology to be defined. A Cretaceous example is located in block 6306/2 south of the Halten Terrace on the western flank of the Frøya High. A strong set of NW-SE magnetic linears join the onshore Lillehammer - Kristiansand Line and these are seen to transect the Froya magnetic high offshore. Excellent correlation is observed with the presence of an erosional canyon at base Cretaceous level. The canyon floor has a complex topography and there are indications that sand prone facies have been deposited on a "bench" in the mid to upper reaches of the system. Additional canyons are also recognized. The Lower Tertiary examples are located in block 35/12 which lies on the Horda Platform. Here a suite of canyon features trend NW and E-W. Seismically, distinct, high amplitude, dipping edges define the envelopes of the canyons and the character of the fill varies considerably becoming progressively more complex downslope.

Where attempts are being made to find sand entry points within slope systems, it may be stated that potential fields and satellite image data is in general under-utilized. Where canyon presence is confirmed, there is some evidence that not all of the sand prone material is deposited at the mouth of the system. Features such as benches and bends may cause significant volumes of reservoir to be deposited in the mid-reaches of the slope canyons. Moreover, from an exploration perspective, the location of belts of canyon incision into highstand slopes may enable the recognition of prospective slope-channel and basin floor fan plays down-dip. In general, observations from these examples would agree with retrogradational backstepping fill models of canyon sequences. The intra-canyon reservoirs being deposited as components of a slope fan system during lowstand to early sea level rise.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil