Imaging Large-Scale Stratigraphic Traps in Lower Cretaceous Rocks, NPR-A Using 3-D Seismic Data

The Lower Cretaceous Brookian sequence in the Western Colville foreland basin on Alaska’s North Slope records significant subsidence followed by rapid sediment influx. This rapid deposition is reflected by large >1800 ft. clinoforms generally filling the basin west to east (Houseknecht et. al, 2008). Sands within these clinoform packages are shed mostly from a distal marine provenance, are fine to very fine grained and are compositionally immature. Maximum burial depth is the primary control of reservoir quality for these sands.

Extensively mapped turbidite traps likely represent tight oil and gas reservoirs that will require enhanced fracture simulation to produce. However the extensive size (>25,000 acres) of the traps somewhat compensates for the anticipated marginal reservoir quality making the turbidite play an attractive exploration target within the National Petroleum Reserve-Alaska (NPR-A).

Brookian shelf-edge delta prospects are perhaps the most prospective oil play fairway in the NPR-A. The shelf-edge delta play fairway has undergone relatively shallow (<7,000 ft.) total burial depth and is predicted to maintain good reservoir quality. The shelf-edge delta plays, like the turbidite plays, are stratigraphically positioned to receive and trap high gravity oil from nearby HRZ source rocks. Some of the untested shelf-edge delta prospects are sizable and have estimated recoverable resource that could rival or exceed reserves produced from the nearby Jurassic-aged Alpine Field that was discovered some 15 years ago.

Turbidite and shelf-edge delta stratigraphic plays in these Brookian clinoform packages are directly imaged on 3-D seismic. Approximately 2,200 mi^2 of 3-D seismic data were processed and interpreted to define stratigraphic traps in the Lower Cretaceous Brookian sequence. Seismic Far Angle stacks (25° - 45°) have proved to be a robust sand indicator and highlight channel fill and lobe deposits in shelf, slope and basin floor settings. Amplitude Variation with Offset (AVO) 3-D class volumes characterize these stratigraphic features and appears to qualitatively differentiate reservoir quality.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California