Controls on the Distribution of Reservoir Properties in the Tarn Field, North Slope Alaska

By

W.R. Morris, D. Hastings, and S. Moothart (Phillips Alaska, Inc.)

The Tarn reservoir consists of Cenomanian-aged turbidite deposits that occur along the base of slope within the Brooks Range foreland basin complex, North Slope Alaska. These turbidite deposits occur in two slope apron systems that were fed by four slope gullies that incise the slope.

The southern slope apron system developed within a shallow silled basin near the base of slope and consists of 7 sequences that were fed from three conduits. The oldest two sequences consist largely of lobe deposits with channel deposits limited to the mouth of the southernmost slope gully. As the basin began to fill, channel levee systems (sequences 3 to 4) extend further into the basin, breaching the sill. The last phase (sequences 5 to 7) of turbidite deposition consists largely of lobe deposits that appear to have a much lower sand content.

The northern slope apron deposits are unconfined at the base of slope and consist of 4 sequences fed by a single conduit. The basal sequence also consists largely of lobe deposits. The overlying two sequences contain significant channel-levee systems and show an offstacking relationship. The final sequence consists of unconfined muddy turbidites.

Reservoir properties (porosity, permeability, SW and N/G ratios) vary significantly between these different sequences and turbidite elements. Permeability can vary by an order of magnitude between different sequences and by two orders of magnitude between different turbidite elements. Typically, porosity, permeability, oil saturation and N/G ratios decrease from channel to amalgamated lobe, to layered lobe to crevasse splay to levee, respectively.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.