GRAVITY AND SEISMIC EXPRESSION OF THE SOUTHERN YUKON FLATS BASIN, EAST-CENTRAL ALASKA

Gravity and seismic studies in Yukon Flats, east-central Alaska, characterize the geophysics of the area and reveal a basin with potential for petroleum production. Yukon Flats is a large area of lowland and rolling hills along the Yukon River in east-central Alaska. Because the region is mostly covered by Quaternary alluvium and there are no deep wells to constrain physical properties of the rocks, geophysical studies are useful for interpreting the subsurface geology. Yukon Flats is geophysically characterized by an oblong, oval-shaped isostatic gravity low that measures about 250 km east-west and 100 km north-south with an amplitude of about -25 mGal. This anomaly closely corresponds with the topographic outline of Yukon Flats. There are several smaller areal gravity lows of about -25 mGal within the broad low that indicate the presence of lower density rocks such as silicic intrusive or basin-filling sedimentary rocks. One of these anomalies, a moderate sized gravity low along the southern edge of the Yukon Flats broad gravity low is probably a sedimentary basin with potential for petroleum production. High-resolution seismic survey lines were made by Exxon-Mobil across this gravity low to evaluate the subsurface stratigraphy. We forward model gravity along one of these lines, which goes from the southern edge of this gravity low to the northwest for about 70 km, to compare this model to the seismic interpretation. The boundaries of the model bodies were derived from interpreted reflectors from a depth-migrated seismic section. We produced initial model densities for the bodies by converting depth-migrated velocities to densities. Outcrops near the basin margins suggest that the basin fill consists of Tertiary fluvial and lacustrine deposits underlain by Devonian to Jurassic rocks of oceanic origin. The gravity model along with the seismic data along this seismic line indicate a fault-bounded 7,600 m (25,000 ft) deep basin near the southeastern end of the line, filled with low to medium density, probably sedimentary rocks. The interpretation of the upper part of this deep basin suggests structures with potential petroleum traps with anomalously low rock densities. Along the northwestern part of the line a 1,200-2,700 m (4,000-9,000 ft) thick section of fairly flat lying, low-density sediments are interpreted.