THICKNESS OF SEDIMENTARY ROCKS IN THE YUKON FLATS BASIN, EAST-CENTRAL ALASKA, AS ESTIMATED USING CONSTRAINED ITERATIVE GRAVITY INVERSION

The Yukon Flats basin of east-central Alaska is a potential source of hydrocarbon resources for local consumption and possible export. Knowledge of the subsurface configuration of the basin is largely restricted to a few proprietary seismic reflection profiles covering a limited area and one 700-m-deep well, which has not been logged for velocity or density. The seismic profiles were reprocessed and reinterpreted by the U.S. Geological Survey in preparation for an assessment of the oil and gas resources of the basin (Stanley and others, 2004). The assessment effort required knowledge of the basin configuration between and away from the seismic profiles.

To extend the interpretation of the basin thickness across the entire area of the basin, we used a version of iterative Jachens-Moring gravity inversion (Jachens and Moring, 1990) based on gridded gravity data. At each iteration, the gravity field was separated into basin and basement components, and the basin component was inverted to estimate thickness. The inversion was easily constrained to agree with the interpreted basement surface along the seismic profiles. A single density contrast of –0.17 g/cm³, based on a 2-D model along one seismic profile, was used in the inversion. We explored the effects of lowering the density contrast to –0.13 g/cm³, a median value for 2-D models along all seismic profiles. The smaller density contrast increased basin thickness away from the seismic profiles but produced unstable results in the Crooked Creek sub-basin, which led us to reject this solution.

In addition to the main sedimentary depocenter interpreted from the seismic data with over 8 km of fill, the preferred gravity inversion indicates four apparent depocenters with over 4 km of fill in the western Yukon Flats basin, a similar depocenter in the eastern Yukon Flats basin, and a large depocenter with over 7 km of fill in the Crooked Creek sub-basin. However, unexposed felsic intrusions in the basement could be responsible for some of the gravity lows we have interpreted as depocenters; granitic rocks are exposed near the western end of the basin. Results for the Crooked Creek sub-basin are consistent with magnetic and magnetotelluric modeling, but this and the other apparent depocenters require further testing by drilling and seismic profiling.