Estimating Relative Intrabasinal Tertiary Erosion Using Geophysical Logs in Upper Cook Inlet Basin, Alaska

Peterson, C. Shaun ¹; Helmold, Kenneth P.¹; Shellenbaum, Diane P.¹; LePain, David L.^2
1 Alaska Division of Oil and Gas, Anchorage, AK.
² Alaska Division of Geological and Geophysical Surveys, Fairbanks, AK.

Semilogarithmic profiles of sonic travel time (Δt) versus true vertical depth (tvd) for 60 wells in Upper Cook Inlet Basin, Alaska were used to estimate relative magnitudes of intrabasinal Tertiary erosion. Bishop Creek Unit #11-11 (BCU 11-11) yields the slowest extrapolated surface sonic transit time (Δt_o) of 202.6 µs/ft, which is within 1.5% of the widely accepted value of 200 µs/ft in basins considered to have undergone minimal erosion. All other wells in this study, when compared to BCU 11-11, register minimum relative erosion estimates in the range of 1,500-10,500 ft. Prior to creating Δt versus tvd profiles, spontaneous potential (SP) curves for all 60 wells were baseline shifted to 100 millivolts (mv) to minimize inherent drift. Only sonic transit time (Δt) and tvd values corresponding to SP baseline shifted values ≥95-100 mv were included in an effort to limit analysis to a mud-rich (shale) lithology. Further editing removed Δt and tvd values from over-pressured zones and unreliable data intervals. Although site specific uplift histories can be variable and strongly influenced by local faulting, folding, and sediment deposition rates, this study identifies BCU 11-11 as the most appropriate well against which relative erosion in other Upper Cook Inlet wells may be compared. Further research will focus on incorporating these erosion estimates in geohistory models for specific Upper Cook Inlet Basin wells.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009