THERMAL MATURITY AND TIMING OF DEFORMATION IN THE FORELAND BASIN AT KURUPA ANTICLINE, BROOKS RANGE FOOTHILLS, ALASKA

As evaluated by the U.S. Geological Survey, petroleum potential of the central North Slope foreland basin fold- and thrust-belt is based on a model of petroleum generation by Cretaceous and possibly earliest Tertiary deep sedimentary burial prior to formation of structural traps during deformation and uplift at 60 Ma and again at 45 Ma. Generated petroleum is postulated to have been sequestered first in stratigraphic traps, some of which later re-migrated to structural traps (or was lost to the surface) when some stratigraphic traps were breached by deformation. Structural traps may also contain late-stage hydrocarbon gases that migrated directly from the source rock.

We present a small, relatively well-constrained study based on vitrinite reflectance data from the Kurupa anticline and adjacent Awuna syncline that suggests structural deformation occurred prior to maximum burial. Constrained by geologic mapping and seismic data, a vitrinite reflectance profile in the 11, 060 ft West Kurupa 1 well penetrating Nanushuk and Torok Formations can be closely linked to a profile established in a nearby 6,000 ft Nanushuk outcrop section. When combined, these profiles can be interpreted as showing that vitrinite isograds cut across these folded Albian foreland basin strata, thus suggesting that maximum heating and presumably burial occurred after folding of these strata.

We recognize that vitrinite reflectance is a relatively coarse tool subject to error and uncertainty, including operator bias, misidentification of vitrinite populations, and contamination by caved materials during drilling. However, we think that these problems have been addressed and can be rejected as explaining our observations. We suggest that the thermal, structural, and petroleum migration history of the foreland basin may be more and, perhaps locally, less complex than currently envisioned and recommend that additional detailed studies be conducted to address this issue.