In the 40-year history of hydrocarbon exploration on the Alaskan North Slope, about 21 accumulations with a total in-place volume of more than 60 billion bbl of oil and 35 trillion ft³ of gas have been discovered. Although the density of exploratory drilling in this region is not uniform, enough drilling has been done to show a distinct concentration of oil and gas in the Prudhoe Bay area between the Colville and Canning Rivers. This concentration is also evident when the Prudhoe area resources are compared with the USGS estimates of undiscovered in-place oil and gas resources of the adjacent areas, the National Petroleum Reserve in Alaska and the Arctic National Wildlife Refuge. Most oil and gas in the Prudhoe area accumulated near the present coastline in res rvoirs that overlie a southeasterly plunging basement ridge, the Barrow arch. The location of these accumulations, in low-relief structural-stratigraphic traps midway along the arch and downdip from its apex at Point Barrow, is the paradox.

An answer to this paradox is provided by analysis of two cross sections, one along the Barrow arch and one perpendicular, showing their original structural positions for the beginning, middle, and end of Cretaceous time. In the Early Cretaceous (mid-Neocomian), the crest of the Barrow arch was near sea level along its entire length. Because of northeasterly sediment progradation during later Cretaceous time, the Barrow area became more deeply buried than the Prudhoe area, thus making the Prudhoe area the focal point for migrating oil and gas. Beginning in the early(?) Tertiary, the Barrow area was slowly uplifted while the Prudhoe area subsided, thus beginning the process that resulted in the reversal of their relative elevations and the focus for migrating oil and gas. Studies show t at the Prudhoe Bay field was tilted during the Tertiary, and some oil and gas escaped, migrated toward Barrow, and was trapped in the Kuparuk, West Sak, and Ugnu fields. This analysis suggests that most North Slope oil and gas were generated during the Cretaceous.

End_of_Article - Last_Page 656------------