When the U.S. Geological Survey took over the drilling program in the National Petroleum Reserve in Alaska (NPRA), many explorationists expected that a major oil accumulation would be found on or adjacent to the Barrow arch. This expectation was based on the presumption that the oil source rocks are buried to great depths in the Coville trough and that the oil generated would have migrated north to be trapped in sandstone or carbonate reservoir rocks near the Barrow arch. Subsequent drilling failed to confirm this model for oil occurrence in NPRA. To date, commercial oil generated in the Colville trough appears to be limited to the Prudhoe Bay area. Understanding the geological reasons for localization of major oil occurrence has important implications for oil exploration in northern Alaska, both onshore and offshore.

Consideration of the basic requirements for oil occurrence (source, migration pathway, reservoir, trap, and seal) suggests that source rock adequacy may be a limiting factor in NPRA. Two geochemically distinct types of North Slope oil have been recognized: the Simpson-Umiat type (associated with a "pebble-shale" unit/Torok Formation source) and the Barrow-Prudhoe type (associated with a Shublik Formation/Kingak Shale source). Except for the oil from the Fish Creek 1 well and the reservoirs of Cretaceous age in the Prudhoe area, the Barrow-Prudhoe oil reservoirs are in Ellesmerian sequence rocks on the Barrow arch. The source-reservoir thermal-maturity patterns, the inferred timing of oil generation, and the structural configuration of the Ellesmerian rocks all suggest that the oil sho ld have migrated to the Barrow area as well as to the Prudhoe area. The presence of subcommercial Barrow-Prudhoe-type oil in NPRA also suggests that deficiencies in the "oil plumbing system" do not explain the lack of oil accumulations along the Barrow arch.

The large volumes of oil in the Prudhoe area, as compared to only "oil shows" in the Barrow area, are best explained by the greater amount and better quality of organic matter in the Shublik Formation and the Kingak Shale to the east of NPRA.

End_of_Article - Last_Page 669------------