Depth to 0.6% Vitrinite Reflectance Defines Magnitude of Uplift in Middle Ground Area of the North Slope, Alaska

Abstract

Tectonic uplift plays a critical role in the formation of an unconventional accumulation. Perhaps most fundamentally, uplift removes significant thickness of overburden rock so that natural fracturing can occur in the brittle, thermally mature source rock. Uplift also allows for the development of overpressure that keeps gas in solution and thus provides reservoir drive. Finally, uplift removes thermal stress from the system, thereby shutting off its evolution and allowing retained petroleum to evolve in-situ. To investigate the significance of tectonic uplift on unconventional petroleum systems, we examined the magnitude of uplift in the central North Slope of Alaska. Maximum burial occurred during the Cenozoic, when significant quantities of sediment shed into the Colville Basin pushed five main source rocks into the oil generation window. Of these, at least two are prospective for unconventional exploration—the Triassic Shublik Formation and the conformably overlying lowermost Kingak Shale. Significant Tertiary uplift shut off these petroleum systems. To investigate the effect of uplift in the central North Slope, we used thermal maturity data in 102 wells. We define an empirical method that assumes that the geothermal gradient is everywhere 25°C/km and that the onset temperature for petroleum generation is 100°C. Because oil-prone source rocks containing marine kerogen begin generating oil and gas at an equivalent vitrinite reflectance (Ro) of 0.6%, this method implies that oil is generated at a burial depth of 4 km, or 13,000 ft. We validate this reasoning in the Point Thomson area in the northeast, where maximum burial is occurring today, stratigraphic sections lack evidence of erosion, and well data indicate Ro of 0.6% at 13,000 ft. This is the only area of the North Slope where this is true; the remainder of the area is characterized by Ro of 0.6% at shallower depths. The depth to Ro of 0.6% varies systematically from about 13,000 ft in the east to 5,000 ft in the west, implying up to 8,000 ft of uplift from east to west on the North Slope. The area south of the super-giant Prudhoe Bay oil field where the unconventional resource potential is greatest has experienced 4,000 to 5,000 ft of uplift. These results generally agree with a previous study that calculated uplift in wellbores from porosity-depth trends observed on sonic logs, but we believe that this geochemistry-based method is directly responding to the thermal history of the section.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016