QUANTITATIVE MODELING OF PETROLEUM SYSTEMS ON THE NORTH SLOPE OF ALASKA

The objectives of this work were to (1) locate pods of active source rock, (2) determine critical moments when petroleum was expelled, (3) identify pathways for petroleum migration, and (4) predict volumes, phases, and compositions of major accumulations within a 620 by 250 km area on the prolific North Slope of Alaska. Four significant petroleum systems in the area originate from source rocks in the Mesozoic Shublik Formation, Kingak Shale, pebble shale unit, and Hue-gamma ray zone. Input for modeling using PetroMod software included mapped distributions of reconstructed hydrogen index and total organic carbon for each source rock, activation energy distributions measured on immature equivalents of each source rock, and heat flow histories calibrated using vitrinite reflectance and corrected bottom hole temperatures in control wells.

The modeling shows that progradational deposition of the Cretaceous-Tertiary Brookian sequence from west (120 Ma) to east (33 Ma) resulted in western and eastern depocenters, where the underlying source rocks became mature at different times. Petroleum generation began during the Early Cretaceous in deeply buried rocks in the southwestern part of the study area. Changing geometry of the sedimentary succession resulted in a complex petroleum migration history. Deposition of Nanushuk-Torok Formation induced subsidence in the southwest that drove migrating petroleum to the east. Accumulations remained on highs in the Prudhoe Bay and Simpson Peninsula areas, fed by petroleum charge from the south. Beginning about 40 Ma, subsidence in the eastern depocenter drove migrating petroleum to the west toward Prudhoe Bay. Liquid accumulations in the west degassed to form free gas phases due to uplift and pressure release. Fully PVT-controlled n-component migration modeling allowed simulation of phase behavior for all components during each time step. Phases and compositions were predicted for each major accumulation for both in situ and surface conditions and compared to known accumulations.