OVERPRESSURE IN THE COLVILLE FORELAND BASIN OF ARCTIC ALASKA

Subsurface pressure estimated from mud weights in exploration wells is used to investigate the presence, spatial distribution, and magnitude of overpressure (pressure exceeding hydrostatic; 0.465 psi/ft) in the Colville Foreland Basin of Arctic Alaska. Pressure estimates from mud weights are corroborated with empirical pressure measurements from drill-stem and other pressure tests, as well as other overpressure indicators such as low sonic velocity and low resistivity in mudstone intervals and magnitude of mud-log gas shows.

Each well analyzed is categorized as encountering with depth (1) no overpressure, (2) a single transition into overpressure, (3) multiple transitions into progressively higher overpressure, and (4) abrupt steps into lower pressure. The top of overpressure is defined by a transition zone between a nearly hydrostatic pressure gradient and a deeper, nearly uniform pressure gradient greater than 0.6 psi/ft. This transition zone is characterized by an abrupt increase in pressure with depth and a local pressure gradient greater than 1.0 psi/ft.

Most wells in the northern, shallower, lower thermal maturity (mainly oil window), and undeformed part of the basin are normally pressured whereas wells in the southern, deeper, higher thermal maturity (mainly gas window) and structurally deformed part of the basin transition into one or more progressively higher-pressure zones at depths ranging from 1,000 to 14,000 feet. Mechanisms inferred to contribute to overpressure in various parts of the Colville Foreland Basin include compaction disequilibrium, hydrocarbon (particularly gas) generation, tectonic contraction, and relatively young uplift of sealed pressure compartments. Thick, predominantly mudstone, deep-water successions in the Jurassic Kingak Shale and Cretaceous Torok Formation commonly include seals associated with overpressure.

Wells in the anomalous Point Thomson area near the eastern end of the Barrow Arch and just south of the tectonic hinge of the Canada Basin transition abruptly into highly overpressured zones (>0.8 psi/ft) at depths around 10,000 ft. This anomalous pressure is inferred to be associated with migration of high-pressure gas and other fluids from the southern Canada Basin.