ABSTRACT: Borehole Gravity Surveys in the Cretaceous-Tertiary Sagavanirktok Formation, Kuparuk River Oil Field, Alaska

Larry A. Beyer

Detailed borehole gravity surveys (sponsored by the U. S. Department of Energy) were made in three wells in the Kuparuk River and westernmost Prudhoe Bay oil fields, Alaska, from depths as shallow as 15 m (50 ft) to as great as 1340 m (4397 ft) through permafrost and underlying heavy oil-bearing sandstones of the Sagavanirktok Formation. A subbituminous coal-bearing sequence and the stability field for methane hydrate occur partly within and partly below the permafrost zone, whose base, defined by the 0°C isotherm, varies from 464 to 564 m (1521 to 1850 ft). The surveys provided accurate, large-volume estimates of in-situ bulk density from which equivalent porosity was calculated using independent grain and pore-fluid density information. This density and porosity da a helped to define the rock mass properties within the hydrate stability field and the thermal conductivity, seismic character, and compaction history of the permafrost.

Bulk density of the unconsolidated to poorly consolidated sections ranges mostly from 1.9 to 2.3 g/cm³. The shallow permafrost section appears to be slightly overcompacted in comparison to similar sedimentary sequences in nonpermafrost regions. The cause of this apparent overcompaction is unknown, but may be due to freeze-thaw processes that have similarly affected sea floor and surficial deposits elsewhere in the Arctic. Fluctuations of bulk density appear to be controlled principally by (1) textural variations of the sediments, possibly exaggerated locally within the permafrost zone by excess ice, (2) presence or absence of carbonaceous material, and (3) type of pore-fluid (water-ice vs. water vs. hydrocarbons). As hypothetical models predict, bulk-density is slightly low r opposite one interval of possible methane hydrate. Porosity may be as high as 40-45% for selected coarser grained units within the permafrost zone, and as high as 30-35% in a series of well-sorted, heavy oil-bearing sandstones in the lower part of the Sagavanirktok Formation.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990