Compaction Properties of Sandstones and Shales in the National Petroleum Reserve, Alaska (NPRA)—Significance for Burial and Thermal History Models

By

E.L. Rowan, P.H. Nelson, and D.O. Hayba (U.S. Geological Survey)

Models of burial and thermal history to determine the timing of petroleum generation are critically dependent on compaction curves for the principal lithologies of a basin. The rock’s porosity or water content exerts a primary control on the thermal conductivity of the bulk rock; conductivity together with heat flow determines a thermal gradient and controls the timing of source rock maturation. Comparison of calculated with measured porosity-depth values provides an important constraint on maximum burial and amounts of uplift and erosion. To refine the models that we have constructed for wells and along seismic cross-sections in the NPRA, we developed compaction curves from log-derived parameters.

Sonic travel time logs were used to calculate porosities within the siliciclastic formations at 6 inch intervals over a depth range of 400 to 16,000 ft; derived porosities range from 0 to 43%. Shale and sand fractions in each interval were determined from gamma-ray logs. The near-continuous well logs provide a more complete record of porosity than individual measurements on core plugs, generally made to characterize potential reservoir rocks. We selected four offshore wells (Antares, Fireweed, Phoenix, and Mukluk) where uplift and erosion have been minimal or non-existent to create master compaction pro- files for sandstone and shale lithologic end-members.We fit exponential curves to the porosity-depth data sets derived from geophysical logs at these wells. These porosity-depth curves were used regionally, across the NPRA, to determine burial and thermal histories for more than 40 wells including many in areas of major uplift and erosion.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.