Compaction Trends and Implications for Uplift, National Petroleum Reserve, Alaska

By

P.H. Nelson and K.J. Bird (U.S. Geological Survey)

Porosity decreases with depth in sedimentary basins due to the effects of compaction and diagenesis. The porosity profile in an individual well depends upon the maximum effective stress, which is usually determined by the maximum depth of burial. Consequently, present-day porosity profiles can be interpreted in terms of maximum burial and subsequent uplift. In 1992, Issler examined uplift and overpressure in the Mackenzie Delta of northwestern Canada, computing porosity in mudstones from sonic logs. Issler’s approach is extended here to include sandstones as well as mudstones, and is applied in wells within and adjacent to the National Petroleum Reserve, Alaska.

Evaluating the mudstone-rich Early Cretaceous Torok Formation, we find that the porosity-depth dependence can be fit with one or two straight lines in each well. The slopes of these lines from wells in the coastal plain are similar to those determined by Issler for Mackenzie Delta wells. However, the porosity-depth lines are offset among wells from different areas within the coastal plain, reflecting differences in uplift along the Barrow Arch. Slopes from wells in the foothills are steeper than in the coastal plain (porosity decreases less rapidly with depth in the foothills); these steeper slopes are the consequence of deep burial and subsequent uplift. When the line segments are replotted in terms of porosity against maximum burial temperature (derived from vitrinite reflectance) instead of present-day depth, the trends from different areas within the coastal plain coalesce, while the trends from wells in the foothills retain the steeper slopes characteristic of deep burial. Thus, maximum burial temperature explains some but not all of the differences in the porosity profiles.

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.