ABSTRACT

Lithic arenites and wackes of the National Petroleum Reserve in Alaska contain a significant volume of ductile grains and have suffered substantial physical compaction with burial. As determined by stratigraphic studies, these rocks were buried to maximum depths of 500 to 4,400 m. Initially the grain fraction (grain volume as a percent of total rock volume) must have been near 59% but increased to over 90% at depth. Statistical analysis reveals that the grain fraction (GF) is primarily a function of burial depth in meters (d), percent ductile grains such as phyllite rock fragments (P), and matrix content (M); multiple regression suggests the following linear equation:

GF = 90 + 0.23P - 0.72M + 0.0018d.

Simple grain rearrangement operates to a depth of 550-800 m and accounts for a 25% increase in grain fraction. At greater depths compaction is by plastic deformation. However, the increase in grain fraction appears to fall off considerably below 1,000 m, and compaction due to plastic deformation increases grain fraction on average by only 7%. Early quartz cementation never progressed beyond a beginning stage and thus was ineffective at preventing compaction. On the other hand, clay matrix drastically impedes compaction. Grain fraction for lithic wackes with an abundant matrix never exceeds 83%.

The compaction law derived in this study may prove useful in further exploration for petroleum reservoirs. For example, lithic arenites with a low ductile content should retain an intergranular porosity of 6% to a burial depth of 2,000 m.