Print this pageIndicators of Formation Overpressure in Brookian Rocks, Brooks Range Foothills of Arctic Alaska
Philip H. Nelson1, David W. Houseknecht2, Daniel
O. Hayba2, and W. Matthew Burns2
1 U.S. Geological Survey, Denver, CO
2 U.S. Geological Survey, Reston, VA
The Lower Cretaceous-Tertiary Brookian megasequence consists of a series of clastic depositional sequences derived from the Brooks Range and deposited to the north and east in the Colville Foreland Basin. Each individual sequence consists of a vertical succession of facies, in ascending order: (1) a basal condensed section with good to excellent source-rock character, (2) a base-of-slope turbidite system containing varying proportions of sandstone and mudstone, (3) a relatively thick marine slope section consisting mostly of mudstone, and (4) a marine shelf-to-shore system that includes sandstone and mudstone. Indicators of overpressure and/or undercompaction in deep wells in the southern half of the basin are observed in facies 1, 2, and 3.
Acoustic and resistivity logs display characteristics associated with undercompacted sediments. That is, velocity and resistivity values within deeply buried mudstones are considerably less than would be expected from extrapolation of velocity and resistivity gradients from shallower, normally pressured mudstones. In five wells penetrating Brookian strata in the foothills, these velocity and resistivity reductions are associated with mud weights exceeding 13 pounds per gallon, vitrinite reflectance values greater than 1.0 percent, and numerous gas shows (C1 >20,000 ppm). In these wells, the low-velocity, low-resistivity zones, which are approximately 2,000 ft thick, are overlain by sand-prone sections. However, in some wells the top of overpressure, as indicated by mud weights and formation pressures determined from drill-stem tests, is located thousands of feet above the top of the low-velocity, low-resistivity zone. To explain this discrepancy, we postulate that the low-velocity zones originated as a result of disequilibrium compaction during initial deposition. These low-velocity zones thus represent “paleo-geopressure” that formed during the rapid deposition of thick, fine-grained sections. Present-day overpressures are a consequence of gas generated from organic-rich sediments buried to sufficient depths.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005