Margaret A. Keller¹, Joe H.S. Macquaker², Paul G. Lillis³

(1) USGS Menlo Park, Menlo Park, CA
(2) Manchester University, Manchester, United Kingdom
(3) USGS, Denver, CO

ABSTRACT: A High Resolution Study of Petroleum Source Rock Variation, Lower Cretaceous (Hauterivian and Barremian) of Mikkelsen Bay, North Slope, Alaska

The pebble shale unit and lower part of the Hue Shale comprise the Lower Cretaceous, relatively condensed, organic-rich mudstone succession of the eastern North Slope of Alaska. The pebble shale unit has been interpreted to be gas-prone in the eastern North Slope region, but oil-prone to the west, where it is considered one of several sources for the Prudhoe area oil fields. The Hue Shale, particularly the lower part, is considered a good oil-prone source rock over the whole area. To evaluate variation in source potential and lithofacies and to determine controls on their deposition in the middle North Slope region, 45 core samples from the Lower Cretaceous section of the Mobil-Phillips Mikkelsen Bay State #1 well were analyzed using Rock-Eval and microscopic techniques.

On the basis of lithology the studied section (11,664-11,606 ft) is divided into the pebble shale unit and the lower part of the Hue Shale, the Hue distinguished by a major increase upsection in the abundance of tuffaceous material. Rock-Eval data indicate that both units are primarily type II, oil-prone source rocks with similar TOC, although, apparently the pebble shale unit has better oil-generating characteristics as indicated by higher hydrogen index (HI) values. The Hue Shale, however, is actively generating petroleum (PI approximately 0.15- 0.35; S1/TOC=0.4-1.0), resulting in reduced HI values, while the pebble shale unit is not (PI < 0.1; S1/TOC approximately 0.3). These data suggest that this Hue Shale interval has a lower activation energy than the pebble shale unit, probably related to differences in their original depositional environments reflected in their varying facies.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado