Abstract: Rock-Eval and Petrographic Results for a Lower Cretaceous, Subsurface Mudstone Succession, Northeastern Alaska: Implications for Petroleum Source Potential and Provenance

KELLER, MARGARET, USGS, Menlo Park, CA; JOE MACQUAKER, University of Manchester, Manchester, UK

To evaluate petroleum source-rock variability over the relatively condensed, mudstone dominated, Lower Cretaceous of the North Slope of Alaska we collected 45 mudstone samples from 70 ft of continuous core from the Mobil-Phillips Mikkelsen Bay State #1 well. Rock-Eval pyrolysis and petrographic results indicate that 3 stratigraphic groups are present. TOC in the lowest stratigraphic interval averages approximately 1.0 wt %, with correspondingly low average S2 of 1.1 mg HC/g rock, hydrogen index (HI) of 103 (64-159) mg HC/g TOC, and production index (PI) of 0.43. In contrast the samples from the middle part of the succession have much higher TOC values averaging 4.0 wt %, and source potential is also good with average values for S2 of 13.6, HI of 336 (278-449), and PI of 0.09. The upper group has an average TOC of 3.5 wt %, S2 of 7.2, HI of 204 (59-397), and PI of 0.23. Maturity based on Tmax is similar throughout the section with an average of 436 °C.

Graphs of S2 versus TOC (Langford and Blanc-Valleron, 1990) indicate that our lower and middle groups of samples (placed within the pebble shale unit) are highly positively correlated (r=0.94) with an average HI of 420. Samples from the upper group (placed within the Hue Shale), excluding 4 outliers, are also highly positively correlated (r=0.89) indicating a different average HI of 310. For the middle and upper groups these average HI’s suggest Type II, oil-prone kerogens, and the S2 values greater than 5 indicate that 32 of 36 samples have good source-rock potential. These data and our lithofacies analysis suggest that samples from the lower and middle stratigraphic groups have similar provenances but differ in either or both the extent of clastic dilution and bioturbation at the time of deposition in contrast to the upper group of samples that probably have a different provenance.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California