VARIATION IN PETROLEUM SOURCE-ROCK POTENTIAL OF THE KINGAK SHALE, NORTH SLOPE ALASKA: GEOPHYSICAL LOG ANALYSIS INFORMED BY ROCK-EVAL PYROLYSIS

USGS petroleum resource assessment of Alaska's North Slope includes evaluation of source-rock potential of the important organic carbon-rich mudstone successions, particularly of Mesozoic and Cenozoic age. One of the major marine mudstone formations on the North Slope, the Jurassic and Lower Cretaceous Kingak Shale is a southward prograding, offlapping unit that was deposited during rifting of Alaska's Arctic margin. The aim of this study is to characterize vertical and lateral variation in source potential of this thick formation (up to 4,000 ft) by analysis of widely available geophysical log data integrated with reported Rock-Eval pyrolysis, paleontologic, and lithofacies data for core and cuttings samples.

The DeltaLogR technique (Passey and others, 1990) was used to determine profiles of total organic carbon (TOC) for 19 wells across the North Slope. These TOC results are based primarily on sonic log response, but also deep resistivity, gamma ray, NPHI and caliper logs. They show that most of the Kingak Shale contains < 2 weight % TOC (cut-off used for effective source). However, organic carbon-rich facies commonly occur in the basal part and locally in the upper part of the unit. For these 19 wells organic-rich strata in the lower part of the Kingak Shale average 2.4-3.5 wt % TOC (individual DeltaLogR values, 2.0-8.0 wt %) over a cumulative thickness of 156-360 ft per well. Rock-Eval pyrolysis results show that some samples of the Kingak Shale, particularly from the lower part have Type II kerogen, but in most wells, the overall succession above this contains primarily Type III.

For the lower part of the Kingak Shale, a distal and/or condensed depositional setting favorable for preservation of organic matter is inferred from geophysical log response, the distribution of log-determined TOC, Rock-Eval pyrolysis, lithologic descriptions of "paper shale", and paleontology. Thickness and organic richness determinations based on geophysical logs, Rock-Eval pyrolysis, and other geologic and paleontologic data indicate that thermally mature Kingak Shale had important oil and gas generation potential in this region.