Source-Rock Evaluation of the Lower Cretaceous Pebble Shale Unit, Northern Alaska

Abstract

The Lower Cretaceous pebble shale unit (PSU), formally known as the Kalubik Formation in the subsurface, is an important petroleum source rock in northern Alaska. The PSU is investigated for source-rock potential at three localities adjacent to and within the Arctic National Wildlife Refuge including: 1) along the west side of the Canning River, 2) along an unnamed tributary east of the Katakturuk River, and 3) at Marsh Creek. Samples were collected and analyzed for total organic carbon (TOC), Rock-Eval, and vitrinite reflectance (Ro), and compared to previously published data from the Mikkelsen Bay State # 1 well. Outcrop data indicate that there are 12- to 31-mthick packages of the PSU that have adequate organic content (2-6 wt. percent TOC) to be considered a good-toexcellent source rock, which is consistent with the Mikkelsen Bay State #1 well (3-6 wt. percent TOC). Rock-Eval data from outcrop samples, however, suggest poor source-rock quality (Hydrogen Index [HI] < 50mg Hydrocarbon [HC]/g TOC) likely caused by elevated thermal maturity as indicated by high Tmax and Ro values. The average vitrinite reflectance from outcrop samples range from 1.28 to 1.79 % Ro, indicating latest oil to gas window maturity. Results are consistent with previous studies indicating that the PSU originally was a good source-rock; however, because of high thermal maturity in the study area the source-rock quality is degraded. A thermal maturity map based on publicly available Ro values from outcrop and subsurface samples of the PSU and high-radioactive zone (HRZ) of the overlying Hue Shale delineates where this source rock interval falls within the oil, gas, and dry gas windows across northern Alaska. Regions that are overmature correlate with deeper parts of the Colville Basin foredeep. High thermal maturity in the southern portion of the Colville Basin, and those recorded in the foothills of the Brooks Range, are interpreted to result from thrust loading and deep burial related to thick accumulations of Brookian sediment. The Barrow Arch was a structural high along the north margin of the Colville Basin, causing a region of low accommodation. Better source rock quality associated with lower Ro values occurs along a northwest-southeast trend consistent with the Barrow Arch structural trend. This investigation narrows the region on the North Slope where liquid petroleum generation from the PSU and HRZ is most plausible, and will help industry avoid areas containing dry gas.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019