ACCUMULATION HISTORY, FACIES ANALYSIS, AND RESERVOIR POTENTIAL OF THE CARBONIFEROUS LISBURNE GROUP, UPPER NANUSHUK RIVER DRAINAGE, CENTRAL BROOKS RANGE, ALASKA

The Carboniferous Lisburne Group is a succession of carbonate rocks that has been identified in surface exposures and the subsurface across northern Alaska. The paleogeographic setting ranges from a carbonate ramp (northeast) to basin (southwest) and records at least five third-order depositional sequences in the Brooks Range.

This study is based on field stratigraphic, petrographic, and conodont data. 75 thin sections were examined to interpret facies and depositional environments. Thin sections were blue epoxy impregnated and stained to evaluate porosity and dolomitization.

The >1300 m thick Nanushuk River section of the Lisburne Group includes the Wachsmuth Limestone (430m) and Alapah Limestone (881m). It was measured, sampled, and described for this study. This section is notably thicker compared with most Lisburne Group outcrops to the east and west, suggesting a significant change in subsidence history from other parts of the Brooks Range.

The Nanushuk River section records carbonate ramp sedimentation that initiated as siliciclastic sediment input of the underlying Kayak Shale gradually tapered off. The lower Wachsmuth records open marine followed by shallow shoal facies. These are succeeded by gradational slope and deep-water sediments, after which the open marine sedimentation of the Alapah ensued. The section is overlain by the Permian Siksikpuk formation. New conodont sampling in the Wachsmuth yielded only long-ranging species including Polygnathus communis (Kinderhookian-Osagean). A late Chesterian to early Morrowan age is assigned for the uppermost Lisburne Group by Gnathodus defectus and Rhachistognathus muricatus. This is consistent with recent ages reported from the uppermost Lisburne at Skimo Creek.

The potential petroleum reservoirs of the Wachsmuth and Alapah are skeletal shoals. This facies exhibits porosity in packages >12 m thick near sequence tops 1-3. Some dolomites in the Wachsmuth reveal intercrystalline and fracture porosity that may have been favorable to hydrocarbon migration.

Variation in accumulation history and subsidence rate may imply a tectonic control. This has important implications for understanding the oil and gas potential in the subsurface north of the central Brooks Range and the Carboniferous paleogeography of northern Alaska.