GEOLOGIC MAP OF THE UMIAT–GUBIK AREA, CENTRAL NORTH SLOPE, ALASKA—INTEGRATION OF FIELDWORK AND SUBSURFACE DATA IN A REGION OF KNOWN OIL AND GAS ACCUMULATIONS

The integration of recent fieldwork with available subsurface data has rendered a new geologic map (1:63,360) of the hydrocarbon-bearing Umiat–Gubik region of the central North Slope. The map spans ~2100 km²and lies at the northern extent of the Brooks Range foothills fold-and-thrust belt in the Colville foreland basin.

Cenomanian–Maastrichtian strata of the Nanushuk, Seabee, Tuluvak, Schrader Bluff, and Prince Creek Formations locally constitute an ~2-km-thick succession that crops out sparsely in the dominantly low-relief, tundra-mantled region; however, exceptional cutbank exposures occur locally. The stratigraphy comprises chiefly marine (Nanushuk, Tuluvak, Schrader Bluff) and nonmarine (Prince Creek) topsets and slope clinoforms (Seabee). This work benefits from and further elucidates recent sequence stratigraphic advances in understanding how this part of the Colville basin was filled during Late Cretaceous time. As an example, the geologic map of this study is the first of the Umiat–Gubik area to distinguish that the three local Schrader Bluff Formation members underlie a regionally significant mid-Campanian unconformity and are entirely older than Schrader Bluff strata that crop out east of the formation’s Anaktuvuk River type section.

The map reveals a series of west- to northwest-trending gentle folds with km-scale wavelengths. Anticlines are locally thrusted and interpreted to be folded above structurally thickened and/or duplexed fine-grained successions of Torok Formation. Three culminations host known yet undeveloped petroleum accumulations at Umiat (mainly oil), East Umiat (gas), and Gubik (gas). These currently stranded discoveries occur along doubly plunging anticlinal traps and the main Umiat culmination is modified by what we interpret as south-dipping reverse faults.

The geologic map was prepared through assimilation of our field data, aerial and satellite imagery, seismic data, and well logs. Near surface formation picks are available or derived for most of the area’s 24 exploration wells and cross sections are constructed along lines of section approximately coincident with publicly available 2D seismic data. This integrative approach yields an improved understanding of the Umiat–Gubik area geology that is constrained by numerous and corroborating datasets.