Oil Potential of the Nuiqsut Area, Colville River, Alaska North Slope
Abdel-Rahman, Mohamed A.
Drilling activities and results are analyzed with emphasis on the area’s stratigraphy and organic geochemistry. Save for a few seismic profiles examined from a CCG - VERITAS multi-client high frequency 3d seismic shoot, data utilized in the evaluation were mostly derived from the State of Alaska data releases and published work.
The Colville River area uniquely combines potential for conventional and unconventional oil accumulations. Various operators have already succeeded in discovering and flowing oil from Beauforian and Brookian sequences on both sides of the River. New acreage has been leased and eight exploration units have been established, seven of which are east of the River. Brookian targets include topsets and deep water turbidite fans. Beauforian objectives encompass the Lower Cretaceous Kuparuk C transgressive sand, the Upper Jurassic Alpine, Nisquit and Netchelik sands. Seismic data in the general area confirm seismic expression of potential conventional targets. Discoveries made to date appear to be readily identifiable and mapable on high quality seismic data aided by seismic attribute analysis.
The USGS mapped the thermal maturity windows for the three major North Slope source rocks, Shublik, L. Kingak and HRZ. The oil fairway occupies a 15-25 mile wide swath that runs ESE-WNW south of the Alpine oilfield. This highlights the potential of the area for yielding unconventional oil from the same source rocks that sourced many billions of barrels to the North Slope oilfields.
Thermal modeling work typically requires information to be input on a multitude of variables and some inferences to be made on others. By selecting one value for each entry, a map with solid boundaries will result. Considering the inherent nature of the 3-dimentional spatial variability of all input variables, a drilling result that differs from an expectation portrayed by a solid line boundary does not necessarily negate the validity of the model on which it was based. For example, activation energy for a carbonate Shublik is much lower than that for shale Shublik and if the Shublik is modeled as one entity the range of its variability will not be represented by one boundary. A Monte Carlo simulation utilizing the range of various input variables can be used and the resulting probability ranges of the various boundaries of oil and gas windows can be portrayed on maps.
At the Alpine field, Alpine reservoir and the overlying Kuparuk C sand are in pressure communication, and are filled by the same 40 API oil. This is a Kuparuk C oil gravity anomaly. Separated only by a few feet of Kuparuk D shale, the HRZ shale overlies Kuparuk C sand.
The Jurassic Lower Kingak Shale is touted, in the literature, to be the source of the Alpine oil based mostly on biomarkers ratios. Examination of published data, indicate that the Alpine oils have Norcholestanes ratios (NDR values) of 1.3 to 1.4, which correspond to the category of being “inconclusive” as age markers, according to the authority on the subject. Further work on the Alpine oil source using individual data points, and not averages, to quantify the variance among data sets before looking at the variance between sets might be in order.
The possibility that the Alpine oil may have actually been sourced, at least partly, from the overlying HRZ shale in the vicinity of where the Kuparuk C sand is filled with light oil cannot be ruled out. Such a scenario may better explain various aspects of the petroleum system in the area.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013