USGS scientists now evaluating western North Slope oil and gas potential
for Petroleum News
The U.S. Geological Survey is embarking on what it refers to as its Western North Slope Assessment, evaluating potential undiscovered, technically recoverable oil and gas resources to the west of the western boundary of the National Petroleum Reserve-Alaska. The region in question includes the Lisburne Peninsula and extends north to Icy Cape on the Chukchi Sea coast. The western NPR-A boundary extends south from Icy Cape.
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The agency anticipates publishing the results of its assessment in January.
In a Nov. 5 meeting of the Alaska Geological Society USGS geologist Dave Houseknecht provided a detailed overview of what is known about the petroleum geology of the assessment area. Houseknecht was seeking feedback from other geologists on the geologic parameters that will inform the assessment.
A series of assessmentsHouseknecht said that the new assessment forms part of a series of North Slope assessments triggered by a 2017 order by then Interior Secretary Ryan Zinke. USGS completed an assessment of the North Slope Torok and Nanushuk formations later that year. And in January 2020 the agency completed an assessment of the central North Slope. In recent years the agency has also completed assessments of potential resource recovery from North Slope oil and gas source rocks, and of potential natural gas recovery from North Slope methane hydrate deposits.
Houseknecht said that USGS had originally planned an assessment of the 1002 area of the Arctic National Wildlife Refuge after the Torok/Nanushuk assessment. However, the Department of the Interior decided to defer that assessment until 3D seismic data becomes available for the 1002 area, Houseknecht said.
And, because the 2017 Torok/Nanushuk assessment encompassed the Western North Slope Assessment area, the new assessment will not include an evaluation of those rock formations in the assessment area, Houseknecht said.
Sparse dataHouseknecht commented that the sparsity of geologic data for the new assessment area will make the assessment quite challenging. There are just three wells in the area: the Eagle Creek, Akulik and Tungak Creek wells. The nearest deep well is the Tunalik well, in the NPR-A a few miles east of Icy Cape. Given the nature of the regional geology, the Klondike well, some distance offshore in the Chukchi Sea, can provide valuable information. There are two important rock outcrops in the assessment area at Redwul and Surprise Creek. Another outcrop, the Kokolik River oil sandstone, lies a short distance inside the NPR-A. Seismic data are limited to about a dozen 1971 vintage 2D seismic lines, Houseknecht said.
Fascinating geologyHowever, the geology of the area is fascinating, lying at the intersection of regional geologic structures of the Chukchi Sea and the western North Slope. The Hanna Trough, a major northwest to southeast trending feature of the Chukchi, trends into the assessment area, where it morphs into a major hollow in the ancient basement rocks that underlie the region. The sedimentary basin to the north of the Brooks Range merges westward into this broad structure. And within this geologic setting lie many of the rock formations familiar elsewhere on the North Slope, albeit with features and characteristic impacted by this particular southwestern North Slope setting.
The Ellesmerian rock sequence, for example, the sequence holding the giant Prudhoe Bay oilfield to the northeast, consists in the assessment area of a series of faulted slabs pushed north by the uplift of the Brooks Range. Here the Saddlerochit Group that hosts the Prudhoe Bay reservoir consists mainly of shale and has some oil and gas source potential. The Shublik, a prolific North Slope oil source rock, is also found in the assessment area and is exposed at Surprise Creek in what appears to be a transition into the Otuk formation, the equivalent formation in the Brooks Range foothills.
Evidence indicates that the presence of rich, thick oil source rocks, spanning the rock sequence from the Shublik up to the base of the Brookian that contains the Nanushuk and Torok, would have resulted in an oil generation kitchen, with oil migrating northward, upwards into a geologic high referred to as the Arctic Platform, Houseknecht commented.
The Nanushuk and Torok formations lie in the Brookian sequence, the youngest and shallowest of the North Slope petroleum bearing rock sequences. In the Brookian of the Western North Slope assessment area, the Mount Kelly tongue of the Fortress Mountain formation lies below the Nanushuk and Torok. There is evidence that in the assessment area the Nanushuk and Torok were deposited in the southwestern extremity of the basin in which these formations were deposited in the northern part of NPR-A.
Some rocks of the Beaufortian sequence, the sequence that hosts the Kuparuk River and Alpine fields, lie between the Mount Kelly Tongue and the underlying Ellesmerian strata.
Oil and gas potentialThere are questions regarding the thermal maturity of the various source rocks in the region, with the older rocks appearing to have mainly been subjected to temperatures where gas rather than oil would have formed. There is the potential for structural oil and gas traps to have formed in the Ellesmerian rocks south of Surprise Creek. However, there appears to be only limited potential for discovering large structural traps below the Torok in the more northerly part of the area, Houseknecht said. He also commented that data from well penetrations relevant to this area suggest poor oil and gas reservoir quality below the Torok.
Houseknecht also presented some initial ideas on how the USGS may subdivide the Western North Slope area for the purposes of carrying out the new assessment. He sees little oil and gas potential in the Lisburne Hills, in the southwestern corner of the area, and does not anticipate conducting a quantitative assessment of this part of the area. A second southern part of the area to the east of the Lisburne hills has some trap potential in the Ellesmerian, although the reservoir quality appears poor. The rocks in more northerly part of the area have good potential as an oil and gas source but, below the Brookian, appear to have poor potential for trapping hydrocarbons and forming reservoirs. Houseknecht said that he is starting to consider this as a source area for oil and gas that would have migrated north, and perhaps northeast. The Brookian in this part of the area has reservoir potential but is excluded from the assessment, having already been assessed in 2017.
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