Oil company interest continues in Nanushuk plays of the North Slope
Alan Bailey
for Petroleum News
Although the COVID-19 pandemic brought an early end to this winter’s exploration season on Alaska’s North Slope, there is continuing interest in oil discovery opportunities in the Nanushuk formation, the focus of major new developments at Pikka and Willow. While new exploration drilling by Oil Search and ConocoPhillips is shedding further light on this intriguing new play, companies such as Armstrong Oil & Gas and Borealis Alaska Oil Inc. are also looking to test further opportunities of a similar character to the established finds, U.S. Geological Survey geologist Dave Houseknecht commented to Petroleum News in a recent overview of Nanushuk exploration activity. The most recent major leasing activity was Armstrong’s purchase of a significant block of leases in the 2019 NPR-A leases sale, Houseknecht said.
Part of the Brookian
The Nanushuk forms part of the Brookian sequence, the youngest and shallowest of the major North Slope petroleum bearing rock sequences. Although rocks of the Brookian are found across the entire North Slope, the Nanushuk is found mainly in the more westerly part of the region. The rocks of the formation were generated from sediment pouring from the west into an ancient marine basin. As these sediments accumulated on and down the basin margin, the margin migrated from west to east, eventually coming to a halt at what is referred to as the ultimate shelf margin, a few miles to the east of what is now the Colville River delta.Following the discoveries of the Prudhoe Bay and Kuparuk River fields in rock reservoirs much older and deeper than the Nanushuk, subsequent exploration mainly focused on these deeper rocks, with Brookian strata such as the Nanushuk generally being ignored. Until that is 2015, when Armstrong Oil & Gas Inc. and Repsol E&P USA Inc., taking a contrarian view of conventional North Slope exploration strategies, made the Pikka discovery to the east of the Colville delta. That discovery upended expectations for potential oil volumes in the Brookian.
The Nanushuk oil reservoirs consist of a series of sand bodies that were deposited at the top and down the upper slope of the ancient marine basin margin, as that margin migrated east. While the sand bodies can be identified in seismic cross sections, anomalously high seismic amplitudes, particularly in high resolution 3D seismic, can point to potential oil accumulations.
Houseknecht said that the potential oil reservoirs have distinctive geometries. Some, such as those of the Pikka discovery, tend to be thick but narrow, fairly elongated and generally oriented north to south. Others, such as ConocoPhillips’ Willow discovery tend to be more oval in shape. These different geometries may lead to different reservoir performances.
Continuing exploration
There has been continuing exploration drilling along the trend of that original Pikka discovery. An oil discovery by the Horseshoe well, drilled in 2017 by Armstrong and Repsol to the south of Pikka, established what became referred to as the Pikka-Horseshoe trend. Evidence of pressure communication in the oil along this trend suggests the existence of an interconnected reservoir system.Successful nearby exploration drilling by ConocoPhillips has resulted in what that company refers to as the Narwhal trend. USGS now refers to this complete system of discoveries and associated sand bodies as the Pikka-Narwhal-Horseshoe trend.
However, although Armstrong has indicated that an oil gravity gradient suggests that the reservoirs at Horseshoe and Pikka are in communication with each other, there has been no public statement regarding possible pressure communication between Horseshoe and the nearby Putu and Stony Hill exploration wells that enabled ConocoPhillips to identify its Narwhal trend, Houseknecht said.
And the possibility of reservoir communication between these various discoveries is intriguing, given the potential for significant complexities in lease unification, development strategies and the determination of resource ownership. Moreover, with the likely need for hydraulic fracturing of the relatively low permeability reservoirs, there is the potential for pathways to open up between adjacent reservoir rock bodies, Houseknecht commented.
Also, the shelf margins that form the reservoirs tend to be more separated to the south, while tending to converge to the north, thus leading to complex reservoir geometry, he added.
To the west of the Colville River delta, in the northeastern National Petroleum Reserve-Alaska, ConocoPhillips had been conducting drilling in its Willow and West Willow Nanushuk discoveries. This winter the company drilled two further appraisal wells, leading to a total of 12 wells in the discoveries and presumably meaning that the discoveries are now well delineated, Houseknecht said.
Publicly available information suggests recoverable oil volumes of 1,000 million to 1,500 million barrels in the Pikka-Narwhal-Horseshoe trend and 400 million to 750 million barrels in Willow and West Willow, Houseknecht said.
This winter’s exploration
This winter Oil Search drilled two exploration wells to the east of the Colville River: the Mitquq and Stirrup wells. The Mitquq well, to the east of Pikka, encountered oil both in the Nanushuk and in deeper Alpine sands. In the Nanushuk the well penetrated 29 feet of gas and 143 feet of oil, but no water, Houseknecht said. The absence of water is intriguing, suggesting that there could be a reasonably sized oil accumulation to the south, depending on how far the reservoir extends, he said. Curiously, the well does not appear to be associated with any seismic amplitude anomaly depicted on Oil Search’s publicly published maps, he said.The Stirrup well on the other hand, located west of the Horseshoe well and some distance southwest of Pikka, is at the northern end of a seismic anomaly that has appeared in an Oil Search publication. The Stirrup well also flowed oil from its Nanushuk target. Intriguingly, Oil Search’s anomaly map for Stirrup overlaps with the northern end of an anomaly mapped by ConocoPhillips, raising questions regarding whether both anomalies represent a single reservoir, Houseknecht commented.
ConocoPhillips drilled one Harpoon well this winter, in a Nanushuk prospect in NPR-A, southwest of Willow. The company has not released any information about the results of this drilling, although there is a rumor that an oil-saturated core was recovered from the well, Houseknecht said. The drilling ended early. But was this because of COVID-19 or because of disappointing findings?
High potential farther west
Some distance west of the area where the Harpoon well was drilled, USGS has used relatively low quality 2D seismic data to identify three large north-south trending exploration play zones where the agency sees significant promise for the discovery of further Nanushuk prospects. One of the surveys, the Ikpikpuk survey, is modern, does contain really good quality data and is available from the Alaska Department of Natural Resources through the state tax credit program.Houseknecht said that he is aware that a few companies have been reprocessing the old data. Armstrong’s lease blocks intersect all three USGS play zones and the company is planning 3D surveying next year. As previously reported in Petroleum News, Armstrong has also bought into some of the Borealis acreage, to the west of Harpoon, thus confirming the company’s bullish view of the exploration outlook in the area.
One impediment to exploration in this part of the NPR-A is the withdrawal from oil and gas leasing of lands to the south, west and east of Teshekpuk Lake. Because of environmental concerns the Department of the Interior withdrew the area around the lake from leasing as part of an NPR-A integrated activity plan issued in 2013. When in 2017 then Interior Secretary Ryan Zinke ordered USGS to conduct updated oil and gas assessments of the North Slope, he also directed the Bureau of Land Management to reevaluate that integrated activity plan. However, a revised plan has yet to emerge.
There are some wells drilled by FEX prior to 2013, and some lease blocks, in the closed area to the west of the lake.
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