Pikka paper publishedArmstrong and Repsol geoscientists provide details about the Nanushuk discovery Alan Bailey for Petroleum News
Geoscientists from Armstrong Oil and Gas and Repsol USA have published a major American Association of Petroleum Geologists paper with comprehensive information about the discovery, features and characteristics of the Pikka oil field on Alaska’s North Slope. The report provides insights into the discovery, geology and production potential of the find.
North Slope discoveries The massive discovery lies in the Nanushuk formation, a rock formation within the Brookian sequence, the youngest and shallowest of the major oil bearing rock sequences on the North Slope. The major Prudhoe Bay and Kuparuk River fields, the discoveries of which initially drove the Arctic Alaska oil industry, have reservoirs in older and deeper rocks. And so, following the discovery of those fields, explorers became intent on making similar finds in deeper rock sequences, thus bypassing the investigation of shallower Brookian rocks. Until relatively recently, that is.
The authors of the new paper point out that there was an element of bad luck in that early exploration, in that some exploration wells near Pikka happened to be drilled at locations that missed the major oil pool that was discovered more recently. In addition the Pikka discovery lies almost entirely below the Colville River flood plain, a factor that renders seismic surveying in the area of the discovery especially challenging. Also, older seismic surveys were designed to delineate deeper potential oil reservoirs.
A shift to Nanushuk interest However, in 2008 Armstrong Energy LLC and GMT Exploration assembled a large acreage lease block in the central North Slope, with Repsol joining this exploration venture in 2011 - Armstrong had begun exploring the region between the Kuparuk River and Alpine fields in 2001. The initial focus in the new lease block was on sand bodies within the Kingak shale, Jurassic in age and relatively deep within the rock sequence. However, the start of oil production in 2008 from the Qannik sandstone in the Nanushuk formation overlying ConocoPhillips’ Alpine field led to an exploration focus on Brookian targets, particularly in the Nanushuk.
Seismic data, in combination with existing well data, demonstrated that sands of the Nanushuk had been laid down as a series of bodies, sigmoidal in cross section, across and down the upper edge of the shelf of an ancient marine basin. The flow of sediment from the west into the basin caused the shelf margin to progressively move towards the east, leaving in its wake a series sigmoidal “clinoforms,” consisting of sand bodies dipping towards the east and elongated in a north-south direction.
It was possible to use existing well and seismic data to map individual sand bodies within the system. For identification, Armstrong assigned numbers to each body, with the Qannik sandstone being designated as the Nanushuk 2 zone. A thin sandstone above the Nanushuk 2 appeared to be oil saturated in many Colville River wells and was designated as Nanushuk 3, the AAPG paper says.
A major discovery In 2013 the new exploration partnership drilled the Qugruk 3 well to the east of the Qannik pool, to test the Nanushuk 3 and a deeper and older target in Alpine sandstone - earlier drilling in combination with seismic data had suggested a thickening of Nanushuk 3 to the east, outboard of the ancient Nanushuk 2 shelf edge. The new well encountered more than 250 feet of oil-saturate sandstone in the Nanushuk 3. The subsequent drilling of a sidetrack well confirmed both the presence of Nanushuk 3 and its reservoir quality. The drilling of Qugruk 1 in the same year demonstrated the extension into the area of the oil bearing Nuiqsut sandstone from the Oooguruk field under the Beaufort Sea to the northeast. And the Qugruk 5 well discovered oil in the Alpine sandstone.
In the winter of 2014-15 the exploration team acquired 3D seismic to the south of the initial discovery, along the trend of the discovery, in what was referred to as the Horseshoe block. In 2017 Armstrong, by then operator of the exploration joint venture, drilled the Horseshoe 1 well, to test a seismic anomaly in the Nanushuk 3, contiguous with the anomaly tested by the Qugruk wells but about 21 miles to the south. The Horseshoe well encountered nearly 70 feet of oil-saturated Nanushuk 3 sandstone - a Horseshoe 1 sidetrack well also confirmed the presence of oil saturated sandstone, the AAPG paper says.
The use of modern seismic technologies to generate high-resolution 3D surveys helped image the shallow Nanushuk oil reservoirs, and became a factor, for example, in the decision to drill the Horseshoe well, the paper says.
In 2018 ConocoPhillips Alaska found oil in the Nanushuk from the drilling of the Putu 2 and 2A wells, and the Stony Hill 1 well, in the area between the Qugruk and Horseshoe wells.
And in 2018 Oil Search Ltd acquired an interest in the exploration joint venture. In 2019 a delineation drilling program was underway in what was by then the Pikka unit.
Geologic characteristics The reservoirs in what is now termed the Pikka field are in the Nanushuk 2 and Nanushuk 3 zones, with most of the oil reserves in the Nanushuk 3 zone. The potential for trapping large quantities of oil in the sand bodies critically depends on the manner in which the sand was deposited. And that in turn depends on the flow of sand into the ancient marine basin together with the rise and fall of the ancient sea level. The exploration research has determined that the Nanushuk 3 was deposited as a shelf-edge river delta, approximately 3.1 miles wide and more than 33 miles long. However, there is no evidence of distributary water channels feeding the delta - it appears that the sand was transported as a sheet across an underwater shelf.
Although the Pikka field is located on a broad anticlinal structure, plunging gently to the east, the Nanushuk 2 and 3 oil reservoirs have what are referred to as stratigraphic traps - the trapping of oil as a consequence of the sequence of sedimentary deposits rather than of the folding or faulting of the strata. Essentially, oil in the Nanushuk sand bodies is trapped in place by adjacent shale strata. An evaluation of the reservoirs has revealed four rock facies - a transgressive facies with very fine-grained sandstone or siltstone, a massive sandstone facies, a laminated facies with thin-bedded sandstone and siltstone, and a blocky slope sandstone facies that has both massive sandstone and finer grained sandstone or siltstone.
Reservoir and oil properties Rock core samples have demonstrated reservoir porosities ranging from 12% to 29%. with an average of 20%, and permeabilities ranging from less than 1 to 690 millidarcies, with an average of 47 millidarcies, the AAPG paper says.
A detailed analysis of seismic data has been providing insights into various characteristic of the reservoir, including the locations of the different rock facies, and hence the estimation of oil resources and the evaluation of development plans.
The exploration joint venture collected downhole fluid samples from wells and measured formation pressures. Oil gravities are relatively light, ranging from 23 to 32.5 API, while the gas to oil ratios range from 262 to 442 standard cubic feet per barrel. The API gravity tends to decrease with depth (lower API gravity oils have higher viscosities).
A 740-foot hydrocarbon column The manner in which the API gravity drops with depth suggests the presence of a 740-foot hydrocarbon column in the Pikka field, the AAPG paper says. A 130-foot gas column at the top is underlain by an oil column at least 610 feet thick. The depth of the gas-oil contact ranges from 3,990 feet to 4,000 feet below sea level. An analysis of more than 100 downhole reservoir pressure measurements from 14 wells across the entire 30-mile by 10-mile extent of the Nanushuk in the Pikka field supports the interpretation of the discovery as a single hydrocarbon accumulation whose pressure has equilibrated over geologic time.
An analysis of oil samples has concluded that the bulk of the oil originated from the Shublik formation, a major oil source rock under the North Slope, with a minor contribution from the Kingak shale, the AAPG paper says.
Estimated resource volumes Reservoir modeling using a combination of seismic and well data indicates the presence of 3,218 million barrels of oil in the area of the planned phase 1 Pikka development. Recoverable resources have been estimated in the range 800 million to 900 million barrels. The original oil in place for the entire field area is estimated to be in the range 9.7 billion to 14.8 billion barrels, with total recoverable oil in the range 1.9 billion to 4.4 billion barrels. However, there are some important uncertainties in these estimates, including uncertainty over the presence and quantity of sand in a northern area where there has been subsurface slumping, and uncertain oil recovery efficiency in some of the reservoir rock facies.
In 2015 the exploration joint venture conducted a horizontal production test well using the Qugruk 301 well, producing more than 30,000 barrels of oil at rates up to 4,600 barrels of oil per day over a period of 18 days. The development plan for the field involves the use of horizontal production wells, with production supported by a combination of submersible pumps and gas lift. A number of injection wells equal to the number of production wells will maintain the reservoir pressure, with water injection initiated along with oil production. Under this scenario, the upper part of the Nanushuk reservoir has the potential to produce oil at rates exceeding 10,000 barrels per day, the AAPG paper says.
The largest field in 40 years The paper says that the Pikka Nanushuk field is the largest oil field discovered on the North Slope in more than 40 years. The discovery established the Nanushuk formation as an exploration target, with subsequent discoveries at Willow and West Willow confirming the Nanushuk’s potential. Initial development at Pikka will focus on the massive sandstone facies near the top of the reservoir, but the very large oil reserves in other rock facies will likely be developed at some time, the paper says.
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