A new NS play?Major Nanushuk oil discovery could point to further opportunities to west ALAN BAILEY Petroleum News
The huge Nanushuk oil development being planned by Armstrong Oil & Gas and Repsol in the Colville Delta area of Alaska’s North Slope marks a significant bright spot amid the current gloom of low oil prices and declining Alaska oil production. But could this particular discovery, sometimes referred to as the Qugruk discovery, point the way to a new, unanticipated oil exploration play in Arctic Alaska, with the potential to bring many more barrels of oil to the northern end of the trans-Alaska pipeline?
In a talk at the Alaska Geological Society’s annual technical conference on April 22, U.S. Geological Survey geologist Dave Houseknecht, an established expert on Arctic Alaska petroleum systems, presented compelling evidence for looking into this new play possibility. Essentially, while rocks of middle Cretaceous age, including the Nanushuk formation, along the Beaufort Sea coast west of the central North Slope, have tended to play second fiddle to plays involving some of the older rocks of the region, the new discovery has revealed the possibility of major undiscovered oil resources along a fairway extending perhaps 100 miles west from that recent discovery, Houseknecht suggested.
Houseknecht characterized the Armstrong/Repsol discovery, with up to 150 feet of net pay sandstone in a 650-foot oil column covering more than 25,000 acres, in reservoir rocks with a porosity of 22 percent, as “pretty astounding.”
Oil sources Geologists have long considered the oil in the producing North Slope oil fields - in Prudhoe Bay and Kuparuk, for example - to have migrated primarily from the south, where the oil source rocks have been buried relatively deeply and have reached temperatures conducive to oil formation. The three major source rocks are the Triassic Shublik formation, the lower Jurassic Kingak shale and the lower Cretaceous Pebble shale/gamma ray zone (or GRZ).
But seismic data from surveys that cross the Beaufort Sea coast show those same source rock intervals dipping steeply to the north, under the nearshore waters of the Beaufort Sea. The rocks appear to have reached depths where, again, the subsurface temperatures could have driven oil formation, Houseknecht said. And with middle Cretaceous sands also dipping northward toward that oil generating kitchen, Houseknecht thinks that oil could have migrated into the relatively shallow Nanushuk reservoir from the north, rather than from the south. That being the case, there is a strong possibility of finding more oil, generated and trapped under similar circumstances, along a zone running many miles to the west.
Middle Cretaceous strata The middle Cretaceous rock sequence at the heart of this play consists of, from older to younger, the Torok and the Nanushuk formations. The rocks were formed as part of what geologists call the Brookian sequence, a massive rock sequence mainly deposited in an ancient marine basin called the Colville basin between what is now the Brooks Range and a structural high called the Barrow Arch along the present-day Beaufort Sea coast.
Houseknecht said that the sediments which formed the Torok and Nanushuk primarily flowed west to east across the Colville basin, eroded from an ancient mountain belt that had formed in what is now eastern Russia. The massive pile of sediments, rather like the leading edge of a gigantic sand and mud slide that lasted several million years, spread to the east, toward what is now the central North Slope, while also tipping north over the Barrow Arch. The result was one of the world’s largest clinoform systems, Houseknecht said. A clinoform is a sloping depositional surface such as might be found within the sediments deposited in a river delta.
But around the area of the current Colville Delta the ancient flow of sediment started to run out of steam, with the nature of the sedimentation changing quite abruptly and a marine shelf margin forming. A similar situation arose on the north side of the Barrow Arch, so that the shelf margin which runs south to north in the Colville River area swings sharply counter clockwise to run in a more west-northwest direction to the north of the present day Beaufort Sea coast.
Houseknecht sees this relict shelf margin as a key to a middle Cretaceous oil play.
In fact, one of the intriguing aspects of this rock system is a condensed shale, deposited on top of the Nanushuk formation as a consequence of a rise in sea level that resulted in water flooding back toward the west across the massive pile of middle Cretaceous sediments. That shale can form a seal for trapping oil and is also a potential oil source, Houseknecht said.
A history of discoveries Toward the northwest end of the shelf margin the U.S. Navy made a small oil discovery more than 60 years ago near a known oil seep at Simpson Bay. Then, toward the southeastern end of the margin, at Umiat, there is a known oil field with a reservoir in the Nanushuk. There have been natural gas discoveries in the Nanushuk in the Brooks Range Foothills. And near the coast there have been several oil discoveries in the Torok and the Nanushuk: the Cassin, Nanuq, Nuna and Moraine pools in the Torok, and the Qannik and Qugruk discoveries in the Nanushuk.
Moving into the younger Seabee formation, of upper Cretaceous age, there have been two producing discoveries: Meltwater and Tarn. Although the Meltwater production has been a little disappointing, Tarn has proved particularly successful, with cumulative production of more than 100 million barrels of oil through 2013, Houseknecht said.
These discoveries all lie in proximity to that Cretaceous relict shelf margin, Houseknecht pointed out. And sand-filled geologic systems associated with estuary-like conditions on the ancient landscape seem to exhibit particularly favorable oil reservoir characteristics in the Nanushuk, he said. In the Torok there are potential reservoir sands in deeper water sediments called turbidites, formed from submarine sediment flows.
The structural setting In the area of the Colville basin a major geologic discontinuity called the lower Cretaceous unconformity, below the middle Cretaceous strata, slopes markedly upward from south to north toward the Barrow Arch. But the infilling of the basin by the sediment deposited from the west has resulted in the Nanushuk strata lying very flat from a north-south perspective. To the north of the Barrow Arch, on the other hand, the Nanushuk dips steeply northward into another basin, referred to as the Canada basin.
The three major North Slope source rock intervals follow this same structural geometry, dipping steeply north to the north of the Barrow Arch and becoming buried to depths where oil might form. That oil should tend to flow back up dip into the Torok and Nanushuk. In fact, unless trapped somewhere along the way, the oil could flow all the way to where the strata outcrop at the surface, a phenomenon that could explain the oil seeps that have been observed along the Beaufort Sea coast, Houseknecht suggested.
The likely timing of oil generation also matches the possibility of oil migration into traps of Cretaceous age, he said.
In terms of the petroleum geology of the region, a series of steeply dipping geologic faults associated with the formation of the Canada basin could have impacted the preservation of oil source rocks and the ability of oil to flow from these rocks into the Torok and Nanushuk. But, while faults may play a role in trapping oil, some oil traps would be stratigraphic, caused by subsurface pinch outs of reservoir rocks, while other traps would involve a combination of structural and stratigraphic features, Houseknecht suggested.
Almost untested But, although the oil play along the middle Cretaceous shelf margin may extend nearly 100 miles west of the Colville Delta area, the play remains almost untested, with only a handful of exploration wells penetrating it offshore. Wells drilled far to the west have demonstrated the presence of the Shublik and lower Kingak source rocks in the region, and have found abundant oil shows in Brookian and older rocks. But none of these wells were drilled in areas where potential traps in the middle Cretaceous rocks could best be evaluated, Houseknecht said.
With Brookian reservoir sands becoming compressed at depth and hence losing some of their porosity, the depths to which the potential reservoir rocks have been buried during their history is important in assessing current reservoir quality. It turns out that, while the middle Cretaceous rocks in the area of the Brooks Range Foothills appear to have been buried to depths of 7,000 to 8,000 feet at some time, the equivalent strata in the Simpson Bay area, for example, seem to have only been buried to around 2,000 to 4,000 feet. And, so, the reservoir quality of potential reservoir sands along the suggested Torok-Nanushuk oil play area should be very good, Houseknecht said.
Key uncertainties associated with the play include forecasting the presence of reservoir-quality sandstone along the play fairway, delineation of subtle traps that may involve both stratigraphic and fault components, and - further to the west offshore - understanding a complex pattern of erosional truncation of the Nanushuk reservoir.
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