A regional phenomenonMassive system led to reservoirs associated with new North Slope discoveries Alan Bailey Petroleum News
Major new Pikka, Willow and Smith Bay oil discoveries in Arctic Alaska have created excitement over potential future oil development. The discoveries involve reservoirs in the Nanushuk and Torok formations, within the Brookian sequence, the youngest of the major petroleum bearing rock sequences in the region.
On May 23 during the American Association of Petroleum Geologists Pacific Region annual conference U.S. Geological Survey geologists Dave Houseknecht and Richard Lease talked about how the sediments that formed the Nanushuk and Torok were laid down, and what the geology of these rocks may mean for continuing oil discoveries.
Houseknecht commented that the recent discoveries have been particularly astonishing because until 2015 the largest discoveries in the two rock formations had been in the range of 3 million to 10 million barrels of oil. There appear to be more than 1.2 billion barrels in the Pikka discovery, more than 300 million barrels in Willow and possibly 6 billion barrels at Smith Bay, Houseknecht said. ConocoPhillips has also made a Torok discovery at Cassin in the northeastern National Petroleum Reserve-Alaska, while Caelus Energy and ConocoPhillips are interested in developing a Torok reservoired oil pool, called Nuna in the Oooguruk unit and Moraine in the Kuparuk River unit.
World’s largest system Essentially, the sediments in the Nanushuk and the Torok were laid down in what geologists term a clinothem, a system of sedimentary deposits that build out from land into a marine basin. But this particular clinothem is the largest of its type found anywhere in the world, containing more than 1 million cubic kilometers of sediment and having a vertical relief of at least 2 kilometers, Lease said. The sediments were deposited in a basin that was forming on the north side of the emerging Brooks Range mountains.
Assigning ages to the various rocks involved and hence piecing together the history of the sedimentation has traditionally involved identifying fossils in the rocks. But radiometric age determination using fragments of the mineral zircon found in the rocks is much more precise, enabling a more accurate picture of the rocks’ histories, Lease said. Analysis of the zircons also provides insights into the origins of the mineral grains, and hence the regions from which the sediments containing the zircons originated.
In the case of the Nanushuk and the Torok, it appears from the zircon evidence that the sediments that formed the rocks poured into the basin longitudinally from the west, probably from an ancient mountain range in what is now eastern Russia. A mountain building episode referred to as the Chukotka orogeny would have resulted in the pouring of vast quantities of sediment across what are now the Chukchi Sea shelf and the western North Slope, mainly during the lower Cretaceous.
Initial surge The zircon evidence indicates a strong surge of sedimentation between 115 million and 105 million years ago, with the sedimentation then slowing somewhat, Lease said. During that initial surge sediment was depositing in the basin at the rate of 1,000 cubic kilometers per thousand years, spreading across a distance of more than 450 kilometers, he said. Sedimentation then slowed somewhat, finally pausing at about 98 million years ago, leaving in place what are now recognized as the Nanushuk and Torok formations. The eastern margin of the marine shelf formed by the sedimentation lay roughly in the area of the present-day Colville River.
The sequence of deposition above Nanushuk and Torok then began at around 95.5 million years ago, with sediments that formed the upper Cretaceous Seabee and Tuluvak formations starting to spread out across the older sediments at about 94 million years ago. The zircons in these younger formations appear to have originated in the Brooks Range, with the sediments appearing to have flowed south to north, in a transverse rather than axial direction across the basin.
Tilting and faulting Houseknecht said that later subsidence of the rocks to the north of the current Beaufort Sea coast resulted in tilting of the Nanushuk and Torok formations, causing surface erosion of the rocks farther south. Massive geologic faulting under the nearshore waters of the Beaufort subsequently dropped oil source rocks that underlie the two formations to considerable depths. Those faults come close to the coast near Smith Bay, where faulted blocks contain the Shublik formation, a prolific North Slope oil source, and possibly some younger oil source intervals, Houseknecht said.
A key geologic feature that controls much of the North Slope petroleum geology is a structural high or ridge called the Barrow Arch that runs east to west roughly along the Beaufort Sea coast.
To the south of the Barrow Arch, evidence from seismic sections indicates that oil source rocks under the massive Brookian clinothem lie within the temperature window where oil would form “over an extraordinarily wide area,” Houseknecht commented. And, with source rocks also likely to be buried deeply enough for oil formation in the faulted blocks to the north of the Beaufort Sea coast, the Barrow Arch can act as a regional focus for oil and gas migration both to the north from onshore and to the south from offshore, he suggested, adding that this bidirectional migration can explain why oil pools in the region appear to contain oil with mixed origins.
Widespread reservoir potential And, given the manner in which the sediments in the clinothem were laid down, spreading eastward down an eastwardly migrating shelf margin, there is the potential for thick reservoir rock units, with interbedded fine-grained sediments that could create stratigraphic oil and gas traps. There have been announcements of reservoirs ranging in thickness from 650 feet to as much as 1,000 feet for the new discoveries, Houseknecht commented. There may be potential for these stratigraphic traps to exist from the ultimate shelf margin in the central North Slope all the way across to the central shelf of the Chukchi Sea, he said.
However, the structure of the sedimentation in the Nanushuk/Torok clinothem does change from west to east, reflecting the manner in which the rate of sedimentation overwhelmed sea level fluctuations in the west, while farther east changes in sea level had a significant impact on the nature of the stratigraphy. The exact nature of the setting in which the sediments were laid down impacts the structure of potential oil and gas reservoirs, as can be illustrated by the detailed structure of the reservoir and trap system in the Pikka discovery.
Lease commented on a cyclicity in the nature of the sedimentation around the Nanushuk/Torok shelf margin, as the relative sea level changed while the sediments were being deposited. Age data from the zircons indicate that this cyclicity had a period of some 450 thousand years, a period that matches the longest of the cycles that result from eccentricities in the Earth’s orbit and rotational tilt. Perhaps this cycle impacted the sea level and the carbon cycle at the time when the sediments laid down, he speculated.
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