Shell builds northern AK geologic model

Timing of faulting, opening of the western Arctic Ocean & upheavals associated with the Brooks Range can all impact petroleum systems

Alan Bailey

Petroleum News

In oil and gas exploration, critical decisions about where to drill wells depend on an understanding of the geology of an exploration region. And in northern Alaska and the adjacent waters of the Beaufort and Chukchi seas a substantial body of geologic insight has built up over the years, helping explorers formulate drilling plans.

However, questions remain over some major features of the region’s geologic history. And as part of its current multibillion dollar foray into Alaska Arctic offshore exploration, Shell has been conducting extensive research into offshore Alaska geology. On Dec. 14, as a follow-up to a presentation he gave to the Alaska Geological Society in November, Tom Homza, Shell’s principal regional geologist for Alaska, talked to Petroleum News about what the company’s team of geologists has found so far — Shell’s regional team, led by Steven Bergman at the Shell lab in Houston, working in conjunction with geoscientists from academia, has proposed some new and revised ideas on the geologic evolution of the region.

Digital light table

For many years geologists have been in broad agreement that four major rock sequences are involved in northern Alaska petroleum systems: from oldest to youngest, the Franklinian, Ellesmerian, Beaufortian and Brookian sequences. Each major oil field on the North Slope has oil and gas reservoirs within one of these sequences, although no significant oil has been produced to date from the Franklinian, a sequence often considered to be the economic “basement” for northern Alaska.

Ellesmerian strata were deposited on the northern margin of an ancient marine basin, to the south of what is now the Beaufort Sea coast, with detritus eroded southward from an ancient landmass in the region of the current Beaufort Sea. The subsequent Beaufortian sequence is associated with what is termed “rifting,” the pulling apart and faulting of the Earth’s crust in a manner rather like what is currently happening in the African Rift Valley. At some point, the crust was completely ripped apart to form the Canada basin, the segment of the Arctic Ocean to the north of Alaska. Subsequently, detritus from the emerging Brooks mountain range flowed into a deep basin, known as the Colville basin, under the North Slope, with this detritus eventually spilling over into the area of what is now the Beaufort Sea. The vast mass of detritus washed down from the emerging Brooks Range formed the Brookian sequence.

Barrow Arch

The timing and mechanisms involved in forming these various geologic features is of great importance in gaining an understanding of the petroleum systems of northern Alaska. There has, for example, been a long debate over the timing of the rifting associated with the Beaufortian sequence.

Data from 2-D and 3-D seismic surveys indicate that, in the area of the central North Slope, tilting of Triassic strata occurred before the deposition of sediment during the lower part of the Jurassic period, Homza said. Since the tilting of the Triassic appears associated with the Beaufortian rifting, the onset of rifting would have occurred early in the Jurassic or perhaps sooner, a significantly earlier timing than some geologists have proposed, he said.

One consequence of the rifting was the deposition of sands in some deep, faulted blocks, now offshore the northern Alaska coast — the potential for oil in these sands remains an untested play, Homza said.

With the transition from continental rifting to “sea-floor spreading” in the Canada basin likely being a lengthy and complex process, the Shell team thinks that oceanic crust may not have formed until the late Jurassic, around the time that the sands that form the reservoir rocks of the Alpine field were being deposited (see sidebar “The Amerasian basin — a geologic enigma”).

Nature of unconformity

The use of a south-to-north seismic cross section of the North Slope to reconstruct the orientation of older rock strata at the time that the lower Cretaceous unconformity formed points to an alternative explanation for the unconformity, Homza said. To the south of the North Slope the Brooks Range was emerging. And the massive weight of the mountain range pushing down on the Earth’s outer layers would have tipped, bent, and broken the crust, causing it to bend upwards in the area of the Barrow Arch, rather like the rising side of a seesaw. This upward tilting is visible in the seismic section. And the subsequent erosion of exposed, tilted rock strata along the arch would have caused the unconformity to form.

This theory would explain the apparent absence of the lower Cretaceous unconformity in the Canadian Arctic Islands — with no Brooks Range in Canada, the mountain-loaded tilting of pre-Cretaceous strata would not have occurred, Homza said.

And the flexing of the crust along Alaska’s Beaufort Sea coast would have re-activated some geologic faults originally formed during the earlier rifting period, triggering the deposition of sands to form, for example, the main reservoir in the Pt. McIntyre oil field, Homza said.

Chukchi Sea

The central part of the Chukchi shelf is dominated by the Hanna Trough, the remnant of an ancient marine basin containing strata equivalent to the Ellesmerian sequence. And in the southern part of the shelf is a major uplifted area known as the Herald Arch, equivalent to the Brooks Range onshore. Further, a series of northeast trending faults cutting through strata in the Hanna Trough marks the rifting observed farther east and associated with the Beaufortian sequence. However, in the Chukchi Sea it appears that the rifting action failed, and oceanic crust never formed.

Although the lower Cretaceous unconformity can be traced into the northern part of the Chukchi Sea, the unconformity does not exist across much of the Chukchi shelf, Homza said. But during the upper Cretaceous the emerging Brooks Range pushed the strata in the Hanna Trough into an upwards bulge, in effect inverting the trough. Erosion of the bulge caused a major unconformity between strata of the upper Cretaceous and strata of the early Tertiary period.

However, quite early in the Tertiary there was a massive collapse of the bulge in the Hanna Trough, with strata dropping along major geologic faults. This collapse had a significant impact on many of the prospective areas in the Chukchi, including the focal point of Shell’s exploration plans, the Burger prospect, which is known to contain hydrocarbons. However, the sands that form the reservoir at Burger were deposited long before the Burger structure formed and are broadly equivalent in age to the main reservoir sand in the Kuparuk River field.

So, although previous exploration in the Chukchi Sea has only resulted in five wells, the Shell team is in a good position to continually refine their Chukchi geologic models based on new findings throughout the Arctic region, Homza said.

The Amerasian basin — a geologic enigma

Unlike other oceans such as the Atlantic Ocean, where evidence such as the records in the ocean floor of changes in the Earth’s magnetic field enable quite accurate time estimates of ocean formation, relatively little is known about the age of the Amerasian basin, the part of the Arctic Ocean that lies between North America and Russia, Tom Homza, Shell’s principal regional geologist for Alaska, told Petroleum News Dec. 14. Nor is there universal agreement on the mechanism by which the basin developed. In fact, this is the last large ocean basin without a generally accepted model for its formation, he said.

In common with other ocean basins, the Amerasian basin would have formed from the splitting apart of an ancient continent, to open up a region of oceanic crust. Although there is considerable uncertainty over exactly when this splitting occurred, gravity and magnetic data from the basin floor and geologic evidence from the basin margins suggest that the basin opened fairly rapidly at some time in the late Jurassic, Homza said.

And, as a mechanism for the opening of the basin, a team of Shell geologists researching northern Alaska geology favors a model in which what is now the North Slope rotated counterclockwise relative to what is now Arctic Canada, rather like a huge windshield wiper blade moving from its parked position. Data viewed in a computer based “digital light table” shows relatively low gravity readings along a faint line bisecting the southern part of the basin — this line could be a relic of the zone along which the basin opened. And, although thick sediments have blurred the pattern of seafloor magnetism in the northern part of the basin, there is some faint magnetic signal in the southern basin that could support the rotational basin opening concept, Homza said.

Chukchi Borderland

Some geologists have questioned the rotational model because of the existence in some of the older rocks in northern Alaska of fossils that show close affinities with fossils of similar age in Siberia. How could a Siberian fauna end up in Alaska, if Alaska has rotated counterclockwise from Canada, these geologists ask.

Homza said that the Shell team thinks that the answer to this enigma is what is called the “Ellesmerian orogeny,” a vast mountain building event that resulted from the collisions between ancient continents in the Paleozoic era and that extends from northern Alaska across northern Canada and, perhaps, into Europe. Movements of the Earth’s crust during that orogeny could have carried rocks with Siberian fossils into what is now northern Alaska, before it separated from Arctic Canada, Homza said.

—Alan Bailey