The fact that Alaska’s North Slope is a major oil province is well known. But how did oil end up in the various oil fields in the region? And how does that relate to the region’s geologic history?
In a talk to the Alaska Geological Society on Dec. 16, geological consultant Dallam Masterson described the results of some research into the North Slope oil systems. The information that Masterson presented came from a paper coauthored with colleague Albert Holba, published by the American Association of Petroleum Geologists in June.
The analysis mainly related to the established, producing fields, rather than recent discoveries.
Three rock sequences
In general terms, there are three major rock sequences involved in the North Slope petroleum system: the Ellesmerian, the oldest and deepest of the sequences, contains the Ivishak sandstone, the primary reservoir for the giant Prudhoe Bay field. At the time of deposition of the Ivishak, the North Slope was connected to what is now the north coast of Arctic Canada, with the Ivishak being deposited southward in a marine basin, to the south of what is now the Beaufort Sea coast.
In late Jurassic and early Cretaceous, Arctic Alaska drifted away from Arctic Canada, with what is now the Beaufort Sea opening up. The associated rifting of the continental crust resulted in the second major North Slope rock sequence, the Beaufortian sequence. The reservoirs for the Kuparuk River and Alpine fields are situated in this sequence.
Then, between the Cretaceous and the early Cenozoic, a massive amount of sedimentary material flowed into a trough between the emerging Brooks Range and what is now the Beaufort Sea coast, and ultimately across a geologic high along the coast. This sediment flow formed the Brookian sequence, the youngest and shallowest of the rock sequences. Recent major oil discoveries in the Nanushuk formation are in the Brookian.
There are three major oil source rocks: the Shublik, the Kingak and the HRZ. The Shublik lies in the Ellesmerian, immediately above the Ivishak. The Kingak is also in the Ellesmerian, above the Shublik. The HRZ, Cretaceous in age, lies at the boundary between the Beaufortian and the Brookian.
Oil signatures
The research that Masterson presented involved determining signatures for identifying the oil from each of the oil sources and using these signatures to figure out where the oil in each North Slope oil field had originated. The results can be used to assess a model for how the North Slope geology has evolved over time - the model predicts how much oil from each oil source would have ended up in each field reservoir.
A key signature for characterizing an oil source relates to the fact that there are two hydrogen isotopes with different atomic weights within hydrocarbons, Masterson said. The formation of hydrocarbons involves the fractionation of the isotopes into differing proportions. And the use of gas chromatography to separate components of the oil into components of different weights results in each oil from each oil source having a distinctive pattern of components across the component weight range.
Gas chromatography of oil from a specific field reservoir can then match the component pattern for that oil with the component patterns of potential source oils.
The component pattern for the Tarn field in the Kuparuk River unit, for example, clearly indicates that the oil in this field came from the HRZ, Masterson said. The Kuparuk River field itself shows an origin in the Shublik. The oil in the Alpine field, on the other hand, exhibits a pattern distinctive of a Kingak source. The Fiord field, northeast of Alpine, also appears to have a Kingak source.
The Prudhoe Bay field is more complicated. The component pattern for much of the field is fairly close to the Shublik, but slightly towards the HRZ, suggesting a dominant Shublik component but with some HRZ oil. However, oil at the western end of the field exhibits some sourcing from the Kingak.
Bulk oil properties
The bulk properties of the oils such as the sulfur content and API gravity also tell a story. Shublik oils, for example, have a high sulfur content, Masterson said. High gravity oils at Alpine and Tarn, on the other hand, have low sulfur content and low concentrations of asphaltenes, chemicals formed when sulfur combines with organic matter, he said. And this line of research suggests that the Prudhoe oils consist of about 60% Shublik and 40% HRZ and Kingak.
Biomarkers can provide more detailed information. These are trace oil components identified using a mass spectrometer. They indicate the presence of tiny organisms within the organic material that formed the oil. The organisms provide evidence for the age of the organic material and the environment in which the organic material formed.
Assessments of oil biomarker contents have confirmed the findings of the other techniques for the sources of oils in various field reservoirs.
The origin of the thick, viscous oil found in the relatively shallow West Sak reservoir in the central North Slope is particularly intriguing. In this oil, located in a relatively cool, shallow reservoir, microbes have been active, preferentially consuming the lighter, easier to biodegrade hydrocarbon molecules. The biomarker signature for West Sak is almost identical to that of the Prudhoe Bay oil, despite the fact that gas chromatography demonstrates that the West Sak oil is missing the light components that would have been biodegraded away. This supports a theory that the bulk of the West Sak oil had been spilled upwards from the Prudhoe Bay reservoir below.
However, the deeper oils in West Sak are associated with secondary condensates that have an isotope content closely matching that of oil in the underlying Kuparuk River field. This is interpreted as a secondary charge from the Kuparuk field after the West Sak oil had already been biodegraded, Masterson said.
The Trinity Basin Model
To try to make sense of all of this, it is valuable to use a model of how the North Slope geology evolved, to test whether the model predicts the manner in which different field reservoirs have been filled with oil from different sources - Masterson reviewed how a commercial model, the Trinity Basin Model, might explain the North Slope oil sourcing for the major oil fields with Ellesmerian and Beaufortian reservoirs.
According to the model, Kuparuk C sandstones that later formed part of the Kuparuk River reservoir, were deposited during the Cretaceous on the downthrown sides of steep faults formed during the crustal rifting associated with the Beaufortian sequence. At this time the underlying Ivishak sandstone was tilted, to form an emerging “Prudhoe high” towards the east.
By the late Cretaceous, about 66 million years ago, the HRZ, had spread across the basin plain ahead of the incoming sediments of the Brookian sequence, and had submerged the Prudhoe Bay high. Then, as the Brookian sequence flooded across and into the basin forming under what is now the North Slope, the HRZ would have been buried to a sufficient depth to form a hydrocarbon seal above what had by then become the Prudhoe Bay oil trap. At the same time, the oil source rocks would have been pushed down deep enough to be in the temperature window where oil would have formed.
A large Prudhoe Bay trap
At that time, the Prudhoe Bay oil trap would have been substantially larger than at present, while the Kuparuk River trap would not yet have formed. About 70 billion barrels of oil may have flowed upslope through the Ivishak into the Prudhoe Bay trap, while 15 billion barrels may also have flowed into the trap from the HRZ. A closure in the HRZ would also have trapped around 17 billion barrels of oil expelled from the HRZ source.
By the middle Eocene, about 45 million years ago, the rocks had reached their maximum burial depths. A large amount of oil would have been expelled into a huge structure in the northern part of the current Prudhoe Bay field - 32 billion barrels from the Shublik, but with some from the Kingak, and 26 billion barrels from the HRZ.
At this time the Kuparuk River trap was also starting to form. And the model indicates that, possibly supported by the stressing of steep faulting from the rifting era, 6 billion barrels of oil would have been expelled from the Shublik, vertically upwards into the Kuparuk River trap. The Tarn reservoir, on the other hand, would have connected directly with the HRZ - hence its charge with HRZ oil. And the Alpine field would have been located in an area where the Shublik would have been generating gas, while the Kingak, in the oil window, would have fed oil into the Alpine trap.
Tilting to the southeast
Moving forward to the present day, thrust faulting and crustal erosion in the area of the Arctic National Wildlife Refuge caused the rocks under the North Slope to tilt a little towards the southeast. That tilting reduced the size of the Prudhoe Bay trap, with oil spilling to the west and upward into the West Sak, with the oil probably moving through pathways generated by rift faults. Gas would also have moved through younger faults upwards from the Kuparuk River field into West Sak.
It turns out that this model for the evolution of the North Slope accounts for the nature of the oils in the established North Slope fields, thus supporting the credibility of the model. For the Prudhoe Bay field, the oil composition predicted by the model is within 5% of the actual composition, as measured, Masterson said.
And the model indicates that, in total, 250 billion barrels of oil would have been expelled from the oil sources in the model area, with 168 billion barrels of that oil coming within the fetch area of the Prudhoe Bay field, he said.
