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Piecing together the North Slope story Houseknecht describes the evidence for the timing and location of oil generation and migration from northern Alaska source rocks By Alan Bailey Petroleum News
Renowned for its world class source rocks, northern Alaska presents some tantalizing puzzles for oil explorers. What are the most prospective areas for undiscovered oil? And in what rock units is oil likely to exist in different parts of the region?
For many years the U.S. Geological Service has been investigating the petroleum systems of northern Alaska, with the agency periodically providing assessments of what oil and gas resources might remain undiscovered under the North Slope tundra. On June 16 Dave Houseknecht, a leading member of the team of USGS geologists specializing in northern Alaska, told Petroleum News about the agency’s latest view on the geologic history of the region, and the implications of that history for future oil and gas finds.
Essentially there are four major rock sequences in northern Alaska: the Franklinian, the Ellesmerian, the Beaufortian and the Brookian, with the huge Prudhoe Bay oil field situated in the Ellesmerian; the Beaufortian hosting fields such as Kuparuk and Alpine; and the Brookian hosting fields including Badami and Meltwater.
Oil sources Onshore, the main oil source rocks consist of the Triassic age Shublik formation, in the Ellesmerian sequence, the Jurassic age lower Kingak, and source rocks at the base of the Brookian, Houseknecht explained. Although these source rocks formed at different times, they occur in relative proximity within the overall rock sequence, so that all of them would have been heated simultaneously to form oil, he said.
And the deep burial of the rocks below huge thicknesses of later formed sediments drove that oil generation, as heat flowing from within the Earth caused elevated temperatures at depth.
The first major episode of deep burial and heating of the source rocks happened under what is now the National Petroleum Reserve-Alaska during the Cretaceous, mostly between 110 million years and 70 to 80 million years ago, Houseknecht said. The burial resulted from the deposition of vast amounts of sediment into what geologists refer to as the Colville basin, a huge east-west aligned trough under the North Slope. Many geologists now think that during the Cretaceous the sediment flowed into the basin east from an emerging ancient mountain system known as the Chukotka orogenic belt, in what is now eastern Siberia.
Oil flow A reconstruction of the configuration of the rock strata during the upper Cretaceous, when peak oil generation occurred under the western North Slope, indicates that the oil forming deep below the Colville basin would have flowed north-east towards what is now Harrison Bay, to the northwest of the Prudhoe Bay field, with some oil also migrating northwest towards Point Barrow and the northern Chukchi Sea, Houseknecht said. Much oil in the Harrison Bay area would have later been displaced, perhaps to the west.
Oil would likely have migrated into the Prudhoe Bay field from a variety of directions over a long time span, with oil generation south and southeast of the field mostly taking place during the late Cretaceous and early Tertiary.
During the Cretaceous phase of oil and gas formation under the western North Slope, the source rocks deep under the Colville basin, towards the southern side of NPR-A became progressively hotter, passing through temperatures conducive to oil formation and eventually becoming hot enough to generate natural gas rather than oil. Gas generation would have led, for example, to the huge quantities of gas thought to exist under pressure in the Cretaceous Torok formation, in the Brooks Range foothills region, on the south side of the North Slope.
NPR-A uplift Examinations of well data, seismic data and surface rock outcrops reveal that large sections of the Cretaceous and Tertiary rock sequence are missing in northern NPR-A, with the missing rocks becoming increasingly older east to west. This evidence, together with an assessment of the current tilting of the strata, indicates that during the Tertiary the strata were pushed upwards perhaps 5,000 to 7,000 feet to form a huge dome-shaped structure, centered southwest of the town of Barrow. A technique known as apatite fission-track thermochronology reveals two pulses of uplift, one at around 65 million years ago and another at around 35 million years ago, Houseknecht said. Uplift in this type of geologic setting seems rather common worldwide during the Tertiary, but the reason for the uplift remains an enigma, he said.
Houseknecht thinks that the Tertiary uplift caused gas to flow north from the high pressure gas reservoirs on the south side of NPR-A, displacing oil from reservoir rocks to the north, thus accounting for the increasingly high levels of gas found in exploration wells east to west in northern NPR-A. This theory of gas migration, coupled with an understanding that oil generation in the NPR-A area had essentially completed before the uplift took place, has led to the downplaying of likely undiscovered oil resources in the latest USGS NPR-A resource assessment.
Tertiary oil generation The next major phase of sediment deposition, and hence deep burial of source rocks leading to oil and gas generation, occurred during the Tertiary and was driven by the emergence of the Brooks Range to the south of the North Slope, Houseknecht said. However, much of this sediment flowed north over what is now the Beaufort Sea coastline, to deeply bury and heat source rocks under what is now the Beaufort Sea continental shelf. There would also have been some burial and oil generation under the more easterly sections of the North Slope.
And the sediment feed from the Brooks Range tended to divert around the uplifted area in northern NPR-A, with one big feed of sediment pouring into a basin under what is now the northern Chukchi Sea, and with another big feed pouring more to the east, Houseknecht said.
The huge depths of Tertiary sediment under the Beaufort Sea would have pushed source rocks into appropriate temperatures to generate oil, with oil likely to have formed earlier at the western end of the Beaufort Sea and becoming progressively younger to the east.
Offshore sources However, because of the relative disposition of ancient landmasses and sea at the time that the Ellesmerian and Beaufortian rocks were deposited, there are big question marks over the existence of Shublik or Kingak source rocks under the Beaufort Sea continental shelf. The likely sources for Beaufort Sea oil would be the rocks in the Brookian, in particular at the base of the Brookian sequence and rocks equivalent to the upper Cretaceous Seabee formation, Houseknecht said. A regional Tertiary rock unit found from drilling in the Lomonosov Ridge of the Arctic Ocean and characterized by the abundant remains of azolla plants is another potential offshore source rock.
With huge thicknesses of folded and faulted sand-rich rock under the Beaufort Sea coast, there is excellent potential for oil and gas reservoirs and traps charged with oil under the Beaufort Sea, Houseknecht said. The Kuvlum and Hammerhead (now called Sivulliq) fields, discovered some years ago, may exemplify Beaufort Sea fields reservoired in Tertiary sands. And on a structural high, close to the current coastline, older rocks with reservoir potential would also have existed in faulted blocks at relatively shallow depths, in proximity to the source rocks at the time of Tertiary oil formation, Houseknecht said.
“That late charge (in the Tertiary) is the best of both worlds because all of the rocks that are at drillable depths in terms of reservoir and trap geometries would have been in favorable positions to be charged by this Tertiary oil generation,” Houseknecht said. “So it’s quite an optimistic scenario.”
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