An overlooked play?
Line of Beaufort Sea wells provide evidence for oil potential in east
While interest in North Slope oil exploration of late has tended to focus on the western part of the region, with major new finds in the Nanushuk and Torok formations, there are other intriguing possibilities more towards the east. And moves towards the conducting of oil and gas lease sales in the coastal plain of the Arctic National Wildlife Refuge will presumably drive interest in that easterly direction.
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For some time now geologist Robert Blodgett and oil industry consultant Steve Sutherlin have been investigating what Blodgett terms the Six Sisters play, a play relating to a chain of six exploration wells drilled quite a few years ago along a group of Beaufort Sea barrier islands, immediately west of where the Canning River enters the sea. The play is close to the northwestern corner of ANWR.
The Stinson and A1 wellsThe play particularly revolves around findings from the Stinson No. 1 well, drilled by ARCO in 1989, and the Alaska State A1 well, drilled by Exxon in 1975. The Stinson well was drilled offshore, at the eastern end of the Six Sisters trend, while the A1 well was drilled to the west of the Stinson well, from North Star Island.
The Stinson well tested a flow rate of 430 barrels per day of light oil and condensate from quartzite in the ancient Paleozoic basement at a depth of around 15,000 feet. According to a report by Blodgett and Sutherlin, oil flow from the find could rise to 700 to 800 barrels of oil under clean well hole conditions. Below the quartzite lies a more mixed rock assemblage, including shales and a carbonate rock called dolostone. The well also penetrated and found an oil show in the early Tertiary Canning formation immediately above the basement.
A substantial oil findThe A1 well encountered a similar stratigraphic sequence to that in the Stinson well, although this well did not encounter quartzite at the top of the basement sequence - the quartzite apparently thins out to the west of the Stinson well. However, the other basement rocks did have hydrocarbon shows in the A1 well. And the well found a substantial oil resource in sands of the Canning: A test conducted for the A1 well flowed oil at 2,507 barrels of oil per day, Blodgett told Petroleum News. Blodgett said that, at the time, this had been the highest producing exploration well in the North Slope region, with a flow rate exceeding that of the Prudhoe Bay field discovery well.
“It’s very economically attractive,” Blodgett said. However, it appears that at the time of the drilling, the oil companies did not view the discoveries as viable for development.
To the west of the A1 and Stinson wells is a line of four other wells that have penetrated similar geology: the Alaska State D1, the Alaska State F1, the Alaska Island No. 1 and the Challenge Island No. 1. Two tests in the F1 well flowed condensate at rates of 152 and 284 barrels per day from the basement rocks. The most westerly of the wells, the Challenge Island well, found some Cretaceous rocks below the Canning and above the basement, including sands equivalent to the reservoir rock in the Point Thomson gas and condensate field to the south.
Intriguing geologyThe geology of the Six Sisters trend is particularly intriguing and could provide some insights to the petroleum geology of ANWR to the east, Blodgett and Sutherlin think. The wells lie near the crest of a major geologic anticline or arch, known as the Mikkelsen High, that plunges down towards the east. The carbonate rocks in the basement sequence yield fossils, including very early forms of shellfish that indicate an early Cambrian age. The quartzite found in the Stinson well consists of almost pure quartz, a phenomenon that raises interesting questions over the origin of the material. The purity of the quartz suggests the derivation of sediment from an older region of stable continent. That, in turn, raises questions regarding the disposition of the plates of the Earth’s crust at the time the quartz was deposited, Blodgett suggested.
The quartzite found in the Stinson well is heavily fractured, with oil trapped inside the fractures. The fractures display a distinctive orientation pattern, a factor that would drive a need for careful planning when it comes to the orientation of any wells used for oil production, Sutherlin said. The fluids in the rocks are at extremely high pressures, he added.
Oil source uncertainThe origin of the oil found in the quartzite is enigmatic - it may have come from a widespread North Slope source rock referred to as the HRZ, or from the Canning formation, Blodgett said. The Shublik, another prolific regional source rock, is also a possibility, although the nearest known occurrences of the Shublik are some distance away. Whatever the source, the oil has likely come from a higher level in stratigraphy and been squeezed down into the quartzite fractures, Blodgett said.
There is a similar and apparently analogous oil play that is actually being developed in Russia, in eastern Siberia, Blodgett said. And the evidence from the Six Sisters wells suggests the existence of a significant Cambrian oil play in the coastal area bordering the ANWR coastal plain, he and Sutherlin think.
When it comes to the oil found in the Canning formation, the trapping mechanism has not been fully determined but is likely to be stratigraphic, with hydrocarbon traps formed by the manner in which the sedimentary strata were deposited.
Implications for regional geologyThe conventional concept for the deposition of the sediments in the lower part of the Tertiary sequence, such as the Canning, in the eastern area of the North Slope and offshore under the Beaufort Sea, is that the sediments poured north down the edge and across the floor of an ancient marine basin. In particular, the sediments would have stacked into layered sands and shales known as turbidites. However, rock samples from the Canning formation, where it is penetrated by the Stinson and A1 wells, show features more characteristic of non-marine deposition. In particular, the rock contains fragments of freshwater plants, which appear similar to plants found in lower Tertiary rocks on the Lomonosov Ridge, under the Arctic Ocean near the North Pole.
The appearance of these non-marine rocks under the nearshore waters of the Beaufort Sea raises interesting and as yet unanswered questions regarding the geography of the region at the time the rocks were forming.
One enigma associated with Arctic Alaska geology is the mechanism whereby the Canada Basin, the sector of the Arctic Ocean north of Alaska, formed. One prevailing theory is that the basin formed from the counterclockwise rotation of the northern part of Alaska away from what is now the northern coast of Canada. Blodgett thinks that similarities between the Cambrian geology in eastern Siberia and that found by the Six Sisters wells may be evidence that, in fact, northern Alaska rifted from Siberia, and not from Canada.
Steve Sutherlin is an owner of Petroleum News
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