Petroleum Systems Offer New Targets
Source Ideas Boost Arctic Promise
Call it a case of seeing something old in a new and excitingly different light.
Arctic Alaska is a recognized world-class petroleum province with plenty of potential for future discoveries, according to David Houseknecht, research geololgist at the U.S. Geological Survey in Reston, Va.
But the wells of the future likely won’t produce hydrocarbons sourced from the same-old, same-old, i.e., the Triassic Shublik formation and the Cretaceous- Tertiary Hue Shale.
“There’s been a lot of exploration in Arctic Alaska focused on these source rocks,” Houseknecht noted. “As exploration expands into frontier parts of the Arctic, these source rock systems probably aren’t going to work because either they’re not present or the hydrocarbons have been destroyed by high temperature related to deep burial.”
So, you ask, what’s left to lure the explorers?
The good news is there are at least three other candidate source rocks of high quality oil that either have been proven already or else probable to be widespread – not only around Arctic Alaska, but perhaps more broadly around the entire Arctic Basin.
The Kingak’s Importance
In the middle 1990s, a drillstem test in a wildcat well not far from Prudhoe Bay recovered oil that for the first time was typed unequivocally to the Kingak Shale of early Jurassic age.
“It was a gassy, high gravity and low sulfur oil,” Houseknecht said. “No one had been able to find an accumulation that was exclusively Kingak oil until Alpine Field was discovered.”
The discovery well was drilled in 1994 but not announced until 1996.
Besides discovering the first economic volumes of oil that could be typed specifically to the Kingak as a source rock, the Alpine discovery was notable for first tapping into the previously unknown Alpine sandstone and trap type, Houseknecht noted: It’s a stratigraphic trap with beach sands deposited in incised accommodation space.
The Kingak-sourced hydrocarbons became a focus of attention because they are high gravity, low sulfur oils. Many of the reservoirs there are relatively fine-grained, so high gravity oil moves more readily and is easier to produce.
Additional discoveries of what is believed to be Kingak-sourced oil have occurred in the National Petroleum Reserve-Alaska, where ConocoPhillips and Anadarko have announced several discovery wells that have recovered oil, condensate and gas.
Test results released indicate hydrocarbon phases that are consistent with the Alpine oil and probably an extension of the oil system generated from Kingak Shale, according to Houseknecht.
“The importance of that to exploration both onshore and perhaps offshore in the Chukchi Sea,” he said, “is that there is an additional source rock that could represent an active petroleum system that would make prospects viable that had previously not been considered so because of source problems.
“I consider the Kingak source rock to already be a proved system,” Houseknecht added, “because Alpine represents the biggest onshore discovery in the last quarter-century in North America – greater than 500 million barrels recoverable – and additional discoveries have been made. So I think it’s fair to say that a number of companies exploring Arctic Alaska, both on- and offshore, are incorporating the Kingak petroleum system into their thinking as they look toward additional exploration.”
The Turonian lower Seabee formation long has been recognized as an oil-prone source rock that is immature or eroded beneath much of the western North Slope. It was distributed above flooding surfaces that originally were present across all of the North Slope, and probably across the entire Chukchi Sea.
“The lower part of the Seabee has not entered in to exploration thinking in the past,” Houseknecht said, “because onshore, where it’s present both in outcrop and where penetrated by exploration wells, it’s immature, as it wasn’t heated enough to generate oil.”
It’s not even present across much of the western North Slope, he added, because it’s been eroded beneath an unconformity.
“But seismic stratigraphy indicates the flooding surface beneath the source interval is more regional than previously recognized, extending across the northern Chukchi shelf,” Houseknecht continued. “Turonian source rocks may be buried into – and locally through – the petroleum window in Tertiary sub-basins offshore.
“It’s definitely a source rock that’s oil prone that could generate probably high gravity, low sulfur oil in the right situations.”
In fact, USGS research conducted on the lower Seabee indicates it’s a rich source rock, e.g., 3-6 percent TOC is common, and the hydrogen index can be greater than 400 in samples of the unit.
An Emerging Concept
An emerging concept in the Arctic entails Tertiary age source rocks, which have generated oil discovered in the Mackenzie Delta of Canada. At least three discoveries just offshore from the Arctic National Wildlife Refuge on the Alaska side of the border also have Tertiary oils in them. The source rock, however, has not been penetrated by drilling.
In 2004, a scientific expedition that was part of the deep sea Integrated Ocean Drilling Program (IODP) recovered almost 400 meters of core from Lomonosov Ridge, which is a high-standing, narrow sliver of European continental crust that rifted away from the rest of Europe during the early Tertiary. The core contains a significant organic-rich condensed section of mostly Paleocene and early Eocene age.
“The important part of this is there is a thick, condensed section of fine-grained sediment that has TOC that is typically 2-5 percent and ranges as high as 14 percent,” Houseknecht noted. “There are intervals within that section with hydrocarbon index values in excess of 300.
“The Azolla horizon is one very unique highly organic layer within this larger condensed section,” he said. “The Azolla is important because, overall, the condensed section was deposited mostly during a greenhouse earth when the global temperature was significantly higher than now.”
Included in the greenhouse earth interval are deposits that have been correlated to what is called the Paleocene Eocene Thermal Maximum (PETM). One of the published papers related to the IODP expedition indicates the surface temperature of the water near the North Pole during the PETM was perhaps as high as 73 to 75 degrees F.
Indications are the Arctic Ocean at this time was not only very warm but was mostly isolated from world ocean circulation as well, according to Houseknecht.
“The implication of the IODP core drilling to petroleum geology of the Arctic,” he said, “is that evidence suggests the whole Arctic Ocean may have been a very highly organic productive basin characterized by anoxic conditions during much of the late Paleocene and early Eocene.”
This suggests the potential for widespread, highly organic potential source rock across the entire Arctic Basin, including along the margins of all continents that border the Arctic.
“The Azolla is very unique because it suggests fresh water conditions – at least in surface waters – during its deposition,” Houseknecht noted. “It’s also important because it’s been found all around the Arctic in a number of related basins, linked by narrow straits to the Arctic Ocean.”