This month in history: USGS assesses central Slope oil and gas
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20 years ago: There may be 4 billion barrels of undiscovered oil, 37.5 trillion cubic feet undiscovered gas between NPR-A and ANWR
Alan Bailey for Petroleum News
Editor's note: This story first appeared in the May 15, 2005, issue of Petroleum News.
The U.S. Geological Survey has published the results of its new assessment of potential oil and gas reserves under the central North Slope of Alaska. The assessment area lies between the National Petroleum Reserve-Alaska and the Arctic National Wildlife Refuge. The area includes mainly state and Native lands extending northward from the Brooks Range to the state-federal offshore boundary in the Beaufort Sea.
The assessment, published May 13, 2005, indicates the possibility of between 2.6 billion and 5.9 billion barrels of undiscovered, technically recoverable oil in the area, with a mean average of 4 billion barrels. There is a possibility of between 23.9 trillion and 44.9 trillion cubic feet of undiscovered, non-associate natural gas, with a mean of 33.3 tcf. The assessment estimates a mean of 4.2 tcf of associated natural gas and 387 million cubic feet of natural gas liquids.
USGS excluded from its assessment oil accumulations of less than 5 million barrels of recoverable oil or 100 billion cubic feet of recoverable gas.
NPR-A and ANWR A 2002 USGS assessment of NPR-A indicated mean recoverable oil reserves of 10.6 billion barrels and mean recoverable gas reserves of 61.4 billion cubic feet for that region. And the 1998 USGS assessment for ANWR estimated 10.4 billion barrels of oil there. In a 2002 presentation the U.S. Minerals Management Service reported an estimated mean of 6.94 billion barrels of conventionally recoverable oil and natural gas liquids, and 32.07 tcf of conventionally recoverable natural gas under the outer continental shelf of the Beaufort Sea.
Add up all these estimates for northern Alaska and you obtain volumes of nearly 32 billion barrels of oil and more than 142 tcf of natural gas for the region. The gas figure doesn't include unconventional sources such as gas hydrates and coalbed methane -- there may be as much as 519 tcf of additional gas in gas hydrates beneath the permafrost of northern Alaska. And then there's the Chukchi Sea with possibly 15 billion barrels of conventionally recoverable oil and 60 tcf of conventionally recoverable natural gas, according to an MMS assessment.
The new USGS estimates for the central North Slope do not include reserves from producing oil fields in northern Alaska -- most of these fields lie within the area of the new assessment and hold substantial reserves of oil and gas.
24 plays identified The USGS estimates for the central North Slope come from an analysis of 24 oil and gas plays that encompass virtually all of the stratigraphic and structural elements of the North Slope geology. USGS viewed the whole region as one large petroleum system.
"It's a very complicated system and a leaky system, so that oil or gas generated from one source rock ... doesn't necessarily stay in the same set of strata ... it gets spread around," USGS geologist Ken Bird told Petroleum News in a May 2005 interview.
Oil and gas plays include the Prudhoe Bay and Kuparuk River field situations in geologic sequences known as the Ellesmerian and Beaufortian, along a prominent structural high known as the Barrow Arch on the Beaufort Sea coast. USGS thinks that these plays contain nearly 800 million barrels of technically recoverable oil and about 1.4 tcf of technically recoverable gas, over and above reserves in existing fields or known accumulations.
A Jurassic stratigraphic play associated with the Alpine field occurs on the west and east sides of the north-central North Slope. This play, together with a play known as the Beaufortian clinoform play, accounts for 264 million barrels of oil and 1.7 tcf of gas in the USGS estimates.
Huge Brookian potential USGS sees the potential for finding particularly large volumes of oil and gas in a relatively young rock sequence known as the Brookian. Stratigraphic plays called the Brookian topset and clinoform plays involve potential sandstone reservoirs that occur over a wide area of the North Slope. These sandstones lie in an ideal stratigraphic setting to gather oil and gas from some of the region's most prolific source rocks, Bird said.
"Basically it's the same play that includes known accumulations like the recent Nanuq accumulation ... the Tarn and Meltwater accumulations, Badami ... and probably Sourdough," Bird said. "What we're talking about here are the deepwater turbidite-type sand reservoirs."
Brookian plays, including both stratigraphic and structural plays, account for 2.5 billion barrels of oil and nearly 15 tcf of gas in the USGS assessment.
Because the sediments that formed the Brookian sequence derived from the Brooks Range, USGS geologists think that the potential Brookian reservoirs could be thicker towards the southern end of the assessment area.
"We think there certainly is a chance for the thicker reservoirs to be developed (in the south) and perhaps even (for) a more frequent occurrence of these turbidite sands," Bird. "We did try to factor things like that into our estimate of field sizes."
However, USGS has followed a generally accepted view that the southern portion of the North Slope is likely to be gas prone because of the high thermal maturity of the rocks in that part of the region.
"It's also born out by the oil and gas indications that are found in the exploratory wells across the region," Bird said.
So, USGS expects relatively small amounts of oil and the preponderance of non-associated gas to occur south of a zone nearly halfway down the assessment area. Explore north of that zone and you're more likely to encounter oil and associated gas.
Economics and field size USGS data relate to technically recoverable reserves -- reserves that can be extracted from reservoirs using known technology. There's a whole other question regarding how much of the reserves can be recovered economically. That question has yet to be addressed.
The sizes of the potential oil and gas accumulations play a central role in any economic analysis -- there's normally a minimum economic accumulation size that makes development viable.
Recognizing that the assessment area includes a well-developed oil and gas infrastructure, USGS geologists assumed relatively small minimum accumulation sizes in its analysis. But the geologists also found that there are unlikely to be oil accumulations larger than 250 million barrels and that most oil or gas accumulations are likely to be fairly modest in size, Bird said. USGS will be publishing its analysis of the accumulation size estimates.
But in whatever way the economics play out it, it's clear that the new USGS assessment confirms the likely existence of major quantities of oil and gas that remain to be discovered under Alaska's North Slope.
Reports from the USGS assessment can be found at http://energy.usgs.gov/ak-cns.html.
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