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Resource estimates up for Alaska’s Yukon Flats USGS: East central basin may contain similar amounts of gas to Cook Inlet Alan Bailey Petroleum News Staff Writer
The oil and gas potential of the Yukon Flats in east central Alaska has long intrigued geologists. This 13,500 square mile lowland area around the Yukon River, between the trans-Alaska pipeline and the Canadian border, lies over a deep basin of sediments that probably contain natural gas and might contain oil.
Last week Richard Stanley from the U.S. Geological Survey visited Anchorage to present the results of a new USGS assessment of the Yukon Flats basin. The assessment has concluded that there may be much more oil and gas in the basin than previously thought. However, the assessment supports previous views that the basin is more likely to yield gas than oil.
But, with almost no well data within the basin, there’s considerable uncertainty how much hydrocarbon resource the basin might contain. The assessment has estimated that technically recoverable oil reserves could lie anywhere within a range of zero to almost 600 million barrels, with a mean of about 173 million barrels. The corresponding figures for natural gas consist of a range from zero to almost 15 trillion cubic feet, with a mean of about 5.5 trillion cubic feet. Natural gas liquids range from zero to 350 million barrels, with a mean of almost 127 million barrels.
The estimates for gas reserves include biogenic gas stored in rock reservoirs but do not include unconventional gas such as coalbed methane, Stanley said.
Two previous USGS assessments in 1995 and 1989 covered a vast swathe of land across the whole of central Alaska and included several oil and gas basins in addition to the Yukon Flats basin. However, the new estimates for oil and gas in the Yukon Flats basin by itself exceed the total estimates for the whole of central Alaska in the earlier assessments. For example, the 1995 assessment for central Alaska estimated a mean of just 2.76 trillion cubic feet of conventional gas.
Stanley attributes the increased estimates to better data.
“This time around we have much better data for the Yukon Flats — we have much better geological information from this area than we did from anywhere in central Alaska during the previous assessments,” he said. Gas from a core hole Lake and river deposits obscure the bedrock in the flats, so there’s little means other than drilling to find evidence of an active petroleum system in the basin. The USGS team did investigate a reported oil seep on the Porcupine River at the east end of the basin but chemical analysis proved that the apparent oil at this location is in fact pond scum from some form of pollution.
Three wells drilled to the east of the basin in the 1970s failed to find any oil or gas shows but some occurrences of bitumen and oil shale have been found to the southeast of the basin.
It’s long been known that loose pieces of a very organic-rich shale known as tasmanite lie in the uplands northeast of the Yukon Flats. Many people have speculated about the possibility of this tasmanite occurring as a hydrocarbon source rock under the Yukon Flats basin. But not enough is known about the subsurface geology to support this idea.
A tantalizing indication of the hydrocarbon potential of the area turned up in 1994 when gas bubbled from coal pulled up from a 1,281-foot core hole at Fort Yukon. A well drilled at the same site earlier this year should provide some interesting information about the subsurface.
“Earlier this year that hole was re-entered by a consortium of federal and state agencies and other groups as a well and it was deepened to a total depth of 2,287 feet,” Stanley said. “The results from that hole are not published yet.” Re-evaluating the seismic Almost all the subsurface information for the Yukon Flats comes from seismic data — oil companies have shot 10 seismic lines within the flats, Stanley said. Five of the lines, shot in 1972, are of poor to moderate quality but can be purchased through a broker. The other five lines date from 1988; they’re of excellent quality but remain confidential. To carry out its assessment, the USGS obtained access to all of the seismic data through purchase or confidentiality agreement. Total line length for the seismic amounts to 418 miles.
USGS geoscientists did new interpretations of all of the lines, while Petrotechnical Resources of Alaska and Doyon also made interpretations of some of the lines.
“For five of those lines we have three different interpretations, for eight of those lines we have two different interpretations, and there are only two lines for which we have only one set of interpretations,” Stanley said.
One particularly telling seismic section from the older survey shows about 25,000 feet of sediment in south-sloping strata in the southern part of the basin. These strata abut the so-called Tintina fault system at the south side of the basin. To the north the sediments thin to 3,000 feet. Geologists believe that the sediments are Cenozoic and possibly late Cretaceous mudstones, sandstones and shales that were deposited from ancient rivers and lakes.
“They were deposited some time between about 70 million years ago and now,” Stanley said.
From the seismic section it’s possible to pick out some folds within the strata. And Stanley said that the more recent, higher-quality sections show a multitude of structures that could trap hydrocarbons.
The Tintina fault along the south perimeter of the basin marks a zone along which the earth’s crust has probably slipped sideways for hundreds of kilometers over the past 60 to 70 million years — it is a fault system that is very similar to the San Andreas fault in California, Stanley said. The seismic evidence indicates that folding and tilting of the sediments in the Yukon Flats basin resulted from movement along the fault.
USGS scientists have been able to use a so-called cloud map, a map that uses colors to depict a combination of gravity and magnetic data, to assess what lies under the Cenozoic sediments at the bottom of the basin. Areas of distinctive color and texture on the map show the existence of domains that contain rocks with consistent characteristics.
“There’s a single domain of dense and magnetic rocks that underlies nearly the entire flats,” Stanley said.
It is possible to trace that domain to areas where the rocks of the domain appear at the surface outside the Yukon Flats. It turns out that this particular domain consists of oceanic sediments of the so-called Angayucham and Tozitna terranes. Past alteration of these rocks as a result of high temperatures and pressures have all but eliminated any possibility of finding oil and gas in them. So, the USGS has concluded that there’s little or no oil and gas potential in the rocks below the Cenozoic under the Yukon Flats basin. Sources and reservoirs But what’s the potential of the Cenozoic sediments?
To help answer this question the USGS team sampled Cenozoic surface rocks around the edges of the basin and also obtained data from previous research in the area. The idea was to find surface rocks that are directly analogous to what geologists believe lies deep in the basin.
“In total we have in our database now more than 600 rock samples from outcrop and shallow core holes from the margins of the Yukon Flats basin,” Stanley said.
The team found abundant examples of coal seams and organic-rich shales that range in age from 24 million to 65 million years. These rocks exhibit strong potential to act as petroleum source rocks, mainly for gas but some with oil potential, Stanley said.
The team also found sandstones and conglomerates that would make excellent oil or gas reservoirs. For example, the Yukon River downstream from the Yukon Flats has cut down through Eocene sandstone and conglomerate with good reservoir properties.
The close juxtaposition of potential reservoir sandstone and potential hydrocarbon source rock also suggests that there could be a very simple petroleum system within the Yukon Flats basin — the oil or gas wouldn’t have to migrate far to reach a reservoir from a source.
“In a number of outcrops we noticed that the sandstones and conglomerates, the potential reservoir rocks, are in close association with the coals and the mudstones,” Stanley said.
In assessing the petroleum potential of the Cenozoic sediments in the basin, the team also looked at equivalent strata exposed at the surface elsewhere in Alaska. For example, very similar sediments of similar age occur around the Healy coal mining area, about 200 miles southwest of the Yukon Flats. On hillsides near Healy it is possible to see thousands of feet of nonmarine Cenozoic sediments that include sandstone and shale strata with coal seams.
“We believe rocks like this … are good analogues of what might occur in the Yukon Flats,” Stanley said.
Perhaps even more intriguing are comparisons with similar geology around the Cook Inlet, where river-deposited conglomerates and lake sediments of similar age to the Yukon Flats sediments have created excellent reservoirs for major oil and gas fields. Oil and gas kitchens The operation of an effective petroleum system depends on the burial of the source rocks to depths where the temperatures are high enough to cook organic material into oil and gas. The oil and gas then flows from the hot “kitchens” into the reservoir rocks. It is possible to model the subsurface temperatures in a basin such as Yukon Flats, to determine the burial depths needed to form oil and gas kitchens.
“According to some modeling, you’d have to bury these rocks to depths of about 2,500 to 3,000 meters before they’ll start generating oil and gas,” Stanley said.
The USGS used gravity measurements to determine where the Cenozoic sediments might lie deep enough for oil and gas to generate. Because the sediments are less dense than the surrounding rocks, a thick section of the sediments causes a relatively low gravity reading at the surface. A gravity map of the Yukon Flats shows several areas where the sediments sit more than 3,000 meters deep; the most extensive area lies in the south of the basin, around the 25,000 feet deep section near the Tintina fault system.
So it is likely that oil and gas have formed deep within the basin. Estimating the reserves But how much oil and gas might there be?
That depends on the quantity and size of potential reservoirs containing hydrocarbon accumulations — the USGS geologists used the seismic and geological data to identify some possible oil and gas accumulation scenarios and estimate the quantities of oil and gas in these scenarios.
To extrapolate these scenario estimates into estimated reserves for the whole basin, the geologists had to perform some statistical analysis using distributions of numbers and sizes of known hydrocarbon accumulations in established oil and gas provinces.
For this analysis the USGS looked at the distributions of actual oil and gas accumulations in the Cook Inlet and on the coast of California. The Cook Inlet Cenozoic geology bears striking similarities to that of the Yukon Flats. The geological setting below the California Coast near the San Andreas fault seems similar to that of the Yukon Flats basin near the Tintina fault: the geological structures of the two areas should hold hydrocarbon accumulations with similar styles of size distribution.
By plugging estimated accumulation numbers and sizes from Yukon Flats into real accumulation number and size trends from the established provinces the USGS geologists could make their statistical predictions of 173 million barrels of oil and 5.5 trillion cubic feet of gas for the Yukon Flats basin.
To put these numbers into some perspective Stanley made comparisons between the Yukon Flats and other areas of Alaska. In recent years the USGS has performed assessments of the National Petroleum Reserve-Alaska and the Arctic National Wildlife Refuge 1002 area in northern Alaska. Both of these areas probably hold substantially more oil that the Yukon Flats basin — 10.6 billion barrels of oil in NPR-A, for example. And, with estimated reserves of 61 trillion cubic feet, NPR-A probably holds much more gas than the Yukon Flats. However, the Yukon Flats may actually hold more gas than the ANWR 1002 area, partly because of a larger land area in the flats.
It is particularly interesting to note that the estimated gas reserves for the Yukon Flats have now come in fairly close to the 6.4 trillion cubic feet of gas produced in the Cook Inlet by 2003, Stanley said.
“The Yukon Flats is no North Slope but could very well be comparable to Cook Inlet,” Stanley said.
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