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Data from 500 wells was analyzed by the USGS to map the shale potential geologically across the North Slope of Alaska.
With mean averages of up to 940 million barrels of oil and 42 trillion cubic feet (Tcf) of natural gas, the North Slope of Alaska ranks second in oil and fourth in natural gas potential of undiscovered, technically recoverable onshore shale oil and gas resources among plays assessed by the U.S. Geological Survey (USGS).
The estimates range from zero up to 2.0 billion barrels of oil and from 0 up to 80 Tcf of gas -- representing technically recoverable oil and gas resources, which are producible using currently available technology and industry practices, regardless of economic or accessibility considerations, USGS noted.
The study, called “Assessment Of Potential Oil And Gas Resources In Source Rocks Of The Alaska North Slope, 2012,” was released on Feb. 24.
As the fact sheet pointed out, these source rocks are known to have generated oil and gas that migrated into conventional accumulations, including the Prudhoe Bay field. However, no attempts have been made to produce oil or gas directly from these source rocks, primarily due to economic and infrastructure considerations.
“Providing scientifically sound, publicly available assessments of the quantity of new, untapped oil and gas resources in frontier areas is but the first step in weighing the potential contributions to energy supplies as well as the impacts of recovering them,” said USGS director Marcia McNutt. “This information can help leaders from both government and industry make good decisions for the long term, anticipate environmental issues in advance of development and guide wise investments.”
Three source rocks of the Alaska North Slope were assessed in this: 1) The Triassic Shublik formation; 2) the lower part of the Jurassic–Lower Cretaceous Kingak shale; and (3) the Cretaceous pebble-shale unit and Hue Shale, together called Brookian shale in this fact sheet.
“The Shublik formation mostly contains a mixture of Type I and IIS kerogen, and oil in conventional accumulations sourced from the Shublik that is of relatively low gravity (23º-39° API) and high sulfur (more than 1.5%).
“In contrast, the Kingak and Brookian shales mostly contain a mixture of Type II and III kerogen, and oil in conventional accumulations sourced from those rocks is of relatively high gravity (35º-42° API) and low sulfur (less than 0.3%), noted the fact sheet.
“These source rocks occur at depths that range from less than 3,000 ft along the northern coast to more than 20,000 ft in the Brooks Range foothills. Over this range of depth, thermal maturity of the source rocks grades from the onset of oil generation in the north, through the oil window, and well into the dry gas window in the south,” the report continued.
Vitrinite reflectance data plus the results of time-temperature and petroleum-generation modeling were used to estimate a boundary between oil in the northern and gas in the southern assessment units on the North Slope for each source rock .
“The presence of brittle reservoir lithologies is an important consideration. Both the Shublik and Brookian source rocks include rock types that are brittle and in which natural fractures are common.
Oil resources are about equally distributed between the Shublik and Brookian source rocks with significantly less potential in the Kingak shale.
For gas resources, the largest concentrations are in Shublik source rocks with significantly less gas potential in the Brookian and small gas potential in the Kingak, according to the report.
“Additional geological information was used to define assessment units and to evaluate the potential for oil and gas to occur in the source rocks. Maps of gamma-ray response in geophysical logs from exploration wells, regional distribution of overpressure, and reconstructed history of burial, uplift, and cooling of North Slope source rocks were among the most important considerations,” noted the USGS.
“Shale-oil and shale-gas formations in the Lower 48 States were used as geologic and engineering analogs in the assessment,” the fact sheet continued.
The North Slope assessment ranks the area just behind the Bakken shale for oil potential, explained Dave Housenecht, USGS research geologist, and behind the Marcellus (81 Tcf), Haynesville (61 Tcf) and Eagle Ford (50 Tcf).
In performing the study, the researchers found that the Shublik and Kingak shales in the far north and northeast of the North Slope have been eroded and are absent source rock, he said.
“We started the study in fall 2008,” he noted. “This kind of information will allow us to construct scenarios that put boundaries on the number of wells” and pads, for example.
The USGS worked with the Alaska Department of Natural Resources, the University of Alaska-Fairbanks, and others knowledgeable about North Slope geology for input into the geological models of the petroleum source rocks.
To learn more about this or other geologic assessments, visit the Energy Resources Program website.
Contact the author, Scott Weeden, at email@example.com.