That celebratory shout you may have noticed reverberating through a part of the oil patch recently may have been the one triggered quietly by a news release.
Not just any news release, of course. This one was from the U.S. Geological Survey, and it announced that its assessment of the Bakken formation in the Williston Basin in Montana and North Dakota revealed about 3.65 billion barrels of undiscovered technically recoverable oil.
In other words, the largest single deposit in the United States except for Alaska.
Along with the oil, the agency tacked on 1.85 tcf of associated/dissolved natural gas and 148 mbo of natural gas liquids.
Some skeptics are questioning the numbers – a USGS assessment of the Bakken in 1995 estimated about 151 million barrels of oil could be found there, making the new figures larger by about 25-fold.
But the assessment was conducted using tried and true geology-based assessment methodology, noted AAPG member Rich Pollastro, a USGS geologist and the Bakken formation task leader.
“We’re mandated by law from the Energy Policy Conservation Act of 2000 to provide these assessments of 32 priority basins in the United States,” Pollastro said.
“These priority basins hold about 96 to 98 percent of the known oil and gas resources for the U.S.,” he noted. “They are prioritized based on not just resource potential but also on federal land percentage.”
The Williston Basin is only about six-and-a-half percent federal lands in the Bakken assessment, Pollastro pointed out, but there are a lot of tribal lands.
“The subsequent Energy Policy Act of 2005 states that we have to use the same methodology in producing these assessments,” he said, “so the Bakken assessment was performed like all others and could not be modified in any way.”
Putting It Together
The USGS findings, as expected, made a huge impact when released in early April, with stories that went far beyond the usual industry publications.
“Billions of Barrels of Oil May Lie Under Northern Plains,” read the headline in the New York Times.
Pollastro spent much of April talking to the media about the findings, and was a popular target for journalists at the recent AAPG Annual Convention and Exhibition in San Antonio.
Speaking to the EXPLORER about the report’s methodology and the area’s geology, he said the assessment is based on geologic elements of a “total petroleum system” that include:
♦ Source rock distribution, thickness, organic richness, maturation, petroleum generation and migration.
♦ Reservoir rock type (whether continual, i.e., unconventional, or conventional), distribution and quality.
♦ Character of traps and time of formation with regard to petroleum generation and migration.
To aid in the estimate of the Bakken resource, detailed framework studies in stratigraphy and structural geology along with petroleum geochemistry modeling were combined with historical exploration and production analyses.
The area involved was delineated into assessment units (AU). Pollastro noted the geologic model used to define the AUs and assess the Bakken formation resources generally includes:
♦ Thermal maturity of the Bakken shale source rocks.
♦ Petrophysical character of the middle sandstone member (ordinarily referred to as a dolomite by industry participants).
♦ Structural complexity of the basin.
The area of the oil generation window for the Bakken continuous reservoir was determined and then divided into five continuous AUs. A sixth hypothetical conventional middle sandstone member AU was defined external to the area of oil generation.
The final assessment numbers released included all of the AUs.
“For the assessment process you have to convert geology and the geologic models into numbers, look at all the engineering aspects, the EURs on the wells, the spacings and so on,” Pollastro said. “It’s very involved for each of these units.
‘Geologic Uncertainty’
The methodology to assess the continuous accumulations – or what the industry calls unconventional – is different from the conventional, Pollastro noted.
“In conventional accumulations we look at sizes and numbers for the accumulations,” he said. “In the continuous type, we assume the hydrocarbons are everywhere – it’s just a matter of how successful you’ll be with recovery.”
The assessment of the Bakken formation indicates that most of the undiscovered oil resides within a continuous composite reservoir that’s distributed across the entire area of the oil generation window.
Also, it includes all members of the Bakken formation – the upper and lower shales and the middle member designated sandstone by the USGS but ordinarily referred to as a dolomite by the industry.
“At this time, only a limited number of wells have produced from the Bakken in the Central Basin-Poplar Dome AU, the Eastern Expulsion Threshold AU and the Northwest Expulsion Threshold AU,” he added.
“This means there is significant geologic uncertainty in these estimates, which is reflected in the range of estimates for oil.”