By John Kemp
LONDON May 1 The Williston Basin underneath
North Dakota, Montana and South Dakota contains twice as much
crude and three times more gas than previously thought,
according to an updated assessment published by the U.S.
Geological Survey (USGS) on Tuesday.
USGS assesses that at least 7 billion barrels of oil, and
6.4 billion cubic feet of natural gas, could ultimately be
recovered from the two formations, with at least a 50 percent
That is up from 3.6 billion barrels and 1.7 billion cubic
feet at the time of its last assessment published in 2008.
The increases are smaller than some market participants
anticipated, and remain far more conservative than estimates by
some of the companies drilling in the area.
But USGS estimates are generally and deliberately
conservative. Historically, the amount of oil ultimately
recovered has exceeded them in many cases, as technology
improves and more becomes known about remote parts of the
formations. So there may be scope for even more crude and gas to
USGS made only minor revisions to its estimate of the
undiscovered but technically recoverable resources (UTRR) of oil
contained in the basin's Bakken formation. But it boosted its
estimates for associated gas. More importantly, it thinks there
is at least as much oil and gas that can be recovered from a
lower layer of rocks known as the Three Forks formation that was
not included in the last assessment published in 2008.
The Bakken formation is actually three layers of rock. Upper
and lower layers of thick black marine shale which have a total
organic content varying from 1 percent to as much as 35 percent,
and a middle layer of sandstone, siltstone, mudstone and
The upper shale layer (which is up to 58 feet thick) and the
lower one (up to 56 feet thick) are thought to be the source of
the oil and gas in the area. But some of the crude and gas has
been expelled and has migrated into the middle layer where it
forms small conventional oil and gas fields.
Large quantities also appear to have migrated downwards into
the underlying Three Forks formation. Three Forks consists of
several layers of tight rock and is as much as 270 feet thick in
the central part of the basin.
USGS undiscovered oil and gas assessments
Continental Resources on Three Forks:
Three Forks' oil and gas potential has attracted increasing
interest from exploration and production companies like
Continental Resources, which pioneered the Bakken shale
Companies hope to drill down vertically through both
formations, with separate laterals at each layer, maximising
efficiency and oil recovery.
Continental outlined the potential in an extensive
presentation to investors in September 2012, which also outlined
its proposed well-spacing plan.
In its own assessment, Continental estimated that including
the lower parts of the Three Forks increased the total amount of
oil originally in place (OOIP) from 577 billion barrels of oil
to 903 billion, and the amount that is technically recoverable
from 20 billion barrels to as much as 32 billion, 36 billion or
even 45 billion.
Continental is much more optimistic about the total amount
of oil that could ultimately be recovered than the USGS, which
is not surprising given its vested interest in the play.
But the company has already drilled a quarter of all the
wells so far into the top part of the Three Forks. It has
completed its first producing well into the next layer down, and
is starting to test wells drilled to even greater depths into
Three Forks is a thicker and more extensive layer than the
Bakken - extending further east across North Dakota, west into
Montana and as far south as South Dakota. However, only 5
percent of the formation area has been tested, compared with
14-33 percent in some of the better known parts of the Bakken.
As a result there is greater uncertainty about how much oil and
gas it might contain and how much could ultimately be recovered.
USGS thinks there could be between 2.8 and 4.6 billion
undiscovered barrels of oil ultimately recoverable from the
Bakken, but its estimate for the Three Forks ranges from 1.6
billion to 6.8 billion.
For the two formations, USGS estimates for technically
recoverable oil resources therefore range widely from 4.4
billion to as much as 11.4 billion barrels.
USGS employs a geology-based assessment methodology. It
examines factors like the thermal maturity of the source rocks,
organic content, porosity, and the geographical extent of the
formations in the oil window. It then estimates technically
recoverable resources by comparison with similar formations
More than 4,000 wells have been drilled, and roughly 450
million barrels of oil produced, since the 2008 USGS assessment.
Most has come from the Bakken, which has provided more data on
the region's geology, and allowed the assessors to improve their
estimates for the Bakken and make a first pass at estimating
recoverable resources from Three Forks.
"We agree with the range of numbers," the director of North
Dakota's Department of Mineral Resources said in a press
statement, "...and think the high estimate of 11 billion barrels
is a reasonable target as technology and exploration of the
Three Forks continues."