The Bakken formation play in the Williston basin is writing a new chapter in the oil reserves history of the US onshore Lower 48, thought by many to be a mature and fast-depleting province.
The play is spearheading production and reserves increases in Montana and North Dakota and prompting reevaluation of earlier resource assessments for the play, said the US Energy Information Administration.
EIA’s report attributed to the US Geological Survey an estimate of the resource for the entire Bakken formation at 271-503 billion bbl in place, with a mean of 413 billion bbl. USGS noted that it has never published those figures, which are preliminary results from research that was never completed and therefore never technically and scientifically peer-reviewed.
The USGS’s 1995 assessment stated that the Bakken might have generated billions of barrels of oil but included no specific figures. A new USGS assessment of the entire basin, due out in about a year, will contain a numerical estimate of the technically recoverable oil resource in the Bakken, the USGS said.
“If the potential bears out, this could (depending on recovery factors) increase the estimate of technically recoverable crude oil resources in the US by billions of barrels,” EIA said in what it promised to be the first in a series of reports on technology-based oil and natural gas plays. By comparison, the most recent estimate of the technically recoverable crude oil resource of the entire US is 174.67 billion bbl, excluding the Bakken. US proved reserves at Dec. 31, 2005, were 21.757 billion bbl (OGJ Online, Oct. 23, 2006).
Play’s largest field
Elm Coulee field, in Richland County, Mont., is at 529 sq miles the largest discovered field in the Middle Bakken formation to date.
Headington Oil Co., private Dallas operator, has estimated oil in place at 5 million bbl/sq mile in the Bakken in this field.
“With an assumed 10% average recovery factor, Headington estimates primary oil recovery could be 500 million bbl from Elm Coulee field,” EIA said. The 1995 USGS assessment estimated ultimate Bakken recovery at 151 million bbl.
EIA predicted, “It is likely that other energy resources also await discovery or rebirth in the US as new technology allows us to locate, define, interpret, and extract them economically.”
The continuous nature of the Bakken formation means its hydrocarbons, deposited during the Upper Devonian and Lower Mississippian periods, have not accumulated into discreet reservoirs of limited areal extent, but the Bakken remains to be penetrated across a considerable portion of the basin.
The Bakken play
Current US activity in the Bakken is taking place in the “badlands” of Richland County, Montana, and McKenzie, Golden Valley, and Billings counties, North Dakota, EIA noted.
The Bakken play started after operators analyzed geologic data on a decades-old producing area, identified an untapped resource, and applied horizontal drilling and fracturing technology to exploit it (OGJ, Dec. 10, 2001, p. 44).
Elm Coulee field, discovered in 2000, produced 15 million bbl of oil (41,000 b/d) in 2005 and is yielding nearly 50,000 b/d. This is about half Montana’s crude oil production.
Current drilling is focused on the middle member of the Bakken, which is more porous and permeable than the overlying and underlying Bakken shales. The Bakken is at 11,000 ft in the basin depocenter in southwestern North Dakota and 3,100 ft in Manitoba and Saskatchewan.
As the Williston basin subsided, EIA explained, massive low-permeability carbonates above and below the Bakken acted as seals. The increased temperatures and pressures that accompanied subsidence thermally converted the kerogen content of the shales into oil.
“Sealed with no conduit or high-permeability formation adjacent to the Bakken formation to allow the petroleum to escape, the internal fluid pressure within the shales rose and eventually fractured the shales and the middle member from within,” EIA explained.
Reserves, production growth
Proved reserves at the end of 2005 were 427 million bbl in Montana, 106% above the state’s 1999 reserves. Operators added 63 million bbl of the increase from field extensions in 2005 alone. Proved reserves at the end of 2005 were 418 million bbl in North Dakota, up 59% since 1999, with 29 million bbl added in 2005 from field extensions.
The increases were almost exclusively from Bakken development and Ordovician Red River fields on the Cedar Creek anticline, and EIA said further gains are expected.
Early Bakken drilling targeted the shales, which swell on contact with water. The formation contains iron pyrite, which forms an iron hydroxide precipitate when exposed to the common fracturing fluid hydrochloric acid.
“There are reported cases of this phenomenon causing irreparable well damage,” EIA said.
The former Lyco Energy Corp., Dallas, and Halliburton Co. frac horizontal Bakken intervals by aligning the horizontal leg so that induced fractures have longitudinal orientation to the well bore. The formation produces through an uncemented, preperforated liner.