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Frontier Areas for Coalbed-Gas Exploration in Utah
By
David E. Tabet1 and Jeffrey C. Quick1
Search and Discovery Article #10057 (2004)
*Adapted from “extended abstract” for presentation at the AAPG Annual Meeting, Salt Lake City, Utah, May 11-14, 2003.
Two other articles on coalbed methane accompany this presentation; they are: Coalbed Methane Potential and Activity of the Illinois Basin and Coalbed Methane Potential and Activity of the Western Interior Basin, both by Steven Tedesco.
1Utah Geological Survey, Salt Lake City, UT ([email protected])
Since 1992, there has been remarkably successful development of Utah’s gas deposits associated with the coals in the Ferron Sandstone Member of the Mancos Shale in central Utah’s Emery coalfield. At the end of 2001, the Ferron coalbed-gas area, or play (Figure 1), had over 490 producing gas wells, and these wells accounted for 28 percent of the state’s total gas production for that year. Encouraged by the success of the Ferron play, petroleum companies are looking elsewhere in the state for new, economic, coalbed-gas plays.
There are currently six prospective coalbed-gas plays in Utah being explored (Figure 1 and Table 1):
1) Blackhawk Formation coals in the Book Cliffs coalfield
2) Coals in the Emery Sandstone Member of the Mancos Shale in the Wasatch Plateau coalfield
3) Neslen Formation coals in the Sego coalfield
4) Dakota Sandstone coals in the Alton-Kolob coalfields
5) Straight Cliffs Formation coals in the Johns Valley-Kaiparowits Plateau coalfields
6) Thick coals in the Frontier and Adaville Formations that extend south from Wyoming into the Henrys Fork coalfield in northern Utah.
These plays have attracted varying levels of industry development interest.
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uJohns Valley-Kaiparowits Plateau play
uJohns Valley-Kaiparowits Plateau play
uJohns Valley-Kaiparowits Plateau play
uJohns Valley-Kaiparowits Plateau play
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The best defined, and most mature, frontier coalbed-gas play in Utah consists of the thick, gassy, high-volatile bituminous coals of the Blackhawk Formation in the Book Cliffs coalfield. Exploration in this play began in the early 1970s, and there was a modest amount of coalbed-gas production during the 1990s. The area may be poised for full-scale development if the current efforts can overcome the water-disposal and well-completion problems that plagued the earlier development efforts. Although the Book Cliffs coalfield has had extensive mining, there are still about 70,000 acres underlain by deeper and unmined coal in the Blackhawk Formation where the aggregate coal thickness ranges from 10 to 70 feet, and drilling depths range from 1500 to 6000 feet (Table 1). The measured gas content of these coals ranges from a modest 60 cubic feet per ton, to an attractive 435 cubic feet per ton of coal. The in-place gas resource in the undeveloped play is estimated to range from 0.8 to 1.9 trillion cubic feet (Tcf). A joint venture of the J.M. Huber Corporation and the Patina Oil and Gas Corporation is redeveloping the previously abandoned Castlegate field, which is reported to contain over 27 billion cubic feet of proven gas reserves for its 17 producing wells. Meanwhile, the J-W Operating Company has recently completed drilling a six-well pilot test of the Blackhawk coals farther to the east.
The coalbeds in the Emery Sandstone Member of the Mancos Shale occur under the Wasatch Plateau and are not exposed at the surface. These coals have never been mined, but are projected from a handful of oil and gas test wells to underlie about 180,000 acres of the Wasatch Plateau of central Utah. The coals are between the overlying Blackhawk Formation coals and underlying Ferron Sandstone coals. Up to 17 coalbeds, with an aggregate thickness of up to 120 feet, occur within the 1600-foot-thick Emery strata. No gas content measurements are available for these high-volatile bituminous coals, but some beds are reported to be gassier than others. The in-place gas resource for this play is estimated to range from 0.8 to 3.2 Tcf. Prima Energy Corporation is actively exploring this play on its 71,000 net acres of leases in the area. Several other petroleum companies have drilled wells in this play in the past few years.
The Neslen Formation coalbeds are known from the Sego coalfield where they are exposed along the Book Cliffs of Grand County; these coalbeds can be traced northward into the subsurface in Uintah County near the Colorado-Utah state line and underlie roughly 240,000 acres. The 400- to 700-foot-thick Neslen Formation contains an average of 14 feet of coal in one to seven beds. Shallow core samples taken near the outcrop of the beds provided gas-content measurements only as high as 48 cubic feet per ton of coal, but there are indications that the deeper coals may be gassier. CDX Rockies, LLC has drilled a well to test the gas content of deeper coalbeds in the Uintah County portion of the play, but no results have been released. The in-place gas resource of this play is estimated to range from 0.3 to 1.8 Tcf.
The coalbeds of the Dakota Sandstone are best developed in southern Utah in the Alton-Kolob coalfields, and this is the area that has drawn company exploration interest. The 140- to 450-foot-thick Dakota Sandstone in this play contains two subbituminous coal zones, an upper one named Smirl and a lower one named Bald Knoll. These two, lenticular coal zones can collectively contain up to 18 feet of coal, and they have an average aggregate thickness of about 13 feet. To date, only two gas-content measurements have been reported for shallow coal core samples, and they revealed low gas contents of 0 to 14 cubic feet per ton. This 400,000-acre play could contain nearly 1 Tcf of gas if the average gas content of the deeper coalbeds is only 100 cubic feet per ton. Several companies have leased 84,000 acres or more in the play. Legend Energy of Utah, the most active of these companies, has staked 23 drill hole locations; drilling began at two locations in November 2002.
Johns Valley-Kaiparowits Plateau Play The thick coals of the Straight Cliffs Formation have attracted some company interest in the Johns Valley area and the northern part of the Kaiparowits Plateau coalfield outside the Grand Staircase-Escalante National Monument in Garfield County. The 130,000-acre play contains an average aggregate thickness of 40 feet of coal within the 600- to 1600-foot-thick John Henry Member of the Straight Cliffs Formation. The 13 gas measurements taken from relatively shallow coal cores were not encouraging, with gas contents of less than 7 cubic feet per ton of coal. A group of investors with a significant fee position in the Johns Valley portion of the play is attempting to put together a drilling program to evaluate the coals and their gas content in this part of the play. They expect that deeper subbituminous coalbeds will be gassier because their gas has not leaked to the surface. This play could contain 0.93 Tcf of gas if the deeper coalbeds contain at least 100 cubic feet of gas per ton of coal.
This frontier coalbed gas play lies north of the Uinta Mountains in eastern Summit County. This is actually a dual play, where the thick coals in both the Frontier and overlying Adaville Formations can be traced south from Wyoming, where they crop out, into northern Utah, where they are poorly exposed or buried by younger rocks. The coals appear to have a high-volatile bituminous rank in Utah, which is slightly higher in rank than the corresponding coals in Wyoming. These two coalbed gas plays occur as parallel, north-trending belts, with the Adaville play covering roughly 35,000 acres, and the Frontier play covering about 15,000 acres. Data from two Utah wells indicate that the Adaville Formation has an average aggregate coal thickness of 100 feet, while coals of the Frontier Formation average an aggregate thickness of 50 feet. No activity is known on the Utah portion of the play, but at least two companies have drilled wells to test the gas content of these coals in Wyoming, although no test results have been released. As a result, little is known of the gas content of these coals. The total in-place coalbed gas resource for these two formations is estimated to range from 0.06 to 0.99 Tcf.
Utah has six frontier coalbed gas plays that are in various stages of testing or development. The undeveloped portions of these six plays are estimated to contain in-place gas resources ranging from 1.96 to 9.78 Tcf. |
