A well-worn play is attracting a fresh look, thanks to the efforts of a small Denver independent. In 1982, Natural Gas of California discovered Horse Trap Field in Wyoming's Lincoln County. The find was on the Darby Thrust, the easternmost plate in the Wyoming Overthrust belt. NGC drilled a structure identified on 2-D seismic and found production in the Pennsylvanian Amsden. The initial well was offset twice, however, and both attempts were dry. After producing some 1.7 billion cubic feet of gas during a 10-year span, the lone well was plugged in 1992. Apparently, almost everyone in the industry forgot about the little field. Most discounted the discovery because of its location on the Darby Plate. While the Absaroka Thrust immediately to the west harbors a multitude of prolific fields, including Anschutz Ranch and Whitney Canyon, the Darby has always been a poor second cousin. The Darby earned its shabby reputation through decades of fruitless exploration, although early signs were promising. As long ago as the 1890s, several small oil fields were developed downdip of oil seeps in the Frontier and Aspen shales, Cretaceous formations that outcrop all along the 90-mile length of the Darby plate. "The best guess is that these outcrop fields produced about 500,000 barrels of oil from very shallow wells," says Bob Kenney, exploration manager, Condor Exploration. The next surge of drilling occurred in the early 1900s, when the great surface anticlines were tested throughout the Rocky Mountain West. A number of wells were drilled on the many such structures that dot the Darby Plate; shows were common, but the reservoirs were wet, says Kenney. The problem: the rocks of the surface plate outcrop or subcrop to the east, which allowed all the shallow features to be flushed. Optimism returned in the heyday of Overthrust exploration in the 1970s and 1980s. Seismic was improving rapidly, and finds were being made all along the neighboring Absaroka Plate. The 2-D methods worked very well on the Absaroka thrust, a plate characterized by fairly thick, ramped anticlines with few faults. The Darby is far more complex, however, with seven or eight imbricated thrust plates and abundant faults. The 2-D data couldn't accurately image the structures in such a convoluted environment. Aside from Horse Trap, the only find on the Darby was Christmas Creek, a noncommercial oil pool. To further sour industry's taste for the Darby, a number of very deep, expensive wells were drilled to test apparent subthrust closures below the thrust plate; all were dry. "The Darby is a special situation," notes Kenney. "When Paleozoic carbonates are thrust over Cretaceous rocks, huge velocity pull-ups are created." The giant structures that appeared to lurk below the thrusted rocks were in fact geophysical illusions. Condor Exploration, formed in 1999, has dedicated its efforts to the much-maligned Darby play. The company decided to redevelop the original Horse Trap discovery, and also to explore the surrounding areas with 3-D seismic. Only that technology could unravel the potential of the stacked reservoirs and stacked thrust plates, it believed. The company now holds some 85,000 acres on the Darby Plate, most of which are in long-term federal leases. It has drilled two successful wells in Horse Trap Field, which flowed at initial rates of 2.8 million and 3.5 million cubic feet per day from Amsden. The company also shot a high-definition, 67.5-square-mile 3-D seismic survey; recommissioned a 10-mile, six-inch existing pipeline; and completed a federal environmental assessment. "We believe that four to five additional offsets could be drilled on the Horse Trap structure, which we think contains recoverable reserves of 10- to 12 Bcf," says Nixon Lange, chief operating officer. "Horse Trap is not a big feature, but it proves that oil and gas have been formed, migrated and trapped on the Darby Plate." Indeed, the seismic has revealed that closures on the shallow Amsden thrust plates are very prevalent. Condor is especially intrigued by a large, four-way structure that it sees in the southern portion of its 3-D survey. This feature has 800 to 1,200 feet of closure and covers several thousand acres. It could contain gas-in-place of about 800 Bcf in the Madison and Big Horn formations. "Clearly, the Darby Plate has giant-field potential," says Lange.