The southeasternmost district in the northern Rocky Mountains includes 85,000 sq mi (220,000 sq km) in eastern Colorado, western Nebraska, southeastern Wyoming, and northeastern New Mexico. The dominant structure is the Denver basin, but the Las Animas arch is an important factor in local accumulations and, with the Raton basin, merits careful study for future potential. Pre-Mississippian systems offer little promise of future discoveries because source beds are lacking, and the Triassic-Jurassic section also is unfavorable. Mississippian, Permian, Pennsylvanian, and Cretaceous reservoirs offer the best possibilities, in that respective order of increasing importance. Subsurface information is scant in most of the district for the pre-Cretaceous rock section, and comparat vely little of the area has been explored for stratigraphic traps. The original reserve of 2.2 billion bbl of oil in place conceivably may be doubled, but an addition of only 410 million bbl is rated as "probable." Future gas reserves may be 980 billion cu ft ("probable" and "possible") and, speculatively, even greater.