Exploratory drilling in eastern and northwestern Colorado increased from 357 wells drilled in 1977 to 574 in 1978. The success rate for exploratory wells decreased from 19.6% in 1977 to 12.5% in 1978. Development drilling decreased from 783 wells in 1977 to 653 in 1978. In the Denver basin, 820 wells were drilled during 1978 with 46.8% success. There were 129 wells drilled in southern Colorado with 25.6% success. Northwestern Colorado had a 59.7% success in 278 wells drilled. Total drilling in the report area increased from 1,140 wells in 1977 to 1,227 in 1978. The overall success rate decreased from 59.2% in 1977 to 47.5% in 1978. Lower Cretaceous Muddy D and J sandstones, Upper Cretaceous Sussex and Shannon sandstones, and Upper Cretaceous Niobrara Chalk were the main o jective reservoir rocks in the Denver basin. Paleozoic rocks remained the major objective in southeastern Colorado, and Cretaceous and Tertiary units were the objectives in northwestern Colorado.