Geochemical Correlation of Paleozoic Oils, Northern Denver Basin—Implications for Exploration: GEOLOGIC NOTES¹ Available to Purchase

Analyses of 22 oil samples produced from Pennsylvanian and Permian rocks across the northern Denver basin suggest that three genetically distinct oil types are present. One type, produced from the Permian Lyons Sandstone in fields located near the axis or west flank of the basin, has pristane-phytane ratios of less than 1 (average 0.8), δ¹³C values for saturated hydrocarbons of –28.5 to 29.1 ppt, low amounts of n-alkanes relative to branched-cyclic alkanes, and absence of pentacyclic biomarker compounds. The second oil type, produced from rocks of Virgilian and Wolfcampian age in northeastern Colorado and southwestern Nebraska, has pristane-phytane ratios of about 1.5, predominantly n-alkanes in the saturated hydrocarbon fraction, abundant hopanoid biomarker compounds, and C₁₅₊ saturated hydrocarbon fractions depleted in carbon-13 compared to the Lyons oil type (δ¹³C values –28.8 to –30.4 ppt). Oil produced from the Pennsylvanian (Missourian) Lansing Group (“F” zone) and Cambrian rocks at Boveau Canyon and Sleepy Hollow fields (Nebraska), respectively, is geochemically similar to this second oil type. The third oil type, produced from rocks of Desmoinesian age along the eastern flank of the Denver basin, has a pristane-phytane ratio near 1, intermediate amounts of n-alkanes relative to isoprenoids compared to the other oil types, and the isotopically heaviest saturated hydrocarbons of the three oil types (δ¹³C values −27.7 to −27.8 ppt). This oil also contains pentacyclic biomarkers, but the pentacyclics do not readily distinguish the Desmoinesian oil type from the Virgilian-Wolfcampian oil type.

These three oil types probably were generated from three different source rocks. The geographic distribution of the Virgilian-Wolfcampian and the Desmoinesian oil types suggest at least two broad areas for possible future exploration: (1) in Desmoinesian rocks, along a trend subparallel to the eastern limit of Desmoinesian rocks in the subsurface from the Nebraska Panhandle to the Las Animas arch, and (2) in Virgilian-Wolfcampian rocks, along a generally east-west trend.