Petroleum Migration in Denver Basin Inferred from Thermal Maturity and Hydrologic Data

Ernest A. Jones, Jr., Donald L. Gautier, Raymond Siever

Cretaceous petroleum accumulations in the Denver basin of eastern Colorado and southwestern Nebraska occur in a productive fairway where potential source rocks are thermally immature for oil generation. Reconstructed potentiometric surfaces, vitrinite reflectance (R_o), and other thermal maturity data suggest that fluids within the basin have migrated hundreds of kilometers from western thermally mature areas (> 0.65% R_o) updip to eastern thermally immature areas (< 0.50% R_o). Oil fields such as Adena and Little Beaver with cumulative production of tens of millions of bbl of oil occur where R_o is below the threshold 0.60% value, the commonly accepted value that indicates the beginning of thermogenic petroleum generation. Variations in cementation, evidenced in the Denver basin by present east-to-west reductions in porosity and permeability, may have affected secondary migration.

Ground-water potentials for the Lower Cretaceous "J" sandstone, calculated from drill-stem test data, decrease from west to east across the basin with a gradient of about 3 m/km. Local potential minima in Morgan and Logan Counties, as well as an increase in formation water salinity from 1,000 ppm to 12,000 ppm toward the basin center, suggest the concentration of formation fluids in those areas. About 65 Ma, when Cretaceous shales first became mature enough to expel hydrocarbons, the initial uplift of the Rocky Mountains created a fluid potential field similar to the present one but of greater magnitude. This ancestral potential caused the generated hydrocarbons to migrate eastward; oil pools then concentrated at paleopotential minima. The analysis of fluid potential gradients makes i possible to determine the dynamics of forces that redistribute fluids in a basin.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.