Thermal Maturation of Eagle Basin, Northwestern Colorado

Vito F. Nuccio, Christopher J. Schenk

Mean random vitrinite reflectance (R_m) was measured on 120 surface samples from Lower Pennsylvanian through Upper Cretaceous rocks in the Eagle basin for the purpose of mapping thermal maturity. The Eagle basin is a trough that formed during the Pennsylvanian between the ancestral Front Range to the northeast and the ancestral Uncompahgre uplift to the southwest.

For comparative purposes, the stratigraphic section in the basin was divided into four intervals, each with a range of R_m values; 2.70-3.70% for Lower Pennsylvanian rocks, 0.70-2.30 for Middle Pennsylvanian rocks, 0.80-1.50 for Upper Pennsylvanian-Triassic rocks, and 0.60-1.70 for Cretaceous rocks. R_m values decrease from Lower Pennsylvanian rocks to Cretaceous rocks throughout the basin. Variability in R_m values within each stratigraphic interval may be due to structural deformation, variations in source material and depositional environment, and spatial and temporal fluctuations in geothermal gradients.

R_m values indicate that most of the Middle Pennsylvanian and Upper Pennsylvanian-Triassic rocks are in the "oil window" (0.60-1.35%) and "gas window" (0.75-3.0%). Although R_m values indicate that Cretaceous rocks are in the oil and gas windows, geochemical data suggest that mainly gas would be generated.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.