Estimation of Paleogeothermal Gradients and Their Relationship to Timing of Petroleum Generation, Eagle Basin, Northwestern Colorado

Vito F. Nuccio, Christopher J. Schenk

Lopatin time-temperature index (TTI) modeling of three locations in the Eagle basin, northwestern Colorado, where vitrinite reflectance (R_m) profiles were obtained, shows that paleogeothermal gradients and the timing of oil generation in the Belden Formation (Pennsylvanian) varied due to differing thickness of the Pennsylvanian section across the basin.

At the Gilman location, where the Pennsylvanian section is thickest (7,900 ft or 2,408 m), two paleogeothermal gradient models were generated that match the average 3.70% R_m and the corresponding TTI value between 40,000 and 50,000. The first model assumes a constant geothermal gradient of 2.4°F/100 ft (43.8°C/km), which places the oil window between 270 and 230 Ma. The second model assumes a changing paleogeothermal gradient of 2.80°F/100 ft (51°C/km) from 320 to 265 Ma and 2.20°F/100 ft (40.2°C/km) from 265 Ma to present, which places the oil window between 275 and 250 Ma.

At the Sweetwater location, where the Pennsylvanian section is 3,900 ft (1,189 m), two models were generated that match the 2.50% R_m with the corresponding TTI value of approximately 2,700. The first assumes a constant gradient of 2.40°F/100 ft (43.8°C/km) through time and places the oil window between 200 and 85 Ma. The second model assumes a gradient of 2.80°F/100 ft (51°C/km) from 320 to 265 Ma and a gradient of 2.40°F/100 ft (43.8°C/km) from 265 to present, placing the oil window between 225 and 90 Ma.

For the Glenwood location, where the Pennsylvanian section is 4,960 ft (1,512 m), a constant paleogeothermal gradient of 1.80°F/100 ft (32.9°C/km) works the best in correlating the 2.50% R_m with the corresponding TTI value of approximately 2,700. Using this gradient, the oil window falls between 175 and 75 Ma.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.