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

Abstract

Lopatin time-temperature index (TTI) modeling of three locations in the Eagle basin, northwestern Colorado, where vitrinite reflectance (R/sub 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/sub m/ and the corresponding TTI value between 40,000 and 50,000. The first model assumes a constant geothermal gradient of 2.4/sup 0/F/100 ft (43.8/sup 0/C/km), which places the oil window between 270 and 230 Ma. The second model assumes a changing paleogeothermal gradient of 2.80/sup 0/F/100 ft (51/sup 0/C/km), from 320 to 265 Ma and 2.20/sup 0/F/100 ft (40.2/sup 0/C/km) from 265 Ma to present, which places the oil window between 275 and 250 Ma. For the Glenwood location, where the Pennsylvanian section is 4,960 ft (1,512 m), a constant paleogeothermal gradient of 1.80/sup 0/F/100 ft (32.9/sup 0/C/km) works the best in correlating the 2.50% R/sub m/ with the corresponding TTI value of approximately 2,700. Using this gradient, the oil window falls between 175 andmore » 75 Ma.« less