ABSTRACT

Vitrinite reflectance measurements were made on 227 surface and near surface samples of Tertiary coals from the northern Powder River basin. The data are compared with 1) surface projections of R_m from reflectance measurements in deep wells, 2) estimates of surface R_m computed from Lopatin analysis, and 3) estimates of R_m converted from published proximate analyses of near-surface coals.

In the Montana portion of the basin, the near-surface R_m values range from 0.4 to 0.6%, higher than the theoretical values of 0.2-0.3% derived from the other methods (coal analysis, and Lopatin analysis). However, in Wyoming, the near-surface R_m values of 0.5 to 0.6% generally agree with those from projections of regression lines of deep well data. The discrepancy between the measured and theoretically derived surface R_m in Montana shows only a rough correlation to regional changes in the geothermal gradient; no other known geological factors can account for this discrepancy with the possible exception of weathering of exposed coals and differential erosion.