THE EFFECTS OF STRATIGRAPHY AND COAL AND SHALE THERMAL CONDUCTIVITIES ON TEMPERATURES GRADIENTS IN THE PICEANCE BASIN, COLORADO, USA

The Piceance Basin is a major oil and gas producing basin in western Colorado. More than 27,000 bottom-hole temperatures (BHTs) from oil and gas wells in the basin have been corrected for the drilling disturbance. When these data were plotted as a function of well depth the plots indicated that the geothermal gradients roughly correlated with changes in lithology and thermal conductivity. The two lowest thermal conductivity lithologies in the basin are coal and shale. Coal has the lower thermal conductivity but shale occurs in much thicker sections which typically have a greater thermal resistance and greater effect on deeper temperatures. We have therefore concentrated our initial analysis on mapping isopachs of thick shale sections. Shale lithologies were identified in wells using gamma-ray logs by high gamma-ray signals from high organic-matter content. These identifications were aided by caliper logs as shaley layers tend to collapse and uneven caliper measurements generally indicate shale. Geophysical logs were available through the Colorado Oil and Gas Conservation Commission in the form of LAS files and were read using IHS Petra software. Coal lithologies were primarily identified from density porosity and neutron density logs and because coal has a characteristically low photoelectric factor.

Cretaceous shales have a major effect on the geothermal gradient in the Piceance Basin as calculated from BHTs. Geothermal gradients calculated from the surface to BHTs in the stratigraphic range from the top of Mancos shale to the top of the Dakota are mostly around 40°C/km. Geothermal gradients calculated from the surface to BHTs in the stratigraphic range from the top of the Mississippian to the top of the Precambrian are mostly about 30°C/km. The gradients are higher in the upper section of the basin because of the low thermal conductivity of shales. Geothermal gradients are higher in the southern portion of the basin and we are exploring whether this increase is the result of higher heat flow or lower thermal conductivity.