Thermal structure of the Anadarko Basin
Jaquidon Gallardo and David Blackwell
AAPG Bulletin, Feb. 1999.
The objective of this study is to develop a contemporary regional thermal model for the Anadarko Basin of Oklahoma, a two stage Paleozoic cratonic basin. The basin contains as much as 12+ km of sedimentary fill and is the site of several of the deepest wells drilled in North America. The thermal model is developed from lithologic analysis at 3 m (10 ft) intervals in 63 wells, heat flow measurements at 7 sites, and in situ thermal conductivity calibration of the sediment section at two sites. The in situ calibration of thermal conductivity is accomplished using detailed temperature logs and represents a new technology in the evaluation of sedimentary basin thermal characteristics. Shale exerts the most control on the temperature distribution because it is the most abundant and has the lowest thermal conductivity. Overall shale dominates basin thermal structure. Shale represents 47 % by volume of the rock in the basin and represents about 75 % of the total thermal resistance, directly related to the temperature gradient. Thus the problems in sampling and characterizing the in situ thermal conductivity of shale from laboratory measurements represents a major limitation in basin thermal regime analysis. The results show that the present day temperatures do not mimic the structure of the sediments, i.e. the hottest areas on a given age horizon in the lower Paleozoic are not in the most deeply buried parts of the Anadarko basin. The combination of decreasing heat flow toward the Wichita Mountains and the facies change in the Pennsylvanian units from marine shale (low thermal conductivity) in the basin to "granite wash" (high thermal conductivity) toward the mountains results in the highest temperatures being displaced about 50 km into the basin from its deep southern margin. The pattern of vitrinite reflectance on the Devonian Woodford Formation is virtually identical to the present day reconstructed temperature pattern. Therefore the thermal pattern in the basin is dominated by conductive heat transport. In contrast to the usual approach, BHT data are used as an independent information set and are used to evaluate the results of the analysis, rather than an integral part of the analysis. Error of temperature calculation can be evaluated in this way. The ability to independently test the thermal predictions is a feature on this new approach.