Geothermal anomalies on the eastern flank of the Cherokee basin, southeastern Kansas, USA

Abstract

Bottomhole temperature measurements from oil and gas drilling in southeastern Kansas on the eastern flank of the Cherokee basin, in combination with a suite of about 2,200 differential temperature logs recently obtained from wireline logging in coalbed methane wells, define several higher-temperature anomalies at the top of the Mississippian Subsystem strata. Temperatures slightly in excess of 90 oF (35 oC) at depths of about 900 ft (275 m) correspond to geothermal gradients as high as about 60 oC/km.
 Sparse historical measurements of heat flow in the cratonic Cherokee basin indicate that the thermal anomalies are not likely caused by locally high heat flow. Heat flow in the Cherokee basin is probably in line with most other shallow cratonic basins. The higher-temperature thermal anomalies defined by logging temperatures do not correspond to previously mapped faults or other structural features in the Phanerozoic sedimentary section, but some anomalies are underlain by Precambrian basement lineations that are detectable with aeromagnetic and gravity measurements. Well-log determination of shale content in the Pennsylvanian sedimentary strata overlying the Mississippian limestones indicates that low thermal conductivity caused by higher shale content may cause some of the thermal anomalies. 
 Lateral (advective) movement of warmer, highly saline water from the basin axis cannot account for the anomalies because the anomalies are not characterized by exceptionally highly saline water in Mississippian strata. Similarly, recorded static fluid levels of wells disposing of oilfield saltwater into the Mississippian strata and the deeper Cambrian-Ordovician Arbuckle Group indicate that the deeper Arbuckle strata generally do not have sufficient formation pressure to force Arbuckle formation water upward into the Mississippian through either natural fractures or leaky wellbores. Small-scale changes in salinity, in combination with geologic structuring indicating faulting, make a case for vertical (convective) movement of heated, less saline water from the Arbuckle Group into overlying Mississippian limestones in isolated localities. Buoyancy of the Arbuckle formation water (due to temperature and salinity differences with the cooler and more saline Mississippian water) could also be the primary force behind the convection. Convergence of cooler freshwater moving westward off the Ozark dome and more saline basinal water moving eastward also could be a factor in defining the limits of some thermal anomalies.