Abnormally High Geothermal Gradients in the Cherokee Basin, Southeastern Kansas, USA
Newell, Kerry D.; Merriam, Daniel F.
The Cherokee Basin in southeastern Kansas is a shallow northward extension of the Arkoma Basin in Oklahoma. It is an asymmetric cratonic basin where strata on its eastern flank dip gently west-northwestward at a rate of 4 m per km. The maximum sedimentary thickness is about 1100 m. The western flank is steeply dipping and faulted due to late-Mississippian—early-Pennsylvanian structural movement associated with the south-southwestward plunging Nemaha Uplift.
Approximately 1300 temperature logs were obtained by drilling for coalbed methane (CBM) in the Cherokee Basin in the last 15 years. These wells were mostly air-drilled and targeted several thin coal seams in the Pennsylvanian Series at depths from 250 to 375 m. Each well was drilled in about 1 day and formation waters would then quickly fill the hole before logging. TDs are usually in Mississippian limestones lying unconformably underneath the Pennsylvanian siliciclastics. The temperature profile in the well logs markedly increases at the basal Pennsylvanian unconformity, then gradually increases in to deeper strata. This suggests that relatively thick Pennsylvanian shales have low thermal conductivity, and thus act as a thermal blanket.
Thermal mapping based on the temperature logs define regions, 750-1000 sq.-km in area, that have, for example, 30 deg C fm. T at 300 m depth, which translates to geothermal gradients of ~50 deg C/km. The source of the anomalous heat is conjectured to be radioactivity in the Precambrian crystalline basement complex and possibly northward movement of heated water out of the Arkoma Basin. The temperature anomalies are also manifest by higher ranks of coal in southeastern Kansas as compared to lower-rank coals at similar depth in the Forest City Basin in northeastern Kansas.
The close spacing of the temperature logs (as many as 100 per 36-sq.-mile [93-sq.-km] township) allows for detailed temperature mapping. Formation temperatures derived from the temperature logs are also less variable than that derived by bottom-hole temperatures from earlier oil and gas drilling, hence these thermal anomalies have gone largely undetected until the region was intensely explored for CBM. The thermal anomalies likely extend in to southwestern Missouri and northeastern Oklahoma.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013