Preliminary Geothermal Gradient Mapping in Alaska?s Cook Inlet Forearc Basin

Abstract

Geothermal gradients were calculated in 118 Cook Inlet oil and gas wells using bottom-hole temperature (BHT) and drill stem test fluid temperature (DST) data acquired during drilling and production testing. Only wells with BHTs and DSTs recorded between 8,000-12,000 ft true vertical depth (TVD) were considered. Over this depth interval, average well depth was 9,953 ? 845 (? standard deviation) ft and uncorrected BHTs ranged from 78-210°F. A correction factor ranging from 9.3 to 15.7°F was calculated for the cooling effect of drilling fluid circulation by comparing differences between BHTs and DSTs in wells containing both temperature readings. Gradients calculated from corrected BHTs ranged from 0.95 to 1.83°F/100ft. Geothermal gradients were highest adjacent to active faults and a slight increase in gradients from north to south was observed across the basin; however, the corrected geothermal gradient average was 1.27°F/100ft, slightly lower than the global average (1.37°F/100ft). These slightly depressed gradients are likely the result of cooling typically found in forearc basin settings.