High Heat Flow in the Idaho Thrust Belt: A Hot Sedimentary Geothermal Prospect

摘要

A synthesis of bottom-hole temperature (BHT) and drill stem test (DST) data compiled for the National Geothermal Data System (NGDS) in the vicinity of southeast Idaho's Blackfoot volcanic field (BVF) was used to calculate heat flow for 31 oil and gas exploration wells drilled in the Idaho thrust belt (ITB). The temperature data and heat flow estimates define a previously unrecognized high-temperature geothermal prospect in Jurassic and Triassic sedimentary rocks adjacent to the BVF at depths of 3-4 km, approximately 25-50 km north of the late Quaternary (58 ka) China Hat rhyolite domes of the BVF. The rhyolite magma, at a depth of 12-14 km, and/or its associated parent mafic magma is believed to be the heat source responsible for driving hydrothermal fluids and heat into the ITB. Several BHT correction methods were tested against DST data and an aggregate average of the best methods was computed and applied to all BHT data. Formation thermal conductivities were also evaluated to calculate more accurate heat flow. An area greater than 150 km~2 has heat flow greater than 120 mW/m~2 and temperatures in excess of 150°C at 3 km. Another localized area defined by a single well also exhibits anomalously high heat flow and subsurface temperatures (116 mW/m~2 and 170°C at 3.5 km, respectively). The major ion chemistry of hot brines and saline formation fluids indicates they are the product of dissolution of evaporite beds in the Jurassic Preuss Sandstone in response to circulating hydrothermal fluids. Their spatial occurrence relative to salt-bearing strata suggests they may play a role in redistributing and storing heat, which could have implications for how these hot sedimentary reservoirs are developed.