Geothermal Setting of the Gulf of Mexico

Seiichi Nagihara
Department of Geosciences, Texas Tech University, Lubbock, TX 79409

The present work provides an overview of the geothermal setting of the Gulf of Mexico, using a recent compilation of bottom-hole temperature (BHT) data from wells widely distributed in the northern continental shelf and heat flow probe data in the continental slope and the abyssal plain. The BHT data have been corrected for drilling-related disturbance. Heat flow in the Gulf varies geographically from less than 20 mW/m² to near 100 mW/m². The variation is controlled primarily by three geologic factors; salt diapirism, clastic sediment influx from the North American continent in the Cenozoic, and the igneous crust-mantle structure shaped in the early stage of the basin’s formation.
Salt diapirs such as the Sigsbee Knolls and others on the Texas-Louisiana continental slope tend to show localized high heat flow anomalies. Thermal conductivity of salt is 2 to 4 times greater than that of other sedimentary rocks. A tall, diapiric salt plug funnels geothermal heat from deep, high-temperature sediment. Laterally extensive bedded salt and salt sheets do not have the heat focusing effect.
Along the northern continental shelf, there is an east-west variation trend in which thermal gradient generally decreases from Alabama to eastern Louisiana, and then increases toward south Texas. The east-west variation is overprinted by another trend in which thermal gradient decreases from inner shelf to outer shelf. These variation trends reflect the accumulation history of massive clastic influx from the North America in the Cenozoic. In a fast sedimentation environment, geothermal heat conduction upward through the sedimentary column cannot keep pace with the rate of sediment thickness increase. That results in a lower thermal gradient than in a slow sedimentation environment. The southeast Louisiana shelf, where geothermal gradient is the lowest, coincides with depocenter locations of the clastic influx since the Miocene. As the continental shelf built up seaward, newer sediment accumulated more toward outer shelf and resulted in even lower thermal gradient there.
Impacts of the Cenozoic clastic influx can also be seen in deepwater. Over the Mississippi submarine fan, heat flow is lowest at its center (< 20 mW/m²) and gradually increases away from it. In the western abyssal plain, where seafloor is deepest and farthest away from major sources of clastic influx, heat flow is in the low 40s mW/m². Heat loss by sedimentation accumulation is still a significant factor there, but is compensated by radiogenic heat production within the sediment. The low-40s-mW/m² heat flow values are consistent with the Late Jurassic age of the oceanic crust underlying the sediment.
In the northeastern margin of the abyssal plain, there is a linear zone across which heat flow abruptly increases toward the Florida escarpment. The location of the zone coincides with the previously suggested boundary between the oceanic crust and the continental crust that was stretched during the initial rifting associated with the formation of the Gulf basin. The higher heat flow over the stretch continental crust indicates that radiogenic heat production within the granitic rock contributes significantly to heat flux in the continental margin.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands