Geothermal gradients of the northern continental shelf of the Gulf of Mexico

A wide, systematic variation of sedimentary geothermal gradients has been previously observed along the northern continental shelf of the Gulf of Mexico. From east to west, geothermal gradients change from 25 to 30 degrees C/km off Alabama to lower values (15-25 degrees C/km) off eastern Louisiana and to higher values (30-60 degrees C/km) off Texas. In order to assess the mechanism responsible for this variation, the present study first compiled an extensive bottom-hole temperature database from over 6000 wells in the northern continental shelf and constructed a more detailed geothermal gradient map than those published previously. Second, basin models were then constructed for three areas within the continental shelf (off Texas, Louisiana, and Alabama) that show differing geothermal gradients. A basin model is a mathematical model that simulates the heat transport through the crust and the sediments of a basin in the context of its geologic evolution. Previous researchers proposed two possible causes for the observed geothermal gradient variation in the northern continental shelf. The first was the thermal effect of sedimentation: areas with faster sediment accumulation result in low geothermal gradients, and vice versa. The second was that basal heat flow (heat flow that enters from the igneous crust to the bottom of the sediments) varied across the continental shelf. The present study finds that sedimentary geothermal gradients in these areas are primarily impacted by two competing mechanisms associated with sediment accumulation. One is the radiogenic heat production within the sediment that adds to the total heat budget upward through the sedimentary column. The other is the transient effect of fast sediment accumulation, which results in reduction in the upward heat flow. Off Louisiana, the transient effect prevails, and hence the area shows the lowest geothermal gradients. Off Texas, due to slower sedimentation, the positive contribution by radiogenic heat is most significant. Off Alabama, because the sediments there are not as thick, the overall contribution of radiogenic heat is less. The models show that the thermal effects of sedimentation are large enough to explain the observed variation in geothermal gradients. Therefore, corresponding variation in basal heat flow is not required.