The Influence of Salt Domes on Paleotemperature Distributions

A characteristic of salt domes is that an enhanced heat flux is associated with them, due to the contrast in thermal conductivity between rock salt and typical sedimentary formations. We present a simple analytical model of heat flow in the vicinity of an isolated salt dome, with numerical evaluations for some sample cases. In addition to an enhanced surface heat flow, these models show that on the upper flanks of a salt dome a temperature higher than the regional trend is predicted, thus enhancing hydrocarbon maturation at these depths. On the lower flanks a temperature lower than the regional trend is predicted, thus inhibiting overmaturation. The magnitude of this temperature anomaly depends upon the position of the measurement point, both vertical and radial, relative to the salt dome; roughly speaking it is confined to within one salt dome radius of the salt-sediment interface. The magnitude of the anomaly can be quite large for typical salt domes. These basic ideas are corroborated by observing downhole measurements from a salt dome in the Gulf of Mexico. The good agreement allows us to bracket the height of that particular dome at between 9 000 and 15 000 m, and also permits us to determine that the total sediment thickness overlying the basal salt is between 13 200 and 19 200 m at the location of the salt dome. These results indicate that the presence of a salt dome can have a significant impact on the subsurface temperature distribution and, hence, on the thermochemical maturation of any source rocks or hydrocarbon accumulations in close proximity to the salt dome. In addition to providing trapping structures, a salt dome also provides an enhanced environment for hydrocarbon maturation on its upper flanks and for inhibiting overmaturation on its lower flanks. Such effects should be included when evaluating the hydrocarbon potential of a salt dome province.