ABSTRACT

We investigate the characteristics of conductive heat flow and the associated temperature distributions around both a highly conductive salt diapir and salt sheet embedded in a lower conductivity host rock. For the salt sheet, particular emphasis is given to temperature in the subsalt formations. Even relatively thin sheets can cause a significant change in the subsalt temperature; a sheet of 1000 m thickness can easily give rise to a temperature decrease of 15°C or more in the underlying formations. Diapiric effects are found to be of similar magnitude.

The surface heat flux profile over a flat, laterally extensive salt sheet shows a broad, antisymmetric peak/trough structure centered over the edge of the salt. The magnitude of this anomaly depends primarily on the thickness and depth of the salt sheet, the contrast in conductivity between salt and the surrounding rocks, and the heat flux across basement. At increasing dip angles the surface heat flux assumes a more symmetric form, becoming more like the profile associated with a salt diapir.

At large distances from any major lateral variations in conductivity, the heat flow problem reduces to a uniform heat flux flowing vertically through a section having a variable thermal conductivity. In this region the solution to the one dimensional heat flow equation provides an excellent approximation. However in practical cases lateral variability is generally significant. Where there are lateral changes in conductivity, such as at the edge of a salt sheet, heat flux lines concentrate to exploit the high conductivity pathway through the salt. For a flat salt sheet, temperatures in the underlying formation are lowest underneath the central portion of the sheet, increasing progressively towards the edge. Dip of the salt sheet also imparts a horizontal component to the flow of heat, resulting in enhanced heating of the updip edge of the salt as well as enhanced cooling of the downdip edge. Depth of burial of the salt impacts significantly only on shallow isotherms.

For salt diapirs, the dominant thermal anomalies are felt to about a few radii laterally from the diapir and typically to 5,000 m or more in depth. Downhole temperature measurements from six wells in the vicinity of an offshore Louisiana salt dome show increasingly higher temperatures with decreasing distance from the dome. We have analyzed this temperature distribution taking into account the effects of lithologic variations, overpressuring and salt dome geometry. Temperatures calculated using a steady-state thermal conduction numerical model are in agreement with measured values. The implication is that the thermal conductivity contrast between salt and surrounding sands and shales can produce all of the observed temperature anomaly; no other heat transfer mechanism, such as fluid flow, is required. The magnitude of the observed temperature increase induced by the salt dome, up to about 30°C, causes a substantial increase in the hydrocarbon maturation rate.