Impact of Heart Flux Anomalies on Hydrocarbon Maturity Around Salt Diapirs and Salt Sheets in Gulf Coast: Models and Observations
J. J. O'Brien, I. Lerche
We investigated the characteristics of conductive heat flow and the
associated temperature distributions around both a highly conductive salt diapir
and a salt sheet embedded in a lower conductivity host rock. For the salt sheet,
temperatures in the subsalt formations are emphasized. Even relatively thin
sheets can cause a significant change in the subsalt temperature; a sheet 1,000
m thick can easily give rise to a temperature decrease of 15°C or more in the
underlying formations. Diapiric effects are 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 toward the edge. Dip of the salt sheet also imparts a horizontal component to the flow of heat, esulting 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 agree 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 r quired. 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.
AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.