The regional geothermal heat flow regime of the north-central Gulf of Mexico continental slope
Eighty-eight oil and gas wells located in the Texas-Louisiana continental slope were analyzed to obtain heat flow and geothermal gradient values. Present-day geothermal heat flow values at 55 of these well locations were determined in order to provide a basis for estimating sedimentary thermal histories and hydrocarbon maturity of the region. The geothermal gradient was calculated for all 88 wells. The heat flow was obtained as the product of the geothermal gradient and the thermal conductivity of the sediment. The thermal gradient was obtained from bottom-hole temperature (BHT) data. Each BHT measurement was corrected for the thermal disturbance caused by drilling. The so-called Honor plot method was used for the temperature correction when there were multiple BHTs obtained at the same depth but at different times. For single BHT measurements, the correction scheme developed by Deming and Chapman, which utilizes the empirical relationship between the well-bore temperature, depth and shut-in time, was used. The conductivity was estimated from a suite of petrophysical logs in a two-step procedure. First, the lithology was determined using a volume of shale calculation. Second, the thermal conductivity was derived for the lithology using the geometric mean law. In addition to the geothermal heat flow, the amount of radiogenic heat production within the sediments was constrained at some of the wells, using the U, Th, and K concentration values derived from spectral gamma ray logs, when these logs were available. The method of Bucker and Rybach, which utilizes the relationship between the total gamma ray log and heat production, was used for wells without spectral gamma ray logs.
The heat flow values from this study and heat flow probe data obtained previously illustrate that regional trends are present. These trends in the heat flow data appear to be dependent on two main factors: accumulation rate of recent sediment and proximity to diapiric salt structures. High Pleistocene sedimentation rates in the western Mississippi Canyon and Atwater Valley protractions have suppressed surface heat flow. The presence of laterally continuous salt sheets appears to have little effect on heat flow. However, isolated salt domes in the upper continental slope serve to cause elevated heat flow values for some wells. Analysis of the combined data set indicates a regional background heat flow of around 40 mWm-2.