Abstract

Geothermal gradients and geopressure gradients were calculated for 249 wells in the western Gulf of Mexico (WGOM) protraction areas of Alaminos Canyon, East Breaks, Mustang Island, Corpus Christi, Port Isabel, and North and South Padre Island. in the WGOM thermal gradients were converted to the depth at which 300°F (149°C) is reached and the geopressure gradients were converted to the depth at which operational overpressure, defined as 0.70 psi/ft (0.016 MPa/m), is reached. Contour maps for each of these hypothetical surfaces were then created and compared with their counterparts for the central Gulf of Mexico (CGOM: Garden Banks, Green Canyon, Keathley Canyon, and Walker Ridge) protraction areas. The depth to the top of overpressure map shows increasing depth from northwest to southeast, first seen in the CGOM, is continued in the WGOM. However, the WGOM is noticeably underpressured in spite of the much higher subsurface temperatures. The two depth-to-300°F-(149°C) contour maps are very different: the WGOM version shows that area to be significantly warmer from geothermal heat flux than the CGOM for no obvious reason. The sediment thickness between the seafloor and magnetic basement is only slightly less than that of the CGOM, the presence of vertical salt is probably insufficient to cause that much heat focusing, and Nagihara et al. (1996) showed radiogenic heat in the study area to be insignificant. The relationship among 513 borehole measurements for temperature and pressure pairs from 249 wells in the WGOM shows a linear regression R² = 0.7179. This plot shows five zones, where in each zone, the pressure gradually increases as the temperature increases to a new threshold value enabling a new chemical reaction or diagenesis; and then the resulting pressure increases to the next level. When compared to the CGOM that shows distinct zonation, the plot suggests that the pressures recorded were not all created in situ. The combination of high temperatures and relatively low pressures, particularly in the offshore Texas upper slope area, suggests that excess pressure dissipated through formation fracturing or along faults.