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Regional overview of deepsedimentary thermal gradientsof the geopressured zoneof the Texas–Louisianacontinental shelf
Seiichi Nagihara and Michael A. Smith
ABSTRACT
Nearly 600 bottom-hole temperature data from the northern continental
shelf of the Gulf of Mexico, each corrected for drilling
disturbance, yielded a regional map of geothermal gradient down to
approximately 6 km (3.7 mi) sub–sea floor. Two geographic trends
can be seen on the map. First, from east to west, the geothermal
gradient changes from values between 0.025 and 0.03 K/m (0.014
and 0.016jF/ft) off the Alabama–Mississippi shore to lower values
of 0.015–0.025 K/m (0.008–0.014jF/ft) off eastern Louisiana
and to higher values of 0.03–0.06 K/m (0.016–0.033jF/ft) off
western Louisiana through Texas. Second, thermal gradients tend to
be lower toward the outer continental shelf (less than 0.02 K/m
[0.0112jF/ft]). We believe that the observed variations are primarily
attributable to the thermal effect of rapid and regionally
variable sediment accumulation during the Cenozoic era, which
resulted in the occurrence of the geopressured zone in the Texas–
Louisiana shelf. In the eastern Louisiana shelf, where accumulation
was fastest, sediments down to about 6 km (3.7 mi) are relatively
young (about <15 Ma) and have not had enough time to fully
equilibrate with deeper, hotter sediments. That resulted in the low
thermal gradient. As the depocenter migrated farther offshore,
younger sediments accumulatedmore in the outer shelf and resulted
in an even lower thermal gradient there. However, this mechanism
alone cannot explain the fact that geothermal gradients in the
Texas shelf are higher than those in the Alabama shelf, where Cenozoic
sedimentation has been much slower. It may be suggested
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