Regional overview of deep sedimentary thermal gradients of the geopressured zone of the Texas-Louisiana continental shelf

摘要

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 0.016 degreeF/ft) off the Alabama-Mississippi shore to lower values of 0.015-0.025 K/m (0.008-0.014 degreeF/ft) off eastern Louisiana and to higher values of 0.03-0.06 K/m (0.016-0.033degreeF/ft) off western Louisiana through Texas. Second, thermal gradients tend to be lower toward the outer continental shelf (less than 0.02 K/m [0.0112 degree F/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 accumulated more 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 that the contrasting sedimentation history between the Texas and Alabama shelves has resulted in some difference in overall thermal conductivity of sediment, and that the geothermal gradients reflect such difference. However, it is more plausible if additional mechanisms enhance heat flow through sediment in the Texas shelf, such as (1) upward migration of pore fluid expelled from deep, overpressured sands and/or (2) a greater amount of heat released from the igneous basement. Deep sedimentary temperatures in the high-thermal-gradient areas suggest higher risks of hydrogen sulfide occurrence and reservoir quality degradation because of quartz cementation.