ALBERT

Description: Eighty-two seafloor heat-flow measurements were recently obtained across the Mississippi Fan region in the deepwater northeastern Gulf of Mexico. These data display an abrupt transition in heat flow between an area near the center of Pleistocene deposition (∼20 mW/m2) and the eastern margin of the fan (∼40 mW/m2). Although deposition of fan sediments has very likely suppressed the shallow subseafloor thermal regime, causing lower seafloor heat-flow values near the center, the magnitude and abruptness of the heat-flow contrast cannot be fully accounted for by the mechanisms related to sedimentary deposition, which include radiogenic heat production in sediments, pore-fluid migration, and presence of salt structures. The most plausible explanation for the sharp heat-flow contrast is that the heat released from the igneous basement is significantly greater in the eastern margin of the fan. The zone of contrasting heat flow lies along a previously suggested boundary between the oceanic crust and the thin transitional crust in the northeastern Gulf of Mexico. The area of higher heat flow coincides with the suggested zone of transitional crust, which, because of its granitic origin, generates greater amounts of radiogenic heat than oceanic crust. This finding opens up the possibility that heat-flow data may be used in delineating crustal lithologic boundaries along continental margins. Seiichi Nagihara is an assistant professor of geophysics and geographic information science at the Department of Geosciences, Texas Tech University. He received his B.S. degree in 1985 and his M.S. degree in 1987, both from Chiba University in Japan. He received a Ph.D. in geological sciences in 1992 from the University of Texas at Austin.Kelly Opre Jones received her Bachelor of Science degree in geology (2001) from Texas A&M University and her Master of Science degree in geoscience (2003) from Texas Tech University. She currently works at Unocal Corporation in Sugar Land as a member of the international new ventures team.

Description: Present-day formation temperatures of the hydrogen sulfide (H2S)-bearing reservoirs in the James Limestone and the Norphlet Sandstone in the continental shelf off Alabama have been determined to be 138–149 and 191–217°C, respectively. Hydrogen sulfide gas in those reservoirs is generated by thermochemical sulfate reduction, a process sensitive to the ambient temperature. Bottom-hole temperature data from 135 wells in the offshore lease areas of Mobile, Main Pass East Addition, and the northern section of Viosca Knoll were examined in the estimation of formation temperatures. The bottom-hole temperatures were corrected for the thermal effect of drill-fluid circulation. Estimation of formation temperatures permitted the determination of the geothermal gradient representative for the study area, leading to a temperature range estimation for the H2S-bearing James and Norphlet reservoirs. Temperatures of offshore Norphlet reservoirs are higher than those reported previously for Norphlet reservoirs onshore. Temperatures of the James Limestone are close to the low-temperature limit for thermochemical sulfate reduction previously suggested. Seiichi Nagihara is an assistant professor at the Department of Geosciences, Texas Tech University. He received his B.S. degree in 1985 and his M.S. degree in 1987, both from Chiba University in Japan. He received a Ph.D. in geological sciences in 1992 from the University of Texas at Austin. His area of specialty includes geophysics, basin analysis, and geographic information science.Mike Smith is the Minerals Management Service Operations geologist responsible for geological reviews of all exploration and development plans and drilling permits for the deep-water Gulf of Mexico. He has also worked as a petroleum geologist, geochemist, and manager for the U.S. Geological Survey, Getty Oil, Texaco, and Geo-Strat.

Description: 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°F/ft) off the Alabama–Mississippi shore to lower values of 0.015–0.025 K/m (0.008–0.014°F/ft) off eastern Louisiana and to higher values of 0.03–0.06 K/m (0.016–0.033°F/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°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. Seiichi Nagihara is an associate professor in the Department of Geosciences, at Texas Tech University. He received his B.S. degree in 1985 and his M.S. degree in 1987, both from Chiba University in Japan. He received a Ph.D. in geological sciences in 1992 from the University of Texas at Austin. His area of specialty includes geophysics, basin analysis, and geographic information science. Mike Smith is the team leader of the Minerals Management Service Operations Geologists responsible for geological reviews of all exploration and development plans and drilling permits for the deep-water Gulf of Mexico. He has also worked as a petroleum geologist, geochemist, and manager with the U.S. Geological Survey, Getty Oil, Texaco, and Geo-Strat.

Description: This article introduces a technique for three-dimensional inverse modeling of geothermal heat conduction through heterogeneous media. The technique is used to determine the basal geometry of a diapiric salt structure found on the continental slope offshore Texas. Salt is two to four times more thermally conductive than other sedimentary rocks. The geothermal field is perturbed by the presence of salt and results in an anomaly in the heat flow through the seafloor. The spatial variation pattern of the anomalous heat flow reflects the geometry of the salt body. The inverse modeling obtains a model for the thermal-conductivity structure that causes the heat-flow anomaly observed on the seafloor. The inversion algorithm systematically searches for an optimal thermal-conductivity model by iteratively minimizing the misfit between the model-predicted and the observed heat flow. To reduce the problem of nonuniqueness, the inversion incorporates a priori information constrained independently, such as the upper surface geometry of the salt and the lateral extent of the salt body, which can be delineated by a limited coverage of two-dimensional seismic data. In addition, it is assumed that the thermal conductivity of the sedimentary strata surrounding the salt body is well constrained. This inversion method is applied to a heat-flow data set obtained over a salt structure on the Texas continental slope. The salt structure was first surveyed with the single-channel seismic reflection, which yielded the a priori information necessary. The base of the salt was not imaged seismically. Then, three dozen heat-flow measurements were obtained on the seafloor over and off the salt feature. The inverse heat-flow modeling performed here shows that this structure is a salt tongue, which has a diapiric root on one side. According to the most optimal thermal-conductivity model obtained, the root seems to extend to 6 km below the seafloor. Refinement in the model geometry and additional constraints on thermal conductivities of the surrounding strata should yield a model that is more detailed and would allow more thorough geological interpretation of the salt structure. Seiichi Nagihara is an assistant professor of geosciences at Texas Tech University. He received his B.S. and M.S. degrees (both in geophysics) from Chiba University, Japan. He received his Ph.D. in geological sciences at the University of Texas at Austin. His research interests are in sedimentary basin modeling, geothermal heat and fluid transport, and geospatial information sciences.

Description: Temperatures of deep (〉 ∼3 km [1.8 mi]) sediments along the Corsair growth-fault zone in the Texas continental shelf are elevated relative to those off the fault zone. This observation is based on a compilation of nearly 400 bottomhole temperatures (BHTs) obtained from about 230 wells widely distributed across the continental shelf. The BHTs have been individually corrected for the thermal disturbance associated with drill-fluid circulation. The isotherm of 140°C (284°F) derived from the corrected BHTs shows more or less continuous peaks along the fault zone. Thermal gradients in the depth range of 3 to 5 km (1.8 to 3.1 mi) shows higher values along the fault zone than off the fault zone. These trends are similar to the previous observations made along the Wilcox growth-fault zone in the Texas coastal plain. Previous studies suggest that the faults of the Wilcox system serve as the conduits for hot fluids expelled from deep, overpressured sediments. A similar mechanism may explain the elevated temperatures along the Corsair fault zone. 2nd revised manuscript received December 18, 2009 Seiichi Nagihara is an associate professor at the Department of Geosciences, Texas Tech University. He received his B.S. degree in 1985 and M.S. degree in 1987, both from Chiba University in Japan. He received a Ph.D. in geological sciences in 1992 from the University of Texas at Austin. His area of specialty includes geophysics, basin analysis, and geographic information science.