Seiichi Nagihara

Three-dimensional gravity inversion using simulated annealing: Constraints on the diapiric roots of allochthonous salt structures

In the northern continental slope of the Gulf of Mexico, large oil and gas reservoirs are often found beneath sheetlike, allochthonous salt structures that are laterally extensive. Some of these salt structures retain their diapiric feeders or roots beneath them. These hidden roots are difficult to image seismically. In this study, we develop a method to locate and constrain the geometry of such roots through 3-D inverse modeling of the gravity anomalies observed over the salt structures. This inversion method utilizes a priori information such as the upper surface topography of the salt, which can be delineated by a limited coverage of 2-D seismic data; the sediment compaction curve in the region; and the continuity of the salt body. The inversion computation is based on the simulated annealing (SA) global optimization algorithm. The SA-based gravity inversion has some advantages over the approach based on damped least-squares inversion. It is computationally efficient, can solve underdetermined inverse problems, can more easily implement complex a priori information, and does not introduce smoothing effects in the final density structure model. We test this inversion method using synthetic gravity data for a type of salt geometry that is common among the allochthonous salt structures in the Gulf of Mexico and show that it is highly effective in constraining the diapiric root. We also show that carrying out multiple inversion runs helps reduce the uncertainty in the final density model.

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

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

Geothermal heat flow in the northeast margin of the Gulf of Mexico

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/m 2 ) and the eastern margin of the fan (∼40 mW/m 2 ). 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.

Three-dimensional inverse modeling of the refractive heat-flow anomaly associated with salt diapirism

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.

Geothermal gradient and temperature of hydrogen sulfide-bearing reservoirs, Alabama continental shelf

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 191–217C, 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.

Characterization of the sedimentary thermal regime along the Corsair growth-fault zone, Texas continental shelf, using corrected bottomhole temperatures

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.

Geothermal gradients of the northern continental shelf of the Gulf of Mexico

A wide, systematic variation of sedimentary geothermal gradients has been previously observed along the northern continental shelf of the Gulf of Mexico. From east to west, geothermal gradients change from 25 to 30 °C/km off Alabama to lower values (15–25 °C/km) off eastern Louisiana and to higher values (30–60 °C/km) off Texas. In order to assess the mechanism responsible for this variation, the present study first compiled an extensive bottom-hole temperature database from over 6000 wells in the northern continental shelf and constructed a more detailed geothermal gradient map than those published previously. Second, basin models were then constructed for three areas within the continental shelf (off Texas, Louisiana, and Alabama) that show differing geothermal gradients. A basin model is a mathematical model that simulates the heat transport through the crust and the sediments of a basin in the context of its geologic evolution. Previous researchers proposed two possible causes for the observed geothermal gradient variation in the northern continental shelf. The first was the thermal effect of sedimentation: areas with faster sediment accumulation result in low geothermal gradients, and vice versa. The second was that basal heat flow (heat flow that enters from the igneous crust to the bottom of the sediments) varied across the continental shelf. The present study finds that sedimentary geothermal gradients in these areas are primarily impacted by two competing mechanisms associated with sediment accumulation. One is the radiogenic heat production within the sediment that adds to the total heat budget upward through the sedimentary column. The other is the transient effect of fast sediment accumulation, which results in reduction in the upward heat flow. Off Louisiana, the transient effect prevails, and hence the area shows the lowest geothermal gradients. Off Texas, due to slower sedimentation, the positive contribution by radiogenic heat is most significant. Off Alabama, because the sediments there are not as thick, the overall contribution of radiogenic heat is less. The models show that the thermal effects of sedimentation are large enough to explain the observed variation in geothermal gradients. Therefore, corresponding variation in basal heat flow is not required.

Examination of the Long‐Term Subsurface Warming Observed at the Apollo 15 and 17 Sites Utilizing the Newly Restored Heat Flow Experiment Data From 1975 to 1977

The Apollo heat flow experiment (HFE) was conducted at landing sites 15 and 17. On Apollo 15, surface and subsurface temperatures were monitored from July 1971 to January 1977. On Apollo 17, monitoring took place from December 1972 to September 1977. The investigators involved in the HFE examined and archived only data from the time of deployment to December 1974. The present authors recovered and restored major portions of the previously unarchived HFE data from January 1975 through September 1977. The HFE investigators noted that temperature of the regolith well below the reach of insolation cycles (~1 m) rose gradually through December 1974 at both sites. The restored data showed that the subsurface warming continued until the end of observations in 1977. Simultaneously, the thermal gradient decreased, because the warming was more pronounced at shallower depths. The present study has examined potential causes for the warming. Recently acquired images of the Lunar Reconnaissance Orbiter Camera over the two landing sites show that the regolith on the paths of the astronauts turned darker, lowering the albedo. We suggest that, as a result of the astronauts’ activities, solar heat intake by the regolith increased slightly on average, and that resulted in the observed warming. Simple analytical heat conduction models with constant regolith thermal properties can show that an abrupt increase in surface temperature of 1.6 to 3.5 K at the time of probe deployment best duplicates the magnitude and the timing of the observed subsurface warmings at both Apollo sites.