Shallow overpressures, and shallow water flow (SWF), are significant hazards to deepwater drilling and facilities. In this paper we demonstrate how a combination of geomorphology, hydrogeology, and seismic interpretation has been successfully used to infer shallow overpressures, to develop a testable hypothesis of pore pressure distribution, and to delineate where to collect in situ data. In situ pore pressure measurements provide ground-truth data validating the model, and can be used to evaluate the hazard of internally driven failure for facilities development scenarios. Several of the slope failures on the Sigsbee Escarpment in the Mad Dog and Atlantis field areas show a geomorphology distinct from "top-down" slope failures. Specifically, these slumps have steep, amphitheatre-shaped headscarps, shallowly sloping bases, sharp inflection points with the surrounding seafloor at both the top and bottom of the headscarp, a linear trend in map view, and linear side walls. These geomorphic characteristics suggest that the slumps are formed by internally driven failure, and that the slumps grow retrogressively by headward migration. Within the Mad Dog and Atlantis field areas, Mad Dog Slump 8 and Atlantis Slump A are the best examples of this morphology; in this paper we will focus our attention on Mad Dog Slump 8 due to its proximity to planned facilities. At any given slope gradient, failure related to internally forcing can be facilitated by a combination of increased pore pressure or decreased sediment strength. In our analysis of the Mad Dog field area, we were not able to identify a mechanism for weakening surficial sediment. We therefore used the seafloor morphology and sub-surface seismic data from Mad Dog Slump 8 to infer both the distribution and magnitude of shallow overpressure through 1D and 2D models. Based upon the results of these models, in situ pore pressure measurements were acquired from a set of boreholes located within, behind, and adjacent to two slumps on the Sigsbee Escarpment. In situ pore pressure measurements were obtained from multiple horizons in each borehole. Measurements indicate that the upper section in the Mad Dog area is normally pressured to very slightly overpressured, but that the section below a prominent regional seismic reflector is overpressured. 2D numerical modeling of the measured pore pressure indicates that there must be significant anisotropy and/or heterogeneous permeability at or immediately below this regional horizon, and that this anisotropy/heterogeneity is capable of projecting overpressures to near the seafloor. A similar analysis of Atlantis Slump E suggested that this section has been hydrologically isolated from the rest of the supra-salt section by a set of salt ridges. In situ pore pressures at two Atlantis boreholes indicate normal pressures throughout the section, although the presence of saline-rich pore fluids from borehole samples (and possible brine seeps) indicates that the flow regime is density driven and related to the presence of salt. Introduction The slumps in Mad Dog and Atlantis show two distinctive types of morphology (Orange et al., 2003, this volume): shallow-seated, near surface failures related to dip-slopes, and deeper seated, amphitheatre-shaped slumps.
The engineering properties of soils are used in the design of various foundation systems to support and anchor offshore oil and gas platforms. Engineering soil properties are also used for detailed geohazard studies. Considering the significance of the facilities located in offshore deepwater environments and the complexity of the geological environments in which their foundations are installed, extensive geotechnical siteinvestigations are performed using many tools and specialized tests. This paper presents the wide variations of the soil properties along the Sigsbee Escarpment. The geologic and geotechnical data resulted in dividing the seafloor into three geologic provinces; the Lower Continental Slope, the Sigsbee Escarpment and the Upper Continental Rise. The geotechnical properties of the normally consolidated and overconsolidated clay soils encountered within these three geological provinces are presented and compared with those observed at other locations in the deepwater GOM. In particular, the interpreted stress history and the normalized shear strength properties are presented and discussed. Introduction Understanding the geotechnical properties of the various soils encountered at complex geological settings in deepwater regions is essential to plan appropriate geotechnical site investigation and laboratory testing programs. Also, documentation of the geotechnical properties of the soils encountered at complex geological provinces is key to improve the evaluation process for future developments. Extensive field geotechnical investigations, including recovery of soil samples using jumbo piston cores and deep soil borings as well as performance of in situ tests, have been conducted at two of BP's deepwater prospects (Mad Dog and Atlantis). As shown in the regional rendering presented in Figure 1, both prospects are located along the Sigsbee Escarpment, in the Green Canyon Area of the Gulf of Mexico. This paper discusses the geotechnical properties of the normally consolidated and the overconsolidated soils encountered at the locations of four soil borings drilled at the Mad Dog Prospect in Blocks 782 and 826 and four soil borings drilled at the Atlantis Prospect in Block 743 of the Green Canyon Area in the Gulf of Mexico. Geologic Setting Mad Dog Prospect The Mad Dog Prospect is located along the Sigsbee Escarpment in the Green Canyon Area at the southern extent of the northern Gulf of Mexico Continental Slope. The Escarpment represents a complex topographic and geologic feature involving steep slopes, faults, and slumps. The upward and lateral movement of underlying salt resulted in the seaward movement of sediments and subsequent deformation and over-steepening of the slope. The steep slope angles lead to slope instability producing gravity driven slumps. The isometric view presented in Figure 2a reveals a total of eleven slump features that have been mapped in the Mad Dog Prospect area. Atlantis Prospect The Atlantis Development is also located along the Sigsbee Escarpment in the Green Canyon Area at the southern extent of the northern Gulf of Mexico Continental Slope. The Escarpment represents a complex topographic and geologic feature involving steep slopes, faults, and slumps. The vertical and horizontal movement of the underlying salt nappes resulted in the deformation of the overlying sediments and over-steepening of the slope.
Shallow overpressures and shallow water flow (SWF) are significant hazards to deepwater drilling and facilities. In this paper we demonstrate how a combination of geomorphology, hydrogeology, and seismic interpretation has been successfully used to infer shallow overpressures, to develop a testable hypothesis of pore pressure distribution, and to delineate where to collect in-situ data. In-situ pore pressure measurements provide ground-truth data that can validate the model and be used to evaluate the hazards that internally driven failure might pose for facilities development.Several slope failures on the Sigsbee Escarpment in the Mad Dog and Atlantis field areas show a geomorphology distinct from "top-down" slope failures. Specifically, these slumps have steep, amphitheater-shaped headscarps, shallowly sloping bases, sharp inflection points with the surrounding seafloor at both the top and bottom of the headscarp, a linear trend in map view, and linear side walls. These geomorphic characteristics suggest that the slumps are formed by internally driven failure, and that the slumps grow retrogressively by headward migration. Within the Mad Dog and Atlantis field areas, Mad Dog Slump 8 and Atlantis Slump A are the best examples of this morphology; in this paper we will focus our attention on Mad Dog Slump 8 due to its proximity to planned facilities.The types of slope failures discussed in this paper are found on many margins worldwide. Their characteristic morphology indicates that the primary mechanism of slope failure is due to internal forcing (overpressure, weakening), rather than by downslope erosional flow. Internally driven slope failures can provide clues about the shallow overpressure regime, and provide exploration targets. Because the ocean is at constant head, and the mud line is a constant head surface, fluid flow will converge on the base of a slump headscarp if the pore pressure regime is above hydrostatic.At any given slope gradient, failure related to internal forcing can be facilitated by a combination of increased pore pressure or decreased sediment strength. In our analysis of the Mad Dog field area, we were not able to identify a mechanism for weakening surficial sediment. We therefore used the seafloor morphology and subsurface seismic data from Mad Dog Slump 8 to infer both the distribution and magnitude of shallow overpressure through 1D and 2D models.Based upon these models, in-situ pore pressure measurements were acquired from boreholes within, behind, and adjacent to two slumps on the Sigsbee Escarpment. In-situ pore pressure measurements, obtained from multiple horizons in each borehole, indicate that the upper section in the Mad Dog area is normally pressured to very slightly over-pressured, but that the section below a prominent regional seismic reflector is overpressured. 2D numerical modeling of the measured pore pressure indicates that there must be significant anisotropy and/or heterogeneous permeability at or immediately below this regional horizon, and that this anisotropy/hete...
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