Measurements and Modeling of the Shallow Pore Pressure Regime at the Sigsbee Escarpment: Successful Prediction of Overpressure and Ground-Truthing with Borehole Measurements

Abstract

Abstract 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.