Measurements and modeling of the shallow pore pressure regime at the Sigsbee Escarpment: Successful prediction of overpressure and ground-truthing with borehold measurements

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 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...

Overpressure and consolidation near the seafloor of Brazos‐Trinity Basin IV, northwest deepwater Gulf of Mexico

“…We suggest the overpressure was driven by this rapid sedimentation. The high pore pressures in the Brazos-Trinity Basin IV region fit into a range of results that have recently documented overpressures in the shallow subseafloor Flemings and Lupa, 2004;Flemings et al, 2008;Orange et al, 2003 [30] At Site U1319, there was almost no loading by the overlying turbidite fill (Figure 3); thus, the overpressure that is present must have been generated by the deposition and consolidation of the mudstone itself. Gibson's [1958] analytical, one-dimensional consolidation model of a clay layer which increases in thickness with time can be used to estimate the amount of overpressure for these sediments [e.g., Flemings et al, 2008].…”