The ability of rock physics models to infer marine in situ pore pressure

Abstract

Pore fluid pressure is an important parameter defining the mechanical strength of marine sediments. Obtaining high spatial resolution in situ pore pressure measurements in marine sediments, however, is a challenge, and as a result, only a handful of in situ pore pressure measurements exist at scientific drill sites. Integrating rock physics models with standard IODP/ODP measurements provides a potentially widely applicable approach for calculating in situ pore pressure. Here we use a rock physics approach to estimate in situ pore pressure at two Scientific Ocean Drill Sites where in situ pressure is well constrained: ODP Site 1173, used as reference for normal (hydrostatic) fluid pressures, and ODP Site 948, where previous studies infer high fluid pressures (k* $ 0.45-0.95, where the pore pressure ratio k* is defined as the pore pressure above hydrostatic divided by the difference between the largest principal stress and hydrostatic stress). Our analysis indicates that the rock physics method provides an accurate, low-precision method for estimating in situ pore pressure at these drill sites, and sensitivity analysis indicates this method can detect modestly high (k* > 0.6) pore pressure at the 95% confidence level. This approach has broad applicability because it provides an inexpensive, high-resolution (meter-scale) method for retrospectively detecting and quantifying high pore pressure at any drill site where quality wireline logs and ocean drilling data exist.

Permeability and pressure measurements in Lesser Antilles submarine slides: Evidence for pressure‐driven slow‐slip failure

“…These porosity changes, however, are accounted for in the rock physics model. The fact that V s model results accurately replicate observations implies that V s values are perhaps a more accurate tool for assessing velocity and pore pressure, as suggested in previous studies [Hornbach and Manga, 2014].…”

“…We apply this approach because it provides meter‐scale depth resolution of pore pressure that more accurately accounts for sediment lithology compared to typical empirical velocity‐porosity‐pressure pressure estimates. This approach is capable of detecting a pore fluid pressure ratio ( λ * ) in excess of ~0.6 and provides a first‐order method for detecting elevated fluid pressures at International Ocean Discovery Program (IODP) sites [ Hornbach and Manga , ]. Here we define the pore fluid pressure ratio ( λ * ) as the fluid overpressure divided by hydrostatic effective stress where P * is the fluid pressure above hydrostatic, σ 1 is the maximum principle stress (assumed vertical), and P h is the hydrostatic stress.…”

“…more accurately accounts for sediment lithology compared to typical empirical velocity-porosity-pressure pressure estimates. This approach is capable of detecting a pore fluid pressure ratio (λ * ) in excess of~0.6 and provides a first-order method for detecting elevated fluid pressures at International Ocean Discovery Program (IODP) sites [Hornbach and Manga, 2014]. Here we define the pore fluid pressure ratio (λ * ) as the fluid overpressure divided by hydrostatic effective stress…”

“…where P * is the fluid pressure above hydrostatic, σ 1 is the maximum principle stress (assumed vertical), and P h is the hydrostatic stress. An important advantage of the rock physics model approach for estimating in situ pore pressure versus other empirical approaches or more costly and time consuming in situ pressure measurement methods is its applicability to any environment where lithology is well constrained, grain contact theory holds, and quality downhole seismic velocity logs exist [Hornbach and Manga, 2014]. As a result, rock physics models have been applied successfully to a diverse range of lithologies, including clay-rich marine sediments, and are routinely used for pore pressure prediction in oil and gas reservoirs and scientific drill holes [Helgerud et al, 1999;Boyd-Gorst et al, 2001;Vanorio et al, 2003;Tsuji et al, 2008].…”

Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank‐collapse to seafloor sediment failure?

“…Also, Hornbach et al [] combined downhole logging data with a rock physics model [ Hornbach and Manga , ] and lithological data to assess pore pressures at Site U1399. They concluded that unusually high excess pore pressures occur at the present day in multiple sand‐rich zones with SLD at this location.…”