Expulsion of abnormally pressured fluids along faults

Abstract

Numerical simulations of fluid flow and heat transport in the South Eugene Island minibasin, offshore Louisiana, show that expulsion of geopressured fluids along faults can produce temperature and pressure anomalies similar to those observed in the area. In the simulations, abnormally pressured fluid moves along the fault through a fracture network. A thermal anomaly forms adjacent to the fault, while a larger fluid pressure anomaly extends into sediments on either side. Results from constant fault permeability simulations indicate that (1) geopressured sediments must be relatively permeable (5 × 10−17 m2) for expulsion to occur, (2) the size of thermal anomalies depend on the depth to which the fault is hydraulically open, and (3) fluid is vertically transported into shallow sediments when fault permeability is high, while lateral transport along deeper sands dominates when fault permeability is low. Excess fluid pressure in abnormally pressured sediments drops to half its original value throughout much of the minibasin after 10,000 years of expulsion; the associated thermal anomaly is also larger than observations, suggesting expulsion is not continuous. Variable fault permeability simulations, in which compaction of fault zone sediments closes the fracture network, indicate that fault permeability decreases by 1–2 orders of magnitude 1–200 years after expulsion begins. Thermal and baric anomalies from variable permeability simulations are smaller than from constant permeability simulations and are more consistent with available data. Faults must remain permeable for 20–30 years to produce thermal and baric anomalies similar to those observed in the area.

Fluid migration and fluid seepage in the Connemara Field, Porcupine Basin interpreted from industrial 3D seismic and well data combined with high-resolution site survey data

“…Similar fluid intrusion patterns are obtained when vertical flow is simulated along a vertical conduit (e.g. a fault or a chimney) enclosed by shale, when the conduit crosses permeable aquifer units (Roberts et al 1996;Lisk et al 2000). The sand fluidization structures only occur within the unlithified sands and were not logged in the wells, therefore there are no lithological parameters (e.g.…”

Gas and gas hydrate distribution around seafloor seeps in Mississippi Canyon, Northern Gulf of Mexico, using multi-resolution seismic imagery

“…The Gulf of Mexico is well known for seafloor methane hydrate accumulations associated with hydrocarbon seeps (e.g., Brooks et al, 1984;MacDonald et al, 1994), and it is well known, at least in a gross sense, that faults provide conduits for the expulsion of pore fluid (Roberts et al, 1996). It is these hydrologic processes that are of interest in this study.…”

“…However, we know that faults can act as high permeability conduits (e.g., Roberts et al, 1996), and we know faults can be inferred from seismic data. If we assume that the gas-related diffractions lie on or near fault controlled conduits, then we may be able to identify the style of faulting responsible for the advection, if not the actual faultconduits.…”

Dissolved noble gases and stable isotopes as tracers of preferential fluid flow along faults in the Lower Rhine Embayment, Germany

“…These barriers are created by the juxtaposition of aquifer-confining units such as clay beds with aquifer media at the location of the fault (Mailloux et al 1999) and/or fault zone deformation processes such as cataclasis (Fulljames et al 1997), diagenesis (Chan et al 2000;Dewhurst and Jones 2003) and clay smearing (Lehner and Pilaar 1997;Bense et al 2003;Egholm et al 2008), which reduce the permeability of the fault zone itself. The role of faults as barriers to groundwater movement has been widely described in the literature but much less documented is the ability of faults to simultaneously act as conduits for sub-vertical flow along the fault (Roberts et al 1996;Wiprut and Zoback 2000;Bense and Person 2006). Hydraulic head observations are typically not sufficient to infer the magnitude and direction of fluid flow along faults, usually due to the sparse distribution of observation boreholes.…”