Hydrogeologic properties of a thrust fault within the Oregon Accretionary Prism

Abstract

Two sets of hydrogeologic tests conducted at Ocean Drilling Program (ODP) Hole 892 on the Oregon Accretionary Prism provided the opportunity to determine hydrogeologic properties of an active accretionary prism fault zone. The first set of tests consisted of shipboard packer tests conducted during ODP Leg 146 (fall 1992), while the second set of tests were constant‐drawdown and constant‐discharge tests conducted in fall 1993 using the submersible Alvin. Pressure response during the first set of tests suggests that fractures remained open until excess fluid pressure (relative to hydrostatic) dropped below 0.315 to 0.325 MPa (λ* ∼ 0.53 to 0.54, where λ* = (pore pressure ‐ hydrostatic)/(lithostatic‐hydrostatic)). Analysis of the packer test data suggested an apparent background pressure of 0.25 MPa (λ* ∼ 0.42 to 0.50). Because the borehole had been open for 12 hours prior to the packer tests, formation pore pressures may have exceeded this value prior to drilling of the borehole. These overpressures dissipated by the time the second set of tests were conducted. One possible explanation for this decay is that the borehole may provide a vertical conduit between the overpressured zone and overlying or underlying sediments that had previously been hydraulically separated from the overpressured zone. The second set of tests were conducted at pressures (≤0.019 MPa or λ* ∼ 0.03) below that estimated to maintain open fractures and yielded transmissivities 1 to 2 orders of magnitude less than estimated for the packer tests (when fractures were open). Constraints on fluid flow rate along the fault are provided by observed displacement in a bottom‐simulating reflector (BSR) at its intersection with the fault zone. The closed‐fracture transmissivities are insufficient to produce flow rates capable of displacing the BSR; therefore open‐fracture transmissivities under conditions of elevated pore pressure are inferred to be necessary for the observed BSR displacement. In addition, calculated rates of specific discharge through the fault zone are 2 to 3 orders of magnitude lower than discharge measured at an associated seafloor vent site; fluid flow must become spatially or temporally focused as it moves up the fault zone toward the seafloor.