Borehole packer tests at multiple depths resolve distinct hydrologic intervals in 3.5-Ma upper oceanic crust on the eastern flank of Juan de Fuca Ridge

Abstract

Single-hole hydrogeologic experiments were conducted with a drillstring packer in Integrated Ocean Drilling Program Hole 1301B, which penetrates 318 m into 3.5-Ma upper oceanic basement on the eastern flank of the Juan de Fuca Ridge. Seven constant-rate injection tests were conducted in open hole, with the packer inflated at three depths in the deepest 166 m of the basement section. The primary pressure data recorded in the test intervals required correction for local variations in seafloor pressure (mainly tides), as well as for changes in the "baseline" pressures during individual tests, probably owing to downhole flow of ocean bottom water induced by drilling. The corrected pressure records were fit to standard, idealized models of borehole response during pumping, leading to estimated basement permeabilities within the upper 318 in of basaltic crust near Hole 1301B of the order of 1 to 3 x 10-12 m2. The upper 207 in of basement around Hole 1301B appears to be more transmissive than the 111 m below this depth. Permeability within the upper interval may be as high as 5 x 10-12 m2, and permeability within a 30-m-thick zone between packer setting depths may be as great as 2 x 10-11 m2. Comparison of the results to packer and thermal data from uppermost basement in nearby Holes 1026B and 1027C suggests that the most transmissive part of the upper crust in this area may be located not immediately adjacent to the sediment-basement transition but deeper in the section. Copyright 2008 by the American Geophysical Union.