Permeability of upper oceanic basement on the eastern flank of the Juan de Fuca Ridge determined with drill‐string packer experiments

Abstract

We report drill‐string packer measurements of permeabilities of uppermost basement at three sites drilled during Leg 168 of the Ocean Drilling Program in 0.9‐ to 3.6‐m.y.‐old crust on the thickly sedimented eastern flank of the Juan de Fuca Ridge. Each of the holes was cased completely through the sediment section and penetrated a few meters to tens of meters into basaltic oceanic basement. The transmissivities and average permeabilities of the open‐hole basement sections were assessed using the packer to seal the hole and controlled pumping from the drillship to apply pressure pulse and constant rate injection tests. The results, namely average permeabilities of the order of 10−14 to 10−11 m2, are generally consistent with prior packer measurements in other young ocean crustal settings, but two new findings are important. First, our new results include very high values in the youngest crust yet tested, suggesting a noteworthy age dependence of the bulk permeability of uppermost basement (at the scale measured with the short‐term packer tests), ranging from about 3×1011 m2 at 0.9 m.y. to about 2×10−14 m2 at 3.6 m.y. Second, significant scale dependence of permeability is suggested by comparison to higher values inferred from (1) thermal indications of uphole flow of formation fluids at one of our sites and (2) in situ pressures and tidal loading effects recorded in long‐term formation pressures measured after the three sites were sealed with hydrologic observatories immediately following the packer measurements.