Permeability Measurements in Hole 896A and Implications for the Lateral Variability of Upper Crustal Permeability at Sites 504 and 896

Abstract

A drill-string packe~ was used during Leg 148 to conduct measurements of the bulk permeabilities of three depth intervals In the upper crustal sectIon peroctrnted by Hole 896A. Located about I km !;()I.lthellSt or Hole 5048. rocar a local heat-flow high. Hole 896A is cased lhrough 179 m of sedimcllt and the top 16 m of b.lemellt. and lhell pcnetnucs 10 469 mbsf. or 290 minto thc upper oce~n.ic c~st . The packer WIIS tlet in ca.~ing a 106 mbsf and ill open hole ut 233 aoo 385 mbsf. and standard slug and constont-rule Illjcclton tests wcre conductcd to assess the lransmissivities and bulk permcabilitic~ of the rcspective intervals 195-469.223-469. and 385-469 mbsf. The re~ullS indicatc thaI the upper 38 m of open hole is quite permeable, on the orncr of 2 x 10-1) ml . The !ltttion below this lOne:. 223-469 mbsf. is less permeable, on the order of 1- 2 x 10- 14 m2 but cenainly pcr- mcabl~ cnough to su~n pa~sive off-ui~ hydrothermal circulation. The permeability ~tructure in Hole 896A is quitc consis- tCIlI with values delermlllcd In the upper ":ZOO m of basement in Hole 5048. ond supporls the infercncc that the uppermost basement is pc~able on a. s:cale ofkilornc:ters around the sites. However. the measurements in Hole 896A show no sign of the sharp reduction In permeablhty previously reponed below .. 250 m ililo basemcnt in Hole 5048.