METHODOLOGY

Laboratory permeability and porosity measurements were made at New England Research, Inc. (NER). Tests were conducted on 2.54-cm diameter minicores, with lengths ranging from 1 to 3.5 cm. All minicore samples were extracted perpendicular to the orientation of the recovered drill core. Details of drilling procedures, core recovery, and minicore extraction are described in Binns, Barriga, Miller, et al. (2002). Permeability measurements were made along the length of the minicores; therefore, the resultant permeability values represent fluid flow along the horizontal axis. No vertical permeability measurements were made because of the limitations in the amount of core recovered. Also, no permeability measurements were made on incoherent material because of the nature of the sampling and measurement procedures; only coherent rock was used for our testing purposes. Samples were selected from depths ranging from 9 to 372 mbsf, with a water depth of ~1675 m (varying between holes). Samples represent the majority of the cored intervals and characterize the primary lithologic units and alteration intensities found in the recovered core, as classified by shipboard analysis. Although there may be some sample bias of the overall system because of limited core recovery (ranging from 0% to 20%, and below 10% on average) (Binns, Barriga, Miller, et al., 2002) and the inability to sample structurally fragmented rock, the sample selections reflect as many of the recovered rock types as possible.

Samples were saturated with 31 g/L sea salt solution using "sea salt" produced by Sigma Chemical Co. for 48 hr prior to permeability testing to saturate the samples. A brine solution representative of seawater was used for saturation as well as for permeability tests to better replicate submarine conditions and to give more reliable results. If gas were used instead of salt water, measurement time could be reduced; however, the possibility of interactions between the salt water and clay minerals could be significant for fluid flow (Karato, 1983a). Because anhydrite was identified in a number of samples and saturation could cause anhydrite dissolution, the saturation fluid was analyzed after the 48-hr period to determine if any minerals present in the sample had dissolved. In all cases, the amount of additional Ca and SO4 detected in the solution was insignificant, and anhydrite dissolution in the samples was determined to have no effect on the permeability or porosity measurements. Porosity was inferred from the difference between the saturated and dry densities for each sample, similar to the shipboard technique employed by ODP. Based on the reproducibility of measurements, errors in porosity estimates are on the order of 0.2 porosity units (p.u.) (i.e., 0.2%). Other sources of measurement error, such as irregular sample geometry, were negligible for these samples. Two samples, 193-1189A-2R-1, 8–11 cm, and 2R-1, 36–38 cm, exhibited large surface pores which tend to drain prior to weighing, leading to systematic underestimates of porosity. Based on historical results, these errors are generally found to be <1 p.u. (G. Boitnott, pers. comm., 2003).

For each sample, the ends were polished flat and parallel to fit properly in the core holder. The samples were placed between two porous steel frits, which uniformly distributed the fluid flow and stress. All permeability measurements were made at room temperature. Permeability was measured with a complex transient method, using the equipment and technique described in Boitnott (1997). In this method, permeability is measured by applying a pressure perturbation to the pore pressure field at the upstream end of the sample and measuring the pressure response at the downstream end. By using a variety of transient frequencies as perturbations, the signal can be optimized while maintaining measurement accuracy. A single-frequency sinusoid was used in most cases, tuned to optimize the signal for each sample. An asymmetrical spike transient was used for higher-permeability samples, whereas a traditional step function (pulse decay) was used for lower-permeability samples.

Permeability was first measured under 5-MPa effective pressure (confining pressure = 10 MPa; pore pressure = 5 MPa). The effective pressure was increased to 50 MPa for each sample and then reduced again to 5 MPa; permeability measurements were made at each 5-MPa interval. After each step increase or decrease in pressure, pore pressure was permitted to reequilibrate. In some cases, particularly for softer, lower-permeability samples, the reequilibration time period was long, with tests taking >1 week to complete. Errors in permeability measurements are difficult to generalize and quantify. Variability between measurements using different transients generally resulted in discrepancies of <5%. By fitting data assuming a wide range of specific storages, errors resulting from uncertainties in specific storage are thought to be less than 10%. Through a direct comparison of measured transient and steady-state permeabilities on selected samples spanning a wide permeability range, measurement discrepancies greater than 30% were observed (G. Boitnott, pers. comm., 2003).

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