Chemistry of waters sampled from oceanic basement boreholes, 1979–1988

Abstract

Waters have been sampled from the basement section of four Deep Sea Drilling Project holes (395A, 396B, 418A, and 504B) since 1979, in an attempt to determine the composition of oceanic basement formation waters. Samples have been obtained by two methods: active, in which water is drawn under negative pressure from the interstices of the wall rocks, and passive, in which it is collected from the open borehole. The most successful sampling has been done on re‐entering the holes months to years after they were last disturbed by drilling and/or pumping of surface seawater downhole. Most of the samples show large changes in composition from seawater that are consistent with alteration of basalts at the in situ temperatures of 10° to 146°C. The samples are typically depleted in Mg2+, K+, Na+, sulfate, and 87Sr, and enriched in Ca2+ and Si. However, all of the samples are contaminated to some extent, mainly by seawater which is either introduced during drilling or sampling, flows downhole due to underpressure in the upper basement, or mixes downhole due to convective overturn in the hole itself. The latter process is apparently common and dilutes any formation water which may enter the hole via diffusion and horizontal convection. In hole 504B, the tritium content of the waters indicates that 24 to 35% of the surface seawater which originally filled the hole is still present after periods of 470 to 1233 days. The more general problem of what has displaced the surface seawater and produced the large chemical changes observed in the borehole waters, whether exchange with formation water or simply reaction of seawater with the wall rocks of the hole, remains unsolved. As a result, no component of true basement formation water has been identified unambiguously in any of the holes. This problem may be solved by (1) compartmentalizing a hole with a series of plugs to limit downhole mixing or (2) using a straddle packer to draw a series of samples from the interstices of the wall rocks of a hole, either shortly after drilling or later following re‐entry. In either case, an array of chemical tracers will have to be measured, either in situ or in the laboratory, to infer the nature of the waters obtained.