New Pressure Results from the Expedition 336 CORKs at North Pond, Western Flank of the Mid-Atlantic Ridge

Abstract:

We report results from a nearly two-year record of pressure data from three CORK hydrogeological observatories in the ~8x15 km “North Pond” sedimented basin in ~7 Ma crust west of the mid‐Atlantic Ridge at 23°N. The new data were recovered during operations with the ROV Jason from the German R/V Maria S. Merian in April of 2014. Two of the CORKs (in Holes U1382A and U1383C) had been installed during IODP Expedition 336 in fall of 2011, with an initial installment of 6-7 months data recovered in spring 2012, also by Jason from R/V Merian. The third hole, U1383B, was instrumented during the 2012 cruise with a “CORK-Lite” deployed by the ROV. All three installations monitor formation pressures in basement beneath the sediment pond. The new data confirm results of the first half-year of data, which suggested a slight formation overpressure (~10 kPa) relative to hydrostatic in the two full CORK installations. This was somewhat surprising given (a) the long history of downhole flow in DSDP Hole 395A that also penetrated basement beneath the sediment pond, and (b) prior observations at more thickly-sedimented eastern Pacific ridge flanks of formation underpressures in sites drilled into basement lows. The new results show a small phase lag and attenuation of formation tidal signals relative to seafloor tides that is the same in all three holes, which confirms that the CORKs are properly sealed at the seafloor. The phase lag and attenuation are also the same among three separate basement intervals in Hole U1383C, which suggests either that the entire drilled section is hydrogeologically well connected or that downhole packers between the intervals do not seal completely. We explore potential models to explain the slight observed overpressures. One possibility is that the geometry of the isolated sediment pond results in higher formation temperatures and less dense formation fluids immediately below the relatively impermeable sediment pond, such that surrounding cooler, denser formation fluids in effect “squeeze” the formation fluids beneath the sediment pond from all sides. While this can produce a slight overpressure beneath the sediment pond, it is not clear if the temperature contrast at North Pond is sufficient to match the magnitude of the observed formation pressure.