Temperature and video logs from the upper oceanic crust, Holes 504B and 896A, Costa Rica Rift flank: Implications for the permeability of upper oceanic crust

In 2001, we revisited thickly sedimented 5.9 Ma crust on the southern flank of the Costa Rica Rift for wireline re-entry of two important ocean crustal boreholes, Holes 504B and 896A, more than 8 years after they were last drilled in 1993. Here we report borehole temperatures measured in both holes within casing through the sediment sections and then into open hole in uppermost basement, as well as a video log from the upper basement section of Hole 896A. Since it first penetrated into oceanic basement in 1979, Hole 504B has been known for downhole flow of ocean bottom water into uppermost basement that was initially strong (≈100 m/h) but then waned; our temperature data indicate a very slow lingering downflow at 0.4 m/h. The pressure differential driving this slow flow was determined to be 11-12 kPa from pressure data acquired when the hole was sealed by our wireline installation of a long-term hydrological observatory. The combination of the flow rate and the pressure differential constrains an estimate for the average permeability of the upper basement section in Hole 504B of 1-5×10^-14 m², a value similar to but slightly less than past determinations. In Hole 896A, which is located ∼1 km away on a sediment-covered basement high, the temperature log indicated uphole flow of formation fluids at an average temperature of 57.8 °C and at a total rate of 12 m/h through casing; it also showed that at least three zones in uppermost basement produce fluids of different temperatures that contribute to this total flow. Although the associated pressure differential could not be measured in Hole 896A, estimates of average permeability of the section with the producing zones can be derived by assuming a differential of ∼20 kPa similar to those measured in other ridge-flank sites in basement highs also known to produce formation fluids; estimated permeability values for uppermost basement in Hole 896A are on the order of 1-4×10^-13 m², again consistent with past packer determinations. The video log in Hole 896A provides unprecedented visual images that document the discrete nature of the permeability of the producing zones. It also suggests an abundance of bacterial floc within the hole that may be either flushed from the formation by the producing fluids or blooming within the hole in response to nutrients advected by the producing fluids.