Abstract
Thermal records from boreholes in young oceanic crust, in which water is flowing up or down, are used to assess formation and borehole flow properties using three analytic equations that describe the transient thermal and barometric influence of downhole or uphole flow. We link these calculations with an iterative model and apply Markov chain Monte Carlo (MCMC) analysis to quantify ranges of possible values. The model is applied to two data sets interpreted in previous studies, from Deep Sea Drilling Project Hole 504B on the southern flank of the Costa Rica Rift and Ocean Drilling Program Hole 1026B on the eastern flank of the Juan de Fuca Ridge, and to two new records collected in Integrated Ocean Drilling Program Holes U1301A and U1301B, also on the eastern flank of the Juan de Fuca Ridge. Our calculations indicate that fluid flow rates when thermal logs were collected were ∼2 L/s in Holes 504B, 1026B, and U1301A, and >20 L/s in Hole U1301B. The median bulk permeabilities determined with MCMC analyses are 4 to 7 × 10−12 m2 around the uppermost parts of Holes 504B, 1026B, and U1301A, and 1.5 × 10−11 m2 around a deeper section of Hole U1301B, with a standard deviation of 0.2 to 0.3 log cycles at each borehole. The consistency of permeability values inferred from these four holes is surprising, given the range of values determined globally and the tendency for permeability to be highly variable in fractured crystalline rock formations such as the upper oceanic crust.