Packer experiments along the décollement of the Barbados accretionary complex: measurements of in situ permeability

Abstract

The first in situ measurements of permeability and fluid pore pressure along the décollement zone (a low-angle detachment fault) between the North American and Caribbean Plates were completed during Ocean Drilling Program Leg 156. Measurements of properties within the fault zone were attempted at two sites using a drill string packer. Interpretation of the packer data was complicated by variations in the baseline formation fluid pressure during the tests, as well as temporal and spatial variations in formation properties. These measurements may be interpreted to reflect a consistent relationship between bulk permeability and vertical effective stress and between bulk permeability and the modified pore-pressure ratio. Measured bulk permeabilities from the most reliable tests varied from 6 × 10 -16 m 2 to 1 × 10 -13 m 2 , whereas vertical effective stress varied from 1.7 to 0.1 MPa and the modified pore-pressure ratio varied from 0.45 to 0.95. These bulk permeability values and trends are consistent with a variety of independent, less direct estimates. These trends may help to explain the transient nature of prism dewatering and fluid flow along faults in this setting.

Coupled Evolution of Deformation, Pore Fluid Pressure, and Fluid Flow in Shallow Subduction Forearcs

“…In these models, upper plate fault‐zone k ranges from 10 −13 –10 −14 m 2 at the seafloor to 10 −16 –10 −17 m 2 where they intersect the décollement at depth. This range of fault permeability is consistent with previous estimates based on in situ single‐ or across‐borehole measurements (Screaton et al, 2000; Fisher & Zwart, 1996, 1997; Kinoshita & Saffer, 2018) and modeling studies of thermal and chemical budgets (Lauer & Saffer, 2012, 2015; Saffer, 2015; Spinelli et al, 2006 and references therein). Modeled flow rates across the seafloor vary over three orders of magnitude, with the peak surface seepage rates (up to 10 cm yr −1 ) corresponding to fault locations.…”

Reanalysis ofin situpermeability measurements in the Barbados décollement

“…These employed: down‐hole packers deployed from the drill‐ship (Fisher et al. 1996; Fisher & Zwart 1997); manipulation of flow through valves at the seafloor from a submersible (Screaton et al. 1997); and interpretation of a recorded pressure pulse caused by nearby drilling and logging operations (Screaton et al.…”

“…Taken together the three campaigns yielded permeability measurements ranging over four orders of magnitude from 5 × 10 −18 to 1.1 × 10 −13 m 2 , for the same location in the décollement. Speculated reasons for the large range in permeability included unusually high formation compressibility (Fisher & Zwart 1997; Screaton et al. 1997), drilling disturbance around the borehole (Screaton et al.…”

“…5) included slug injections, constant flow withdrawals, and constant flow injections. Details of the tests, model assumptions, and references for the analytical methods are provided in Fisher & Zwart (1997). For all of the tests, the recorded pressure values were not available until the entire test sequence was finished and the datalogger retrieved.…”

“…5). A linear fit to the log permeability data and the inferred vertical effective stresses yielded log 10 ( k ) = −12.8–1.3σ v ′ (Fisher & Zwart 1997). The slope of the linear fit was much greater than that observed at the laboratory scale and this was attributed to that larger scale of the tests (Fig.…”