Sandstone Exhumation Effects on Velocity
and Porosity:
Perspectives from the Ferron Sandstone
JARRARD, RICHARD D., and STEPHANIE E. ERICKSON
The fluvial-deltaic sediments of the Cretaceous Ferron Sandstone, Utah, were
buried to 3000-3400 m, with associated compaction and cementation, then uplift
exhumed different portions of the Ferron by 0 to >3400 m. Velocity
and
density-based porosity logs for 24 wells, plus petrophysical analyses of outcrop
plugs, show the petrophysical responses of Ferron exhumation.
Exhumation here is accompanied by an acceleration of carbonate leaching,
probably because of increasing groundwater flux at shallower depths. Velocity
logs show a
velocity
decrease that is even greater than expected based on this
secondary porosity development. Ferron logs deeper than 550 m, as well as
high-pressure measurements of plug
velocity
, fit the time-average
velocity
-porosity relationship, whereas shallower logs are much slower than
predicted from this transform, and outcrop-plug velocities are even slower. We
attribute this
velocity
plunge to reduction of framework modulus, caused mainly
by exhumation-induced pressure release but enhanced in outcrop by freeze-thaw.
Acceleration of carbonate leaching with exhumation, as seen in the Ferron,
may be anticipated only where meteoric water flux, rather than hydrocarbon
migration, controls carbonate dissolution. Velocity
rebound caused by pressure
release probably is a pervasive companion to exhumation. For exhumed formations,
undetected leaching or rebound can cause errors in the following analyses:
estimating amount of exhumation from
velocity
/depth or porosity/depth trends,
extrapolating outcrop-based petrophysical relationships to the subsurface,
calculating reservoir porosity from seismic velocities for depths of <550 m, and
estimating accretionary prism fluid flux using seismic velocities.