Paleosols as Exploration Tool for Fluvial Reservoirs
Mary J. Kraus, David M. Uhlir
Well log or coring programs typically focus on fluvial sand bodies rather than nonproducing overbank deposits. The Eocene Willwood Formation in Wyoming demonstrates the potential of paleosols that formed on overbank deposits for predicting the location and geometry of channel-sand bodies.
Nearly all Willwood overbank deposits were pedogenically modified and consist
of vertically stacked paleosols. Five types are distinguished on the basis of
features recognizable in core, including color, color sequence and contacts,
nodules, and geochemical properties. They represent different stages of maturity
and show a progressive lateral transition from the least to the most mature with
increasing distance from a coeval channel sandstone. Therefore, a specific
paleosol type provides an approximation of distance to a channel-sand body
.
Immature paleosols are abundant close to ancient channel systems and highlight
possible reservoirs. This is potentially useful in predicting the trend of
ribbon sand bodies, which are difficult to correlate. Vertical sequences of
paleosols show upwa d changes in maturity over tens of meters that record the
direction the channel and resulting sand
body
moved through time.
The maturity of larger packages of paleosols varies with basin subsidence.
Packages of less mature paleosols characterize more rapidly subsiding areas of a
basin, where channels tend to locate. Paleosol maturity trends across a basin
can also help establish basin axis location. In addition, sand-body
geometry is
partly regulated by subsidence rate. The maturity of paleosols in a particular
stratigraphic interval is a good indication of subsidence rate and thus
sand-
body
geometry.
AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.