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.