High-Resolution Ground-Penetrating Radar: An Aid to Reservoir Analog Outcrop Studies
Joseph M. Kruger, Alex Martinez, Howard R. Feldman, and D.
Outcrop analog studies are important for defining the small-scale stratigraphic architecture of producing reservoirs, but are commonly insufficient for determining 3-D geometries. Ground-penetrating radar (GPR), which generates an image similar to a seismic section, can be very useful for generating high-resolution images of reservoir analogs. GPR data from eastern Kansas show that in some areas, high- frequency (500 MHz) antennas can achieve vertical resolutions of approximately a decimeter down to depths of 5 m or more. GPR data were acquired over an outcrop of an upper estuarine sandstone facies within an incised valley fill of the Upper Pennsylvanian Tonganoxie Sandstone. The Tonganoxie is a minor reservoir in Kansas, and is an analog for valley fill sandstone reservo rs.
Excellent correlation between outcrop data and two parallel GPR lines less than 6 m from the outcrop face demonstrate that features as small as individual foresets were successfully imaged. The high resolution may result partly from thin (1 mm) clay/micaceous drapes on foresets. The clay and micaceous drapes are conductive and have a dielectric constant different from adjacent sandstone. GPR data reveal a complex sandstone body architecture. Onlap and downlap relationships on GPR profiles result from trough and tabular planar cross bedding. GPR profiles reveal significant lateral and vertical changes in geometry and scale of cross bedding. A decrease in cross bedset thickness upward may reflect channel filling. Future work will involve collection of minipermeameter data in order to co relate GPR responses to changes in permeability.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California