Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 241, part 3
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Conclusions

Two petroleum-reservoir-analog outcrops of limestone units of interest were studied using high-frequency GPR methods to image stratigraphic architecture behind the outcrop face. GPR successfully imaged major bounding surfaces and features such as crossbedding and internal bedding within units in the subsurface to a maximum depth of 3-4 m (9.8-13.1 ft). Careful interpretation of the GPR data and correlation with outcrop information allowed general relationships between reflection characteristics and lithology to be determined. Strong reflections were found to correspond with major bounding surfaces, which consisted of either a decrease in grain size or change in lithology (often enriched with clay or siltstone), both of which resulted in a change in the electromagnetic properties of the rock at these locations.

At the Captain Creek study site, the contact between the Captain Creek Limestone Member and the Vilas Shale and the contact between the lower and upper units of the Captain Creek were successfully imaged using GPR methods. Additional internal features as small as 0.1-0.2 m (0.3-0.7 ft), such as soil-filled fractures and smaller-scale bedding units (including crossbedding within the lower and upper Captain Creek) were also imaged. Thick, clay-rich soil in the central portion of the outcrop greatly hampered GPR-signal penetration and limited the extent of GPR profiles.

At the Plattsburg Limestone study site, the erosional contact between the Bonner Springs Shale and the Merriam Limestone Member was successfully imaged using high-frequency GPR methods. Features as small as 0.1-0.2 m (0.3-0.7 ft), including smaller-scale bedding units and air- and soil-filled fractures within the Merriam Limestone Member, were also imaged. Identification of thinly bedded (less than 0.2 m; 0.7 ft), argillaceous intervals of the Merriam Limestone Member was possible because of the high resolution of 500-MHz GPR. However, signal penetration was reduced in the more argillaceous units of the Merriam member due to higher clay content. High-frequency GPR also had difficulty penetrating and imaging features within the Bonner Springs Shale because of rapid signal attenuation caused by the high conductivity of this unit. Lower-frequency GPR methods may provide more satisfactory results if used at this site to image features associated with the Bonner Springs Shale.

Overall, GPR was successful in imaging detailed stratigraphic architectural elements as small as 0.1-0.2 m (0.3-0.7 ft) at each study site. Detailed correlation between GPR and outcrop data allows the use of pattern recognition when away from outcrops or boreholes. With a nearby outcrop, the shapes and patterns of sedimentary features can be recognized and then extrapolated into the subsurface with some confidence. Further GPR imaging of these reservoir analogs could provide information about three-dimensional changes in stratigraphy and lithology that may be useful in exploitation of valuable petroleum resources.

Acknowledgments

The authors wish to thank Neil Anderson, Mike Roark, and Mike Shoemaker of the University of Missouri-Rolla, for the use of UMR's radar equipment and field assistance in this study. David Leck provided field assistance at the Plattsburg Limestone study site, and Rich Sleezer performed soil sample analyses for the Captain Creek study site. Critical reviews by Rick Miller, Toni Simo, and Roger Young greatly aided us in clarifying our ideas and strengthening the paper, and editing by Liz Brosius helped make the paper more accessible. Lastly, Tim Carr of the Kansas Geological Survey graciously provided funding for this study.

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