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Comparison of Outcrop and Core Samples for Assessing Mudrock Characteristics

Sally J. Sutton1, Previous HitWilliamNext Hit C. Dawson2, Previous HitWilliamTop R. Almon2, and Frank G. Ethridge1
1Geosciences, Colorado State University, Fort Collins, CO
2ETC, Chevron, Houston, TX

Mudrock intervals are rarely cored and hence studies of mudrocks rely heavily on outcrop samples, despite difficulty obtaining samples unaffected by modern weathering. Modern weathering impacts these studies in at least two ways, by altering characteristics being studied and by introducing a sampling bias favoring more resistant intervals. Some characteristics, e.g. sedimentary structures, may be unaffected by weathering. Others more readily affected by weathering include porosity, permeability, volatile content, mineralogy, and chemical composition. Depending on how strongly these parameters are affected by weathering, using surface samples to characterize subsurface flow barriers may be difficult. Similarly, if use of outcrop samples results in sampling bias, results obtained for surface samples may not be representative of sub-surface mudrock properties. We have examined this problem specifically with reference to predictions of mudrock sealing capacity. More broadly, the same questions can be raised about all outcrop-based mudrock studies. Comparison of core and outcrop samples from the Cretaceous Lewis Shale in the Washakie Basin, Wyoming and Graneros Shale in the Denver Basin, Colorado indicate sealing capacity is underestimated from surface samples. The general relationship of sealing capacity to other parameters may, however, be intact. Further, variations in average silt content suggest that siltier intervals are likely to be over represented in outcrop-based studies. Because silt content above about 20% is associated with poor seals, this sampling bias may cause significant underestimates of sealing capacity. Overall, surface samples may preserve useful information about subsurface sealing capacity and other mudrock characteristics, but should be interpreted cautiously.

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas