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Christopher D. White1, Brian J. Willis2
(1) Louisiana State University, Baton Rouge, LA
(2) State University of New York at Oswego, Oswego, NY

Abstract: Estimating shale length distributions from outcrop data

Dimensions of shales and other geologic features must be estimated for models that predict reservoir character and flow behavior. Because it is difficult to infer length distributions from subsurface data, outcrops are often used as a source of analog data for these estimates. However, length distributions tabulated directly from outcrop observations are not accurate because (1) the longest objects often extend beyond the outcrop limits, and (2) shorter objects tend to be under-represented within outcrop boundaries. We propose a method to determine unbiased length distribution estimates based on the frequency that a geologic feature terminates within the limits of an outcrop. Length distributions estimated from termination frequency are unbiased, regardless of the variability in size of the geologic feature or the outcrop geometry. Modeled distributions define mean lengths and estimate length variability. The estimated mean is unbiased for any distribution of lengths.

Maps of bedding architecture constructed from kilometers-long exposures of a tide-influenced deltaic sandstone (Frontier Formation, Wyoming, USA) demonstrate use of termination frequency in an analysis of length observations. Length distributions were calculated from thousands of mapped shales. Agreement between data and the model prediction was excellent. Field observations suggested that shales belong to more than one distinct population, formed by different depositional processes. To model these distributions, observed lengths were classified using Bayes’ rule. Lateral and vertical trends in shale length were observed and quantified; shales are longest low in the sandstone body and at the sandstone body margins. No anisotropy in shale length was observed.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana