Dr. James Willis
Shale distribution analysis in deepwater reservoirs often focuses on the classical Thomas-Stieber model of laminar and dispersed shale, without consideration of structural shale. A laminar-dispersed model however yields the optimistic end-member of calculated reservoir properties versus models that consider the possible presence of structural shale. For example, in what may be overall clearly a laminated sequence, failure to consider structural shale potentially overestimates the laminar shale volume and underestimates the dispersed shale volume, yielding overestimated values of calculated sand fraction total porosity and effective porosity.
We previously developed a revised petrophysical model of shale distribution analysis that constrains the range from optimistic to pessimistic reservoir properties when using triple-combination log data. Core and cuttings can help constrain presence of the three shale distribution types. Logs that can constrain laminar shale volume such as triaxial resistivity and image logs are especially useful in that if the laminar shale volume can be determined, our petrophysical model can then determine the respective volumes of both dispersed and structural shale. If structural shale is present, our three-type distribution model demonstrates that reservoir quality is poorer than that calculated via the Thomas-Stieber laminar-dispersed shale model.
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