Petrophysical Analysis of the Green River Formation, Colorado - a Case Study in Oil Shale Formation Evaluation
- Christopher Skelt (Chevron Energy Technology Company)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- October 2011
- Document Type
- Journal Paper
- 331 - 343
- 2011. Society of Petrophysics and Well Log Analysts
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- 231 since 2007
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The Green River formation in Southwestern Colorado is known as one of the world’s richest oil shales and is the target for several oil companies’ programs aimed at developing and evaluating technology to assess and develop the resource. Petrophysical evaluation of the potential liquid hydrocarbon yield is challenging due to the complex mineralogy that includes high and variable concentrations of minerals seldom encountered in conventional reservoirs. We present a case study of the evaluation of a comprehensive wireline and core data set that included natural, capture and inelastic gamma-ray spectroscopy, and NMR logs, and approximately twelve hundred feet of core-derived elemental and mineralogical analysis supplemented by Fischer Assays and RockEval pyrolysis measurements of the liquid hydrocarbon content.
The borehole was shallow, on gauge and filled with a low salinity drilling fluid and the logs were acquired slowly, in conditions particularly suitable for good spectroscopy log quality. The dataset therefore offered an opportunity to assess the relative accuracy of the elemental yields from natural and induced gamma-ray spectroscopy in the most favorable conditions likely to be encountered in oilfield operations, and as such represents a valuable reference for more general use of these logs. We showed that the match between wireline and core derived elemental concentrations varied considerably from element to element, and used this information to select the elements used as inputs in the detailed mineralogical analysis that followed.
This volumetric compositional analysis into minerals and organic matter was used as a starting point for reproducing Fischer Assay results, the recognized standard for quantifying liquid potential. This step included transformation from liquid hydrocarbon in gallons per ton of the assay samples to the more familiar barrels per acre-foot of the gross rock volume.
Finally, liquid hydrocarbon potential was estimated using simple overlays of square root conductivity against density and compressional slowness to compare the pre-diction accuracy achievable with a limited data set available in most regional wells against the standard obtained with the comprehensive data set used for the initial detailed evaluation.
|File Size||9 MB||Number of Pages||13|