The data and discussion included in this chapter provide strong evidence for a high degree of lateral and vertical compartmentalization within the Lance Formation and, by extension, the upper Mesaverde Group. Such compartmentalization is caused, apparently, both by structural and depositional factors. Structural factors appear to have particular relevance at larger levels of scale, with the possible important exception of fracturing. Depositional factors, on the other hand, are crucial at the well bore scale. Examination of lithology, reservoir quality, well log patterns, mud weight data and other information indicates that compartmentalization within the Lance is below current well spacing (20-40 acres). Examples of closely spaced wells (< 100 ft) show significant changes in sandstone occurrence and thickness. This implies that drainage radius can vary considerably for different productive sandstone zones within a vertical section of Lance. Sand bodies tend to be narrow and elongated; thus closely spaced wells oriented in the strike direction for a particular sand body may drain the same reservoir, but not others. Any thick vertical section of Lance includes sand bodies of different geometry and orientation, a fact confirmed by dipmeter logs.
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