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The Lance Formation at Jonah field comprises a thick succession, locally more than 3500 ft (1100 m), of low-permeability, fine-grained alluvial sandstones. These alluvial sandstones were deposited by modest rivers in a rapidly subsiding basin in the northern portion of the Greater Green River basin of southwest Wyoming. Stacking of these alluvial sand bodies has produced an extremely heterogeneous reservoir in which reservoir sand bodies are from 9 to 15 ft thick (3 to 5 m) and approximately 200–700 ft wide (60–210 m P50 values). Because of the low net/gross of the overall Lance Formation (10–35% overall, 40–80% locally) and the comparatively small sand bodies, it is most likely that wells drilled on 40-ac (0.16-km2) density (1320 ft [402 m] closest spacing between wells) will result in more than 75% reservoir additions and less than 25% rate acceleration. It is also quite likely that wells drilled on increased density, less than 40 ac (0.16 km2), will result in significant reserve additions as well.

Because the Lance Formation at Jonah field is dominated by intercalated, relatively thin alluvial sand bodies and alluvial-plain deposits, much of the reservoir-bearing interval is at or below seismic tuning. As a result, despite the presence of a high-quality, three-dimensional seismic survey, the reservoir cannot be consistently imaged or described from seismic data. In this chapter, a description of the reservoir at Jonah field is developed based on a sedimentological description of the available cores and the integration of well-log data. These data are then used to develop quantitative estimates of the sizes of fluvial systems that resulted in the sand bodies found in Jonah field as well as estimates of the lateral extents of the sand bodies themselves. Traditional techniques for estimating sand body geometry are compared with a probabilistic approach based on numerous analog sand body studies. For subsurface decision making, a probabilistic approach more accurately captures the range of likely outcomes than the more traditional approach of attempting to derive single-point estimates of fluvial dimensions.

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