Reservoir Heterogeneity in Fluvial Sandtones: Lessons from Outcrop Studies of Jurassic Examples, Colorado Plateau

Andrew D. Miall

Heterogeneity in fluvial sandstones occurs on six scales, separated by a six-fold hierarchy of lithosome-bounding surfaces. The largest scale represents units at the member or submember level, bounded by sixth-order surfaces (mappable by wireline log correlation in subsurface). Fifth-order surfaces define major channel bodies, which range from ribbon- to sheet-like in geometry. Sheet sandstones may be mappable using closely spaced well data, but ribbon sandstones are difficult to correlate except in the most well-developed field. Three-dimensional seismic has exciting potential here.

Macroforms (e.g., point bars) are bounded by fourth-order surfaces and may contain accretionary phases separated by low-angle, third-order erosion surfaces. Macroforms are a few hundred meters to several kilometers in areal extent, requiring well spacings of 32 or 16 ha. (80 or 40 acres) for reliable mapping.

Individual lithofacies units or multistory units are bounded by first-and second-order surfaces, respectively. These units may be readily identified in core but can rarely be correlated reliably between wells because of their limited areal extent (less than 10 ha. or 25 acres).

The distribution of each shale depends on the scale of the lithosome with which it is associated and does not follow a simple cumulative probability distribution, e.g., shales associated with sixth-order bounding surfaces may represent basin-wide periods of low fluvial energy; those resting on second-order surfaces are mud drapes over bed forms in small abandoned channels, having areal extents of only a few square meters.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.