Abstract: The True Complexity of Arid-Region Ephemeral-Fluvial Sandstones as Revealed by Outcrop Analysis and Reservoir Simulations

Colin P. North, Katy S. Taylor

Arid-region ephemeral fluvial sequences form major hydrocarbon reservoirs, such as in the Permian, Triassic and Jurassic of the North Sea. Because they are often sand-rich, they have been regarded as internally simple. Yet problems with sand-body and shale correlation, plus production anomalies and history-matching difficulties as fields have matured, show this view is oversimplistic. The sedimentological literature on such deposits is misleading and naive, and biased to small-scale examples. In reality our understanding is based on just a handful of fundamental but old and meagre studies.

Outcrop studies on the Jurassic Kayenta Formation in Utah have been aimed at clarifying the true nature of such deposits. Detailed sedimentological studies, probe-permeameter transects, and laboratory porosity determinations,

reveal considerable variability. The most effective demonstration of the complexities in arid-region fluvial sandstones comes from reservoir simulations of waterflood and gravity drainage based on the outcrop data. Permeability varies rapidly both vertically and horizontally, over a wide range of values (1-1700 mD, with porosity 3-25%), due to rapid variations in grain size and sorting characteristics. This is a consequence of the discontinuous spatial and temporal nature of depositional processes in such systems; for example, bedforms are almost never in equilibrium with hydraulic conditions. Intraformational mudclast conglomerates as permeability barriers both because of the clasts themselves and because they act as sites of preferential cementation. Such features are also seen, and cause production difficulties, in the Triassic sandstones which form the main reservoir of the Wytch Farm oil field in South England.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994