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Evidence for Clustering of Delta-Lobe Reservoirs within Fluvio-Lacustrine Systems, the Jurassic Kayenta Formation, Utah


The spatial relationship of fluvial bodies within the Kayenta Formation, Warner Valley, UT, shows lateral and vertical clustering of delta-lobe sand bodies within a matrix of fine-grained open-lake deposits. Clustering of fluvial channel belts due to non-random stream avulsion is well documented for aggrading high-accommodation fluvial systems operating in alluvial plains. This concept, however, is not established for broadly lacustrine systems with abundant fluvio-deltaic lobes. The delta lobes of the Kayenta Formation appear to bear similar spatial clustering to that observed in fluvial channel belts and argue for extending this clustering concept to lacustrine systems. The Jurassic Kayenta Formation is exposed for over 10 km of continuous outcrop in Warner Valley permitting observation of a long continuous outcrop of mixed delta and open-lake strata and proportion of distribution statistics. Delta lobes are composed predominately large wings of silt to fine grained sandstone that trace to coarser fluvial channel-belt deposits. These are incased in thickly laminated mudshale and siltstone of the open-lake deposits. The fluvial channel belts scour into units while the wings grade laterally into the open lake units. The delta lobes form ledges and cliffs which range in size from .3 meters thick and 10s of meters wide (single lobes) to 10 meters thick and 100s of meters wide (multiple clustered lobes). When multiple fluvial bodies are clustered the younger typically scour into the older either from the top down or laterally, making it difficult to follow any one given fluvial body laterally for long distances. Clusters of delta lobes are separated vertically and laterally by mud-rich open lake deposits bearing scattered single-lobe bodies, or no lobes at all. Recognition of clustering in the Kayenta Formation expands this important concept from strictly fluvial to fluvio-lacustrine systems. The clustering of fluvial bodies within lakes is significant in predictive reservoir models in that it improve connectivity and localization of delta-lobe reservoirs. The cause of clustering in fluvial settings is believed to be a reflection of preferential avulsion fairways adopted by river systems. The clustering of delta lobes in lacustrine system is probably a basinward projection of this same avulsive process.