Understanding Channel Belt Connectivity on a System Scale in Fluvial Settings

Abstract

In this study we use the distribution of uranium deposits as a proxy for connectivity in subsurface system scale fluid flow to help understand potential porosity and permeability variations on a system-wide, regional scale. Uranium distribution in sandstone units is strongly influenced by sandstone body connectivity, and can therefore be used as a proxy for fluid flow. Our study area is the deposits of the Salt Wash system, a Distributive Fluvial System (DFS) in the Jurassic of Utah and Colorado, USA. More than 90% of recorded uranium deposits are located where net sandstone represents 40-55% of the succession and where channel belts form 20-50% of the succession. Below the 40% sandstone and 20% channel belt cut-off levels, uranium deposits are rarely observed, suggesting a loss in effective connectivity allowing the enrichment and precipitation of uranium when all other conditions needed are met. This cut-off coincides with a change in large-scale architecture of the fluvial system from medial to distal DFS facies. It is represented by a change from channel belt deposits with minor amalgamation between channel belts separated by laterally extensive floodplain deposits, to a decrease in channel belt presence with little or no amalgamation between channel belts and an increase in floodplain fines resulting in decreased connectivity between channel deposits. These relationships suggest that a reduction in channel belt connectivity is related directly to downstream facies variations associated with distributive fluvial systems, allowing a quantitative predictive system scale model of channel belt connectivity in fluvial systems to be built. Our data also suggests that average permeability of the formation decreases down DFS as net-gross decreases and connectivity decreases.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016