Ephemeral-fluvial sediments as potential hydrocarbon reservoirs
Although reservoirs formed from ephemeral-fluvial sandstones have previously been considered relatively simple, unresolved problems of sandbody correlation and production anomalies demonstrate the need for improved understanding of their internal complexity. Ephemeral flows occur in direct response to precipitation, receiving little or no water from springs or other long-continued sources. They consequently predominate in dryland regions where precipitation is high in intensity, short lived and of limited areal extent. Resulting flow is high energy, relatively shallow and also restricted in duration and areal coverage. High transmission losses, abundant loose material and sparse vegetation result in highly concentrated flows which dissipate rapidly, causing a downstream decrease in flow discharge. Sediments deposited from these flows include parallel laminated sands, massive sands, scour-fill sands, transitional lower to upper flow regime dunes, and commonly contain numerous erosional discontinuities, scattered mudclasts, rapid grain size changes and deformational features. Large quantities of rainfall falling over longer periods produces steady flows dominated by well sorted, lower flow regime bedforms which have moderately well developed fining-up sequences. High intensity rainfall falling for shorter periods produces unsteady flows which are characterised by more poorly sorted, upper flow regime bedforms and an absence of fining-up sequences. Outcropping ephemeral-fluvial systems have been studied in order to determine the main features and processes occurring in sand-rich ephemeral systems and to identify which features will be of importance in a hydrocarbon reservoir. The Lower Jurassic Upper Moenave and Kayenta Formations of south-eastern Utah and northern Arizona comprise complex series of stacked, sand-dominated sheet-like palaeochannels suggestive of low sinuosity, braided systems.