Understanding Fluvial Architecture in Modern Desert Systems - Key to Modelling Rotliegend Reservoir Geometries

A distinctive fluvial depositional sequence has been identified in the Dutch Rotliegend. This resembles a Bouma-type sequence, but with characteristics identifying the deposits as fluvial flash-flood sediments as modern analogues show. The observed features indicate deposition during waning flow conditions that ranged from massflow or upper flow regime at the base to standing water at the top of a depositional unit. These deposits are interpreted as mostly channelized flash-flood deposits rather than as sheet-flood deposits. The inferred channelized character of the fluvial sediments has a major impact on predicted reservoir geometries in areas where the Rotliegend is dominated by ephemeral fluvial systems. In the axial part of the channel systems extensive fluvial erosion gave rise to lenticular geometries of both fluvial and aeolian sand bodies, rather than lenticular fluvial sands being interbedded with laterally extensive aeolian sands. Our deposition model implies that probabilistic instead of deterministic reservoir modelling should be applied in areas where fluvial and aeolian deposits are interbedded. Though 2nd-3rd order ‘drying & wetting’ cycles can be used to guide correlations on a regional scale, on a field to reservoir scale the proposed fluvial model results in more realistic reservoir models without laterally extensive, highly productive aeolian units.