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Alluvial Fan Interaction with Adjacent and Contemporaneous Arid Continental Environments: Implications for Basin-Scale Fluid Migration and Charge


Proximal continental deposition is dominated by alluvial fan environments, which are prominent throughout basin development. Towards the distal extent of the basin, the alluvial fan interdigitates with contemporaneous arid continental environments of deposition. The unique petrophysical properties of the deposits that form this zone of interaction have the potential to significantly impact upon the basin-scale hydrocarbon migration pathways. This work considers the Cutler Group sediments of the Paradox Basin, western USA. The Cutler Group provides unparalleled exposure from the proximal alluvial fan environments, through to the environments that dominated the distal extent of the basin. This allows for a comprehensive analysis of the deposits that form the zone of interaction between the proximal and distal parts of the basin. Detailed analysis of the zone of interaction demonstrates that where the alluvial fan is dominated by debris-driven depositional mechanisms, winnowing through interactions with wind-blown or fluid-driven processes of the distal environments results in a relative increase in the permeability of the deposits. Debris-rich flows can also influx into small-scale ponds, leading to the development of secondary subaqueous fans that display an elevated permeability. Proximal fan-related fluid-driven systems can also interact with distal aeolian environments. Periodic flooding within the zone of interaction leads to an isolation of the aeolian deposits between fine-grained baffles. Stochastic facies modelling of these interactions outlines the effect they have upon latter-stage fluid flow throughout the basin system. The complex relationships affect potential reservoirs by: 1) interconnecting isolated reservoirs of the distal basin, 2) creating ‘thief zones’ that impact upon stratigraphical trapping potential of distal reservoirs, 3) providing a bypass route to charge the reservoirs, and 4) introducing baffles into an otherwise productive system. Generic facies models and stochastic simulations derived from this work are applied to the sediments of the Brockram Facies, northern UK: a poorly exposed arid continental depositional system dominated by alluvial fan sediments at the basin margin. The application provides significant insight into facies, geometry and connectivity of the zone of interaction and subsequent implications for fluid migration within this basin.