Controls upon depositional architecture and cyclicity of alluvial fan systems and associated environments: implications for hydrocarbon potential

Alluvial fans are key environments in continental basins. Their facies and architecture are influenced by interactions between 1) the varied environments of the fan, from debris-flow dominated, fluid-flow dominated to fan-surface lacustrine and overbank, 2) autocyclic processes of these environments, and 3) allocyclic controls of climate, base level and sediment supply. As some of these controls are cyclic at various scales, fans can include good, but stratigraphically complex, reservoirs associated with fan-related fluvial networks and debris flows, to admissible, localised or extensive seals and baffles provided by fan-surface deposits. Furthermore, fans can be long-lived throughout the history of the basin and thus interact with changing distal environments. Fan sediments may influence basin-scale migration, connect isolated distal reservoirs, or provide bypass to charge of those reservoirs. Thus, an understanding of fan architecture in response to changing controls is crucial to interpreting both fan reservoir potential and fan influence on the basin petroleum system.

We examine well-exposed fans from the Cutler Group of the Paradox Basin, U.S.A., deposited in a continental basin subject to arid-monsoonal climatic cycles, varied sediment supply and changing base level. The fan facies, architecture, relative dominance of fluid over debris deposits, and the connectivity of fluvial networks show spatial and temporal dependence on these competing controls, with cyclicity evident at a variety of scales. At a small scale, oscillating climate is a dominant control. Aridity is characterised by debris flow facies, with elevated permeability, whilst humidity is characterised by finer grained, less permeable facies. At a larger scale, the connectivity of fan-related fluvial networks is influenced initially by changes in base level. The dominance of this control reduces with fill: climate and sediment supply are more dominant controls on younger deposits.

Generalised, spatial and temporal facies models characterising fan development in response to varied controls are developed from the outcomes of this work. These models provide input for both fan-scale reservoir modelling, and basin-scale petroleum systems modelling in continental settings.

The work is applied to the Permian basin of northwest England, to characterise facies, geometry and connectivity of fan-related fluvial networks and potential for hydrocarbon migration within this poorly exposed basin.