Modelling Temporal and Spatial Sedimentary Architectural Complexity in Mixed Aeolian-Fluvial Reservoir Successions

Abstract

Abstract Studies of modern desert dune fields allow geologists to draw conclusions about the controls that govern the development of spatial patterns of arrangement of desert landforms. This knowledge can be applied to predict the likely arrangement of architectural elements in preserved ancient desert successions. This serves as the basis for the development of more sophisticated facies, architectural-element and sequence stratigraphic models that can be applied in reservoir geology. This study presents a series of ten bespoke facies models that demonstrate different types of aeolian-fluvial interaction documented from dune-field margin settings. These ten semi-quantitative models have been developed based on analysis of modern and ancient systems, and via comparison of literature-derived case-study examples of ancient successions using a meta-analysis approach. The presented facies models account for the nature and origin of stratigraphic complexity present in aeolian dune-field margin successions that arose in response to the combined interplay of a series of autogenic and allogenic controls. From an applied perspective, mixed aeolian and fluvial successions are known to form several major reservoirs for hydrocarbons, including the Permian Unayzah Formation of Saudi Arabia. However, quantitative stratigraphic prediction of the three-dimensional form of heterogeneities arising from aeolian and fluvial interaction is notoriously difficult: (i) interactions observed in one-dimensional core and well-log data typically do not yield information regarding the likely lateral extent of sand-bodies; (ii) stratigraphic heterogeneities of these types typically occur on a scale below seismic resolution and cannot be imaged using such techniques. Understanding the nature and surface expression of various types of aeolian and fluvial interaction, and considering their resultant sedimentological expression, is important for prediction and interpretation of preserved deposits of such interactions that might be recognized in the ancient stratigraphic record. Assessment can be made of the spatial scale over which such interactions are likely to occur and this has applied significance; the developed facies models facilitate the prediction of net reservoir sandbody dimensions from subsurface successions by constraining the geometry and lateral and vertical connectivity of sand bodies for specific desert system types. Assuming layer-cake correlations between neighbouring wells within stratigraphically complex reservoirs composed of mixed aeolian and fluvial facies is inappropriate; instead, a range of bespoke facies models should be utilized, each of which considers possible stratigraphic configurations and each of which has implications for likely reservoir performance.