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Reservoir Modeling of Deepwater Lobes: Impact on Connectivity and Production

Abstract

Depositional lobes are the building blocks of many economically important deepwater reservoirs. We compiled a database of lobe geometries, facies, and reservoir properties to build an event-based geostatistcal model applicable to unconfined turbidite systems. We employed flow diagnostic tools to investigate simplified flow behavior of the model. As a first step, analog lobes were placed into a hierarchical stratigraphic framework: lobe elements comprise multiple sedimentation units, each of which is interpreted to represent unconfined deposition of an individual sediment gravity flow. Lobe elements are generally lenticular in cross section and lobate in map view, and exhibit width to thickness ratios of the order of 1000:1. They are interpreted to be bounded by a process of avulsion or abandonment and exhibit predictable changes in grain size, lithofacies, and architecture from proximal to distal: sedimentation units de-amalgamate and become more argillaceous. Argillaceous sandstone exhibits low porosity and permeability compared to cleaner sandstone. Two or more lobe elements stacked in close proximity and of similar facies and architectural style compose a lobe complex. Two end-member reservoir models were constructed: 1) a complex of ‘homogeneous’ lobe elements; and 2) a complex of ‘heterogeneous’ lobe elements. The homogeneous model includes only sandstone and mudstone facies. In this model, sandstone facies lack spatial continuity of properties, which were randomly distributed. The heterogeneous model includes a complex of lobe elements that are built from sedimentation units. Individual sedimentation units are populated with facies and property trends based on subsurface data and conceptual models. Flow diagnostic tools show that lobe heterogeneity has significant impacts on flow: 1) intra-element flow is limited by proximal to distal trends in bed amalgamation and rock properties; and 2) inter-element flow is limited by the connectivity of amalgamated, relatively clean sandstone of proximal lobes. Therefore, detailed characterization of heterogeneity is critical to predicting the production behavior of lobe-dominated reservoirs.