From outcrop analogue to flow simulation: modelling heterogeneity in shallow-marine reservoirs

Abstract

Shallow-marine reservoirs are typically complex, containing hierarchically arranged heterogeneity in inter-well volumes, at scales that are challenging to represent in reservoir models. Permeability contrasts associated with clinoforms are one such heterogeneity, but at present there are no modelling tools available to automate the generation of multiple three-dimensional (3D) clinoform surfaces. Consequently, clinoforms are rarely incorporated in models of shallow-marine reservoirs, even when their potential impact on flow is recognized. A numerical algorithm that generates multiple 3D clinoforms is presented. The algorithm is validated via construction of 3D, surface-based reservoir models of: (1) fluvial-dominated delta-lobe deposits exposed at outcrop (Cretaceous Ferron Sandstone Member, Utah, USA); and (2) a deltaic reservoir using a sparse subsurface dataset (Jurassic Sognefjord Formation, Troll Field, Norwegian North Sea). We use a suite of 3D reservoir models constructed with the clinoform-modelling algorithm and outcrop-analogue data (Cretaceous Ferron Sandstone Member, Utah) to quantify the impact of clinoforms on fluid flow in the context of: (1) other uncertainties in reservoir characterisation, such as the impact of bed-scale heterogeneity on vertical permeability; and (2) reservoir engineering decisions, including oil production rate. Clinoforms are difficult to identify using production data, but our results indicate that they can significantly influence hydrocarbon recovery and their impact can be larger than that of other geological heterogeneities and of reservoir engineering decisions. Vertical permeability within distal delta-front facies comprising interbedded sandstones and shales is found to be an important influence on sweep within clinothems. However, it is difficult to characterize these intervals from subsurface datasets. A digital outcrop modelling method is presented and applied to capture the geometry and architecture of sandbodies in such deposits in an outcrop analogue (G2 parasequence, Grassy Member, Blackhawk Formation, Utah, USA). We use the resulting digital outcrop model to make a preliminary interpretation of 3D gutter cast geometry.