The effects of dolomite geobodies within carbonate clinoforms on fluid flow and connectivity: Insights from an outcrop analogue on Bonaire, The Netherlands (South Caribbean)

Abstract

Carbonate clinoforms are often challenging to characterize and model due to their complex geometries and additional heterogeneity introduced by diagenetic processes. Dolomitization can influence petrophysical properties resulting in either an increase or reduction in porosity and permeability of the host rock and forms geobodies of varied shapes depending on pre-dolomitization permeability patterns and dolomitization mechanism. Therefore, in partially or fully dolomitized successions, the prediction of fluid flow behavior is not trivial. This study uses a well-studied outcrop analogue of Mio-Pliocene partially dolomitized clinoforms at Seru Grandi (Bonaire) to better understand fluid flow in different dolomitization scenarios. Clinothems consist of heterogeneous coralline algal facies overlying bioclastic facies, with dolomite geobodies truncated on their upper and lower bounds by clinoform surfaces. Digital outcrop models were used to characterize geometry and spatial relationship of facies and heterogeneity, such as clinoform dip, length, height, and spacing. Multiple realizations of clinoform and dolomite body geometries are modelled using a surface-based modelling (SBM) approach coupled with an unstructured mesh flow simulator (IC-FERST). Two scenarios are considered, in which dolomitization has resulted in either a decrease in porosity and permeability as observed in outcrop, or a relative increase of porosity and permeability values as a potential subsurface scenario. Flow simulation results reveal an exponential relationship between water breakthrough times and flow rates versus dolomite proportions. Additionally, the arrangement of the dolomite bodies (aligned vs. disjoined) exhibits very similar fluid flow behavior across a wide range of dolomite proportions. Sensitivity of flow behavior to the geological models is strongly dependent on dolomite permeability relative to precursor limestone. Dolomite body arrangement is more important for flow behavior at high dolomite proportions for low permeability dolomite, or at low dolomite proportions for high permeability dolomite. This study emphasizes the significance of having a good understanding of the dolomitization mechanism and dolomite body geometries, reducing uncertainty in dolomite distribution, petrophysical properties, and, therefore, fluid flow behavior.