The fossiliferous Eocene carbonate reservoir interval of the offshore Hasdrubal Field has a diagenetic history in which dolomitisation of micrite-rich facies has significantly enhanced reservoir quality. Using published information on the diagenetic evolution, digital textural representations of the limestones and dolostones are created using the Multi-Component Architecture Method (MCAM). This technique allows integration of images at different length scales (e.g., SEM, micro-/nano- XRT) which provide different forms of information about the pre-dolomitisation rock, including textural and compositional factors, and the arrangements of preserved components. These digital representations of the dolostones, together with their now completely overprinted precursor micritic limestones, permit exploration of the impact on porosity and permeability of uncemented, partly cemented and biomoulds Nummulites, their number per unit volume, and their orientation (i.e., uniformly or randomly orientated) allowing investigation of the petrophysical consequences of different distributions, orientations and abundances of the matrix and fossiliferous components. Viable diagenetic pathways, which were identified by thin section analysis as well as data available in the literature, are linked to petrophysical property evolution. In this way MCAM permits investigation of earlier stages of the diagenetic processes which were overprinted by later diagenetic processes.

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The Eocene El Garia Formation in the offshore Hasdrubal Field was originally a nummulitic limestone in which subsequent burial dolomitization has significantly enhanced permeability. Identification of the reservoir's petrophysical property distributions requires knowledge of the spatial extent of its dolomitization, in turn requiring understanding of the processes that caused the dolomitization. Some of this understanding can be derived from measurements but others need to be simulated. In this study, the former are used as guides and we focus on the latter, evaluating the character of the dolomitizing fluid's movement and temperature patterns by using basin modelling to develop heat-flux simulations to represent the time of dolomitization. Basin modelling reconstructs the region's geology at the time of dolomitization, while heat-flux simulations recreate the appropriate conductive and convective heat and mass transport through these systems. Potential key drivers are rock mass and fault-zone permeability, and the position and shape of any salt domes. The results suggest that salt dome shape and position is the dominant control, the salt dome localizing convective systems which also use convenient faults so that hotter upwelling fluids pass through the Hasdrubal reservoir and are instrumental in the development of burial dolomitization.@en

Field X comprises a giant Palaeogene limestone reservoir with a long production history. An original geomodel used for history matching employed a permeability transform derived directly from core data. However, the resulting permeability model required major modifications, such as horizontal and vertical permeability multipliers, in order to match the historic data. The rationale behind these multipliers is not well understood and not based on geological constraints. Our study employs an integrated near-wellbore upscaling workflow to identify and evaluate the geological heterogeneities that enhanced reservoir permeability. Key among these heterogeneities are mechanically weak zones of solution-enhanced porosity, leached stylolites and associated tension-gashes, which were developed during late-stage diagenetic corrosion. The results of this investigation confirmed the key role of diagenetic corrosion in enhancing the permeability of the reservoir. Insights gained from the available production history, in conjunction with petrophysical data analysis, substantiated the characterization of this solution-enhanced permeability. This study provided valuable insights into the means by which a satisfactory field-level history match for a giant carbonate reservoir can be achieved. Instead of applying artificial permeability multipliers that do not necessarily capture the impacts of geological heterogeneities, our method incorporates representations of fine-scale heterogeneities. Improving the characterization of permeability distribution in the field provided an updated and geologically consistent permeability model that could contribute to the ongoing development plans to maximize incremental oil recovery.@en

Basin modelling tools are widely used to predict reservoir charging but, unfortunately, their application to carbonate reservoirs is far from straightforward. For carbonates, the relationships between porosity/ effective stress/depth, must be addressed before a reliable basin model can be produced. We propose using a synthetic diagenesis approach, where Pore Architecture Models (PAMs) are used to generate representative porosity values for the different diagenetic (backstripped) stages that are recognised from cores or thin sections. The synthetic diagenesis approach is then used to inform more conventional basin modelling. The viability of this method is the topic of this contribution.@en

Carbonate reservoirs have petrophysical property distributions largely controlled by a combination of the depositional, diagenetic, and structural (burial/uplift) histories of the reservoir itself and also of the basins that contain them. Carbonates are very prone to diagenetic alteration; porosity and permeability can be strongly affected by the thermal state, fluid-pressure and pore fluid chemistry through their geological history. We use a novel workflow, adapted from basin modelling, to investigate how the burial/uplift history of an offshore carbonate reservoir and its basin, taken as a system, can have controlled the fluid and heat movement within, into and out of the reservoir. The reservoir rock properties and diagenetic history are assessed, as is the local and regional geological evolution for potential contributory factors to the diagenesis. A model of the potential basin system is developed, observed reservoir diagenetic history being added to the normal basin modelling constraints. This model provides good estimates of geometry and property evolution, and of fluid transport, through geological time. Since fluid and heat fluxes are important in the diagenetic evolution of the carbonate pore system, these results are complemented by simulating the movement of heat and brine in the reservoir using finite element-finite volume simulations. These simulations capture the complex geological structures, especially fault-fracture systems, and better represent the flow physics and chemistry that control reservoir diagenesis. Results from these simulations will later be returned to the basin model to improve the calibration of the timing, depth, and rates of diagenetic events. This new workflow is applied to a Lower Eocene offshore carbonate reservoir with a complex diagenetic history which seems to have a strong basin evolution influence. Importantly this workflow is generic and can be applied to any carbonate reservoir to enhance the link between geological models at the basin scale and reservoir scale models.@en