Lateral heterogeneities in a bioclastic shoal outcrop reservoir analogue: Architectural, cyclical and diagenetic controls on spatial pore distribution

Abstract The distribution of pores in carbonate rocks is controlled by the synergy of different factors such as the initial sediment, environment processes and diagenesis. Prediction of the texture-related pore distribution to evaluate reservoir quality is still a challenging task in geothermal and hydrocarbon exploration. In order to degrease these uncertainties, 3D facies models are produced on the basis of facies types and their relationship to petrophysical properties. However, several studies have shown that these relationships are often rather controlled by diagenesis that is often described as a sequence for an entire reservoir. Two outcrops as part of the same bioclastic shoal reservoir analogue present two different architectural styles: (I) layer-cake architecture and (II) high-angle foreset architecture. In order to identify petrophysical trends and related controlling factors, both outcrops were mapped and studied in detail. Based on the investigation of 41 thin sections, microfacies types were defined and diagenesis was analysed. In addition permeability-porosity was measured and pores were digitally characterized. Although both outcrop successions studied are associated with the same reservoir interval, varied architecture leads to fundamental differences in the porosity-permeability distribution. Petrophysical trends in the layer-cake architecture are mainly controlled by cyclicity and channel features. Compared to layer-cake architecture, trends in the high-angle foreset architecture are dominantly controlled by the depositional environment. Both outcrops show differences in their diagenesis. While layer-cake architecture suffered the entire sequence, large parts of the high-angle foreset architecture were not influenced by late burial and telogenetic processes. As a consequence, primary porosity is persevered within the high-angle foreset architecture. In addition, results of this investigation indicate that primary porosity has a bigger influence on good permeability values compared to telogenetic associated vug-enhancement. This investigation therefore demonstrates that individual carbonate reservoirs can suffer different diagenetic pathways. Moreover, the study highlights the potential of outcrop studies to improve the principal understanding of fundamental trends within potential subsurface reservoirs.