Abstract
AbstractThe properties of carbonate rocks are often the result of multiple, diagenetic events that involve phases of cementation (porosity occlusion) and dissolution (porosity enhancement). This study tests the hypothesis that the order of these events is a major control on final porosity and permeability. A three‐dimensional synthetic model of grainstone is used to quantify trends that show the effect of early cementation, non‐fabric selective dissolution, and then a second‐generation of (post‐dissolution) cement. Models are 3 mm3 with a resolution of 10 μm. Six simple paragenetic sequences are modelled from an identical starting sediment (without accounting for compaction) where the same diagenetic events are placed in different sequences, allowing for quantification of relative changes in the resultant porosity and permeability for each diagenetic event, the trajectory through time, as well as for each final rock. All modelled paragenetic sequences result in reductions in porosity and permeability, but the order of diagenetic events controls the trajectory and final rock properties. Differences in the order of early cement precipitation alone produce variable final values, but all follow the porosity–permeability relationship as expressed by the Kozeny‐Carman equation. However, final values for the sequences which include a phase of dissolution fall on a new curve, which departs from that predicted by the Kozeny‐Carman relationship. This allows an alternative form of porosity–permeability relationship to be proposed: κ = 2280ϕ–30,400, where ϕ is porosity (%) and κ is permeability (mD). Hence while the Kozeny‐Carman relationship predicts porosity–permeability changes that occur with cementation, it is unable to capture accurately changes within the pore network as a result of dissolution. Although the results may be dependent on the properties of the initial carbonate sediment and simplified diagenetic scenarios, it is suggested that this new porosity–permeability relationship may capture some generalized behaviour, which can be tested by modelling further sediment types and diagenetic scenarios.
Highlights
Carbonate rocks are very heterogeneous, due to both the variability of sediment grain types and the complexity of subsequent diagenetic overprint
Cementation alone occludes pore space and so all three scenarios modelled show a decrease of porosity and permeability (Fig. 4)
Results from this study demonstrate that the order of diagenetic events has a marked impact on final carbonate porosity and permeability
Summary
Carbonate rocks are very heterogeneous, due to both the variability of sediment grain types and the complexity of subsequent diagenetic overprint. Many paragenetic sequences composed of differing successive diagenetic events are possible, leading to a International Association of Sedimentologists. Chemical compaction closes pore throats isolating pore bodies and reducing permeability (Budd, 2001). Dissolution can increase the size of both pores and pore throats leading to enhanced porosity and pore topology (or connectivity) and so increasing permeability
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