Abstract Accurate estimates of depositional porosity and in particular permeability are critical initial inputs for process-based models used to predict diagenesis in carbonate reservoirs. Unfortunately, the depositional porosity and permeability of modern carbonate sediments have rarely been measured. In this study, porosity and permeability, using Kozeny-Carman and Lattice-Boltzman algorithms, were calculated on simulated grain packs built from grain-size analyses of carbonate sediments. Five depositional environments on the Caicos Platform were sampled: Tidal Flat, Lagoon, two different Shoals, and Beach. Sediments were analyzed for texture, composition, mineralogy, grain size, and sorting. The depositional environments sampled have a characteristic grain-size distribution and the mean grain size for individual sample locations was relatively uniform. Porosities calculated for grainstones (0.39 to 0.42) were lower than those measured in a separate study on the Great Bahama Bank (0.5 to 0.53). Porosity calculated for tidal-flat sediments (0.31) is significantly lower than measured values (0.61 to 0.78) because intragranular porosity is not calculated. Depositional permeability calculated for grainstones with Lattice-Boltzman (290 to 531 darcies) was greater than with Kozeny-Carman (137 to 285 darcies). Values of permeability for ooid grainstones measured on the Great Bahama Bank (16 to 57 darcies) are an order of magnitude lower, but those sediments are also finer grained (200 versus 700 microns mean grain size).

Abstract The diverse modern carbonate environments and island outcrops on the Caicos Platform create an excellent locale for learning the fundamental principles of carbonate reservoir characterization. This study investigated aspects of early diagenesis at several classic Caicos field stops. The Pleistocene age, shallowing upward parasequence, outcropping as a vertical transition from subtidal reef and shoal to foreshore at Boat Cove, West Caicos, was sampled to describe texture, grain composition and sorting, cement, mineralogy, pore types and visual porosity. Early diagenetic products include the stabilization from aragonite to calcite, cementation and dissolution (fabric and non fabric selective). Pore types are dominantly interparticle with occasional isolated moldic pores and vugs. Measured porosity (11.1 to 33.3%) and permeability (0.8 to 1341 mD) are significantly lower than typical depositional values. Fractures at Boat Cove are dominantly mud filled and are potential baffles to flow. Waters were sampled from several locations including: salinas on West Caicos, Lake Catherine, Mid-Platform Shoals, Central Platform Trough and a variety of sub environments around North Caicos Tidal Flats. The presence of mesohaline to hypersaline brines suggest the potential exists for brine reflux in several Caicos environments. Numerical models of geothermal convection, illustrate subsurface early diagenetic environments and processes which can not be directly observed from the Caicos platform-top. Models predict geothermally driven calcite cementation around the platform margin and dissolution in the platform interior. Results from this study have been used to help understand early diagenesis in reservoir analogs, build a database of diagenetic fluids for reactive transport model studies and have been incorporated into Caicos field schools to enhance instruction of carbonate diagenesis and reservoir quality prediction.

Abstract Dolomitization requires not only favourable thermodynamic and kinetic conditions, but also a fluid-flow mechanism to transport reactants to and products from the site of dolomitization. This paper reviews work that seeks to provide a quantitative framework for conceptual models of dolomitization, using analytical and, particularly, numerical simulation models of fluid flow and rock-water interaction. This approach is starting to yield new insights into the major controls on the rate and pattern of fluid flux, and the resultant dolomitization. Three sets of forces can drive the fluid flow required for dolomitization: elevation (topographic) head of meteoric water and/or seawater; gradients in fluid density due to variation in salinity and/or temperature; and pressure due to sedimentological and/or tectonic compaction. However, in many situations individual flow mechanisms may not operate in isolation. Rather fluid flow will commonly be a product of a number of different drives acting simultaneously. The balance between drives will change over time with variations in relative sea-level, climate, platform geometry and palaeogeography (which collectively comprise the critical boundary conditions). The simplistic prediction of dolomite body geometry from a single driving force may be misleading, as fluid flow will critically depend both on the boundary conditions and the distribution of permeability. Indeed, even for single driving forces, model predictions change significantly as simplistic assumptions are relaxed and these key parameters are specified with increasing realism. The coupled modelling of dolomitization reactions within the flow field is less tractable than that of groundwater circulation because the kinetics of dolomitization are less well understood, particularly at lower temperatures. Dolomitization is likely to occur along a reaction front, where a favourable balance is struck between mass transport and reaction kinetics. For instance, in simulations of geothermal convection dolomitization focuses along the 50–60 °C isotherm. Dolomitization reactions are favoured by higher temperatures in deeper zones, but rates are limited by low flow because of lower permeability. Although flow rates are higher in shallow more permeable carbonates, lower temperatures limit reactions. High flow rates during reflux of platform-top brines give rapid dolomitization. This is associated with porosity occlusion in front of and behind the broad zone of replacement dolomitization driven by anhydrite cementation and overdolomitization, respectively. Lithological heterogeneities strongly affect the pattern of dolomitization, which is highly focused within more permeable beds and those with a higher reactive surface area. While we focus here on dolomitization, models can also provide insights into diagenetic processes such as marine calcite cementation and aragonite, calcite and evaporite dissolution by refluxing brines, and by seawater circulation below the aragonite and calcite compensation depths. However, it is important to be aware of the assumptions and limitations of the numerical model(s) used. Particular attention must be paid to specification of boundary conditions, permeability and reactive surface area. The uncritical application of numerical techniques to particular cases of dolomitization is at best uninformative and at worst misleading. Careful application of these techniques offers great promise for well-constrained field problems, with greater inclusion of natural heterogeneity and time-variant boundary conditions. We also need to model feedbacks between diagenesis and porosity-permeability, and to include platform growth in simulations of slower diagenetic processes.