CARB3D+: A New Forward Simulation Model for Sedimentary Architecture and Near-Surface Diagenesis in Isolated Carbonate Platforms

Smart, Peter L.¹, David Waltham², Graham Felce¹, Fiona F. Whitaker¹ (1) University of Bristol, Bristol, England (2) Royal Holloway University of London, Egham, UK

We have developed a new 3-dimensional forward simulation model for carbonate plat­forms (CARB3D+) which incorporates near-surface (eogenic) diagenesis. This adopts the conceptual approach used in our previous 2-dimensional model, recognising that many fun­damental controls on platform architecture and sedimentology, such as subsidence and sea level, also control diagenetic evolution via the position of hydrologically-defined diagenetic zones (hydro-zones). The sophisticated process-based sedimentological algorithm includes sediment production by reef, shoal margin, interior and pelagic carbonate factories. Sediment is entrained by waves and currents, and transported by currents and by grain avalanching on slopes. Depositional facies, mineralogy, fabric selective porosity and matrix permeability are predicted for incremental sediment units through time in response to changing sea level. Importantly CARB3D+ includes dynamic feedback between platform morphology and sediment production and transport.

In CARB3D+ four hydro-zones are defined in 3D for exposed carbonates using a spatial­ly variable depth-averaged freshwater lens model. The vadose zone lies above the water­table, the freshwater lens zone lies below the water-table and above the transition (mixing) zone to the underlying saline zone. For each hydro-zone the sequence and rates of mineral transformation are specified. Progressive changes in fabric selective porosity, bulk mineral­ogy, cement volume and mineralogy are predicted, and non-fabric selective (secondary) porosity is up-scaled to derive island-scale permeability. Grain-size dependent compaction is also incorporated. Critical feedbacks between the evolving poro-perm characteristics and distribution of the hydro-zones are included.

To aid functionality the model is packaged in a custom Graphical User Interface, and includes plan and cross section displays for environmental variables and sediment attributes. There are also options for chronostratigraphic plots, well logs and a seismic simulator.