DEEP HORIZONS: A GIS TOOL TO FORWARD-MODEL SECONDARY POROSITY DEVELOPMENT IN CARBONATE PLATFORMS

Forward modeling is a technique used to predict the sequence and structure of geologic systems based on assumptions that follow from depositional and erosional processes. Modeling of this type has been used extensively in the petroleum industry to predict reservoir properties. Carbonate platforms are an especially complex type of reservoir, partly because they largely consist of heterogeneous reef and marine assemblages, but also because after formation they are prone to dissolution in the subsurface. This modification results in changes in fluid flow and hydrogeologic properties of the rock, which must be modeled over time as a dynamic geometry. In coastal carbonate systems, extensive conduits develop along the fresh-saltwater mixing zone within an aquifer. As sea level changes, the elevation of conduit development changes relative to it. Horizontal and vertical heterogeneity in bedrock properties and a mixing zone that moves with changes in sea level complicate efforts to accurately model and predict fluid flow. The changing thickness and position of a freshwater lens, controlled by permeability and precipitation, may result in complex yet predictable zones of dissolution. Ultimately, a broadly applicable model of secondary porosity development must consider the interactions between all of these parameters over time and space. DEEP HORIZONS is a first step toward a comprehensive model of conduit formation in a carbonate platform to accurately simulate the changes in aquifer structure over time. It uses global sea level history and user-specified geologic parameters to identify zones that could potentially contain significant conduit development. The tool is written in Python for use in ArcGIS, and will aid in testing hypotheses related to speleogenesis, with potential applications in water resource management and petroleum exploration.