A Cost-Effective Strategy for the Spatial and Temporal Evolution of Porosity in Context of Thermal and Paleohydrologic Evolution

Abstract

Porosity prediction ahead of the drill is a most complex exploration problem in light of the growing need for the industry to explore for hydrocarbons in frontier basins. A cost-effective strategy that makes a sequential use existing basin scale fluid-flow model, and reservoir scale reaction-transport models is based on the understanding of dominant mechanisms of porosity alteration during burial. Pore volume collapse due to mechanical rearrangement of grains during initial stage of burial is calculated on a basin scale via use of a finite difference code, BASIN2 (Bethke, University of Illinois) where physical properties of three rock types are substituted with those of 13 rock types and the heat flow is coupled to fluid flow. Fluid flow velocities so generated due to initial pore volume collapse are then input into a coupled reaction-transport code, REACTRAN (Ortoleva, University of Indiana) for the reservoir of interest at the location of proposed prospect. Porosity changes (gain/loss) due to diagenetic changes are then calculated as a function of increasing time and temperature in the initially open system. Using the timing of onset and termination of paleo-overpressuring calculated by BASIN2, the timing at which a reservoir may be a closed system is assessed. Although inmore » an ideal case all the mechanisms of porosity modification must be handled simultaneously, linking these models in a comprehensive basin model would be enormously computer-intensive. In addition, such linking appears to be unnecessary, since the predicted timing and extent of porosity and diagenetic events seems to agree with the observed timing and extent of porosity and diagenetic events for the areas modeled so far.« less