Prediction of sealing capacity by the equivalent grain size method

Cap rocks do not hold infinite amounts of oil/gas so there must be a physical limitation of the column height of oil/gas that a trap can hold. The sealing capacity of a trap is one of the most important conditions that needs to be evaluated quantitatively. In this paper, we review the basic phenomenon of hydrocarbon trapping by capillary pressure. By evaluating the observed hydrocarbon column height from a field, we can quantify the sealing capacity in terms of pore throat radius. The pore throat radius, however, is not a convenient parameter for geologists, so we convert it into grain size (assuming that the sediments consist of equal-sized spheres). In this conversion, we use a theoretical relation of COEF (ratio of pore throat to grain size) as a function of porosity. We improve this relationship by conducting a physical experiment where we measure the capillary pressure for an artificial seal to derive an empirical relationship between COEF and porosity. Thus we can evaluate top seal capacity in terms of imaginary grain size (this is named ‘equivalent grain size method’). Then we propose a ‘practical’ flow chart for evaluating sealing capacity in a quantitative way. One of the biggest problems for seal evaluation is that it was usually based on the structural and stratigraphical interpretations. For the effective seal evaluation, we propose an integrated approach such as doing all the interpretations at the same time. This ‘multi-disciplinary’ approach would solve the debatable phenomena on the sealing/non-sealing problems to give us a deeper understanding of the petroleum systems.