IDENTIFYING THE CAUSES OF OVERPRESSURE IN HPHT SETTINGS: THE NON-UNIQUENESS OF DENSITY-VELOCITY CROSSPLOTS

Reliable estimates of formation pressure will result in improved exploration success, cost-effective well planning and drilling safety. In order to do this, the cause of the overpressure needs to be known so that the appropriate methods of quantification will be applied. Crossplots of velocity and density of mudstones can be used to highlight and interpret the occurrence of overpressure generation mechanisms. When overpressure is generated solely by disequilibrium compaction, data will plot on the virgin curve. The occurrence of other mechanisms will lead to characteristic deviations from the virgin curve. Having taken into account the rock property relationships under different overpressure mechanisms and vertical effective stress regimes, this study recognizes that the direction of deviation on such plots is simple to relate to the causal mechanism when the evolution of one unit of rock through geological time is considered. However, in practice, data presented on such plots come from a range of depths, often with different stress histories. A generic modelling of unloading (fluid expansion / gas generation) based on known vertical effective stress and rock property relationships indicates that the trends seen on velocity-density crossplots are partially caused by the overpressure generating mechanism, but also influenced by other factors such as the size of the pressure transition zone and amount of overpressure in the system. The current practice of recognizing anomalous trends on velocity-density crossplots as an indication of specific late geopressure mechanisms ought to be done with caution. Additional information such as the laboratory analyses of rock samples would be required to prove the existence of a particular mechanism.