Unloading mechanism indications in overpressure: A Niger delta example, ASSN Field.

摘要

Shale densities and p-wave velocities derived from three ASSN well log data were cross plotted and then subjected to pore pressure analyses using three different techniques. The relationship between the vertical effective stress and shale velocity as the depth increases were also analyzed, primarily to determine the influence of mechanisms other than disequilibrium compaction on the overpressure in the study area. An unloading curve that was estimated and fitted based on observed decrease in effective stress values while shale velocity reversed as depth increases further support the notion that unloading has contributed to overpressure in the study area. A 700ft/s drop in the shale velocity between density of 2.4g/cm3 and 2.425 g/cm3 at depth interval of between 8000ft and 11000ft was attributed to liquid-gas-phase change which is possible in a comparatively high to intermediate temperature gradient northeastern Ugheli debo-belt of the Niger delta basin.