Time-lapse seismic data play a key role in hydrocarbon field monitoring and infill opportunity development. In several deepwater hydrocarbon-producing reservoirs, time-lapse softening changes have been observed at the oil-water contact. Here, we refer to this as contact softening. Contact softening occurs when acoustically softer hydrocarbons above the contact invade and replace the stiffer brine in the pore space below the contact. This typically occurs when pressure depletion from production elsewhere is transmitted through a shared aquifer, causing the expansion of an unproduced hydrocarbon zone. It is shown that the strength and extent of such signals are controlled by the updip fluid volume, rock and fluid compressibility, and amount of pressure depletion transmitted. Such a relationship enables the qualitative and quantitative assessment of pressure depletion and characterization of the updip reservoirs. In some cases, it can be used to estimate a range for fluid compressibility, which has applications in fluid typing. The application of such a relationship could lead to opportunity identification in brownfield development and near-field exploration.

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