When oil and gas is extracted from a reservoir, pore pressure will drop near the production wells.
Gas fields are usually produced with the strategy of rapid pressure depletion with production coming to an end when the reservoir pressure is too low to keep an economically viable flow rate in the wells. In a homogeneous, high-permeable reservoir the measured well pressure gives good information on the whole reservoir.
However, structurally or stratigraphically heterogeneous reservoirs may have more complex pressure depletion patterns across the reservoir volume. Knowledge of the true pressure distribution is never complete. Different reservoir structures will have different seafloor responses in both subsidence and gravity change associated with fluid extraction. Therefore, these measurements can be used to infer depletion and reservoir stiffness in regions not sampled by observation wells.
Also, oil fields and mixed oil & gas fields will experience depletion around the production wells. When water, gas or CO2 is injected for pressure support and improved sweep efficiency, there will be local pressure drops around production wells and simultaneously pressure buildup around injection wells, causing compaction and expansion respectively in the reservoir. The associated mass reduction and mass increase will slightly change the gravity at the surface/seafloor.
These subtle changes are measurable.