Publisher Summary Abnormally low-formation pressures develop in petroleum reservoirs during intensive oil and gas production, or in aquifers as a result of water extraction. The reduction in pore pressure in reservoir rocks may result in the influx of water from the adjoining shales and consequent reduction in shale pore pressure. If the influx of shale water is not sufficient to replace the produced fluids, a greater percentage of the overburden load will be carried by the skeletal structure of the rock (the grain-to-grain stress 1 is increased). This eventually, will be reflected at the earth's surface as subsidence. Compaction occurs in both the reservoir rocks and the associated shales as the skeletal structure adjusts itself to carry the additional stress. The area of subsidence as a result of subsurface fluid production is about twice as large as the area of the reservoir. Subsidence (differential) at the surface causes engineering and ecological problems. These include structural damage, rupture of casings, and disruption of pipelines. Globally, reservoir rock compaction occurs as a result of pore fluid pressure reduction. Geertsma can be considered as the “father” of the theory of land subsidence due to underground fluid withdrawal. Today, subsidence can be measured accurately in situ by wireline tools, such as FSMT, whereas surface subsidence can be recorded by satellite microwave Doppler techniques, such as GPS.