Normal pressure is pore fluid pressure that equals the hydrostatic pressure of a column of formation water extending to the surface. Overpressure is pore fluid pressure greater than normal pressure. However, no standard defin- ition exists for what constitutes high overpressure. What can be said is that high overpressure often means trouble. For an explorationist, it could mean blown reservoir seals; for a driller, it could mean excessive time spent fighting for- mation fluid influxes and/or drilling fluid losses. Apractical upper limit for pore pressure is the overbur- den stress. Pore pressures in this range are on the verge of opening fractures that can vent fluid and bleed off pressure like a pressure relief valve. Therefore, criteria for defining high overpressure are sometimes expressed in terms of a per- centage of the overburden stress, say, pore pressure greater than 90% of the overburden stress. In this article, high over- pressure will be defined simply as pore pressure that approaches the overburden stress. All but one potential cause of overpressure can produce high pressure. Fortunately, the mechanism that cannot gen- erate high pressure is the most common cause of overpres- sure. Therefore, detecting high overpressure basically boils down to determining where extraordinary overpressure mechanisms may be encountered. Overpressure detection is based on the premise that pore pressure affects compaction-dependent geophysical prop- erties such as density, resistivity, and sonic velocity. Shales are the preferred lithology for pore pressure interpretation because they are more responsive to overpressure than most rock types. Consequently, overpressure detection centers around shale deformation behavior.
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