<P> The hydraulic seal occurs in rocks that have a significantly higher displacement pressure such that the pressure required for tension fracturing is actually lower than the pressure required for fluid displacement--for example, in evaporites or very tight shales . The rock will fracture when the pore pressure is greater than both its minimum stress and its tensile strength then reseal when the pressure reduces and the fractures close . </P> <P> After the discovery of a reservoir, a petroleum engineer will seek to build a better picture of the accumulation . In a simple textbook example of a uniform reservoir, the first stage is to conduct a seismic survey to determine the possible size of the trap . Appraisal wells can be used to determine the location of oil - water contact and with it, the height of the oil bearing sands . Often coupled with seismic data, it is possible to estimate the volume of oil bearing reservoir . </P> <P> The next step is to use information from appraisal wells to estimate the porosity of the rock . The porosity, or the percentage of the total volume that contains fluids rather than solid rock, is 20 - 35% or less . It can give information on the actual capacity . Laboratory testing can determine the characteristics of the reservoir fluids, particularly the expansion factor of the oil, or how much the oil expands when brought from high pressure, high temperature of the reservoir to "stock tank" at the surface . </P> <P> With such information, it is possible to estimate how many "stock tank" barrels of oil are located in the reservoir . Such oil is called the stock tank oil initially in place (STOIIP). As a result of studying factors such as the permeability of the rock (how easily fluids can flow through the rock) and possible drive mechanisms, it is possible to estimate the recovery factor, or what proportion of oil in place can be reasonably expected to be produced . The recovery factor is commonly 30 - 35%, giving a value for the recoverable reserves . </P>

To hold greater quantities of liquid or gas a rock should have