<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> <P> The difficulty is that reservoirs are not uniform . They have variable porosities and permeabilities and may be compartmentalised, with fractures and faults breaking them up and complicating fluid flow . For this reason, computer modeling of economically viable reservoirs is often carried out . Geologists, geophysicists and reservoir engineers work together to build a model which allows simulation of the flow of fluids in the reservoir, leading to an improved estimate of reserves . </P> <P> To obtain the contents of the oil reservoir, it is usually necessary to drill into the Earth's crust, although surface oil seeps exist in some parts of the world, such as the La Brea tar pits in California, and numerous seeps in Trinidad . </P> <P> A virgin reservoir may be under sufficient pressure to push hydrocarbons to surface . As the fluids are produced, the pressure will often decline, and production will falter . The reservoir may respond to the withdrawal of fluid in a way that tends to maintain the pressure . Artificial drive methods may be necessary . </P>

What conditions are needed for an oil reservoir to form