Abstract
An oil (or gas)/water contact in an oil and gas field is normally horizontal because of a gravitational control in a petroleum trapping mechanism. Inclined contacts, however, have been observed in a significant number of fields. The Cushing gas-oil field discovered in 1912 in the Central Plateau, Oklahoma, USA, is the first example of the field with the inclined contacts. After the Cushing discovery, many fields of inclined contacts have been discovered in the Great Prairie of USA. The inclined contacts of these fields are caused by gradients of hydraulic heads ; the gradient is caused by a difference between an altitude of mountainous areas (recharge areas for aquifers) and a depth of subsurface aquifers (reservoir at a field in the prairies). Since 1990, oil and gas fields with the inclined contacts have been found in marine environments. The Peciko gas field is the example from the Mahakam Delta Province, Indonesia. A number of oil and gas fields with the inclined contacts have been discovered in the North Viking Basin. The marine type of inclined contacts should be considered to be controlled by lateral gradient of pore-pressures. The gradient is caused by a lateral transfer of overpressures from neighboring, stratigraphically-underlying, overpressured formations.
Hubbert (1953) proposed hydrodynamic mechanism of entrapment of oil and gas fields associated with inclined contacts. The theory has been the most commonly accepted theory for inclined contacts ; a flow of formation fluids hydrodynamically causes a gradient of hydraulic heads that inclines the contacts. Hydrodynamic effect on the gradient, however, should be estimated to be negligible based on a series of model calculations of the velocity heads and friction loss heads. It is because a rate of subsurface flow through aquifer (reservoir) is too small to cause a sufficient magnitude of hydrodynamic effect.
