<P> This is present in the reservoir if there is more gas than can be dissolved in the reservoir . The gas will often migrate to the crest of the structure . It is compressed on top of the oil reserve, as the oil is produced the cap helps to push the oil out . Over time the gas cap moves down and infiltrates the oil and eventually the well will begin to produce more and more gas until it produces only gas . It is best to manage the gas cap effectively; that is, placing the oil wells such that the gas cap will not reach them until the maximum amount of oil is produced . Also a high production rate may cause the gas to migrate downward into the production interval . In this case over time the reservoir pressure depletion is not as steep as in the case of solution based gas drive . In this case the oil rate will not decline as steeply but will depend also on the placement of the well with respect to the gas cap . </P> <P> As with other drive mechanisms, water or gas injection can be used to maintain reservoir pressure . When a gas cap is coupled with water influx the recovery mechanism can be highly efficient . </P> <P> Water (usually salty) may be present below the hydrocarbons . Water, as with all liquids, is compressible to a small degree . As the hydrocarbons are depleted, the reduction in pressure in the reservoir allows the water to expand slightly . Although this unit expansion is minute, if the aquifer is large enough this will translate into a large increase in volume, which will push up on the hydrocarbons, maintaining pressure . </P> <P> With a water - drive reservoir the decline in reservoir pressure is very slight; in some cases the reservoir pressure may remain unchanged . The gas / oil ratio also remains stable . The oil rate will remain fairly stable until the water reaches the well . In time, the water cut will increase and the well will be watered out . </P>

Where is oil found in the earth's crust