NOMENCLATURE OF A TRAP
The highest point of the trap is the crest, or culmination.
The lowest point at which hydrocarbons may be contained in the trap is the spill point; this lies on a horizontal contour, the spill plane.
The vertical distance from crest to spill plane is the closure of the trap.
The zone immediately beneath the petroleum is referred to as the bottom water, and the zone of the reservoir laterally adjacent to the trap as the edge zone.
Within the trap the productive reservoir is termed the pay.
The vertical distance from the top of the reservoir to the petroleum/water contact is termed gross pay.
All of the gross pay does not necessarily consist of productive reservoir, so gross pay is usually differentiated from net pay.
The net pay is the cumulative vertical thickness of a reservoir from which petroleum may be produced. Development of a reservoir necessitates mapping the gross : net pay ratio across the field.
Within the geographic limits of an oil or gas field there may be one or more pools, each with its own fluid contact.
This field contains two pools, with different oil : water contacts (OWC). In the upper pool the net pay is much less than the gross pay because of non-productive shale layers. In the lower pool the net pay is equal to the gross pay.
DISTRIBUTION OF PETROLEUM WITHIN A TRAP
A trap may contain oil, gas, or both. The oil : water contact (OWC) is the deepest level of producible oil. Similarly, the gas : oil contact (GOC) or gas : water contact is the lower limit of producible gas.
Where oil and gas occur together in the same trap, the gas overlies the oil because the gas has a lower density.
Whether a trap contains oil and/or gas depends both on the chemistry and level of maturation of the source rock and on the pressure and temperature of the reservoir itself.
Fields with thick oil columns may show a more subtle gravity variation through the pay zone. Boundaries between oil, gas, and water may be sharp or transitional. Abrupt fluid contacts indicate a permeable reservoir; gradational ones indicate a low permeability with a high capillary pressure. Not only does a gross gravity separation of gas and oil occur within a reservoir, but more subtle chemical variations may also exist.
Fluid contacts in a trap are generally planar, but are by no means always horizontal. Correct identification of the cause of the tilt is necessary for the efficient production of the field. There are several causes of tilted fluid contacts.
They may occur where a hydrodynamic flow of the bottom waters leads to a displacement of the hydrocarbons from a crestal to a flank position. This displacement can happen with varying degrees of severity.
In some fields the OWC has tilted as a result of production, presumably because of fluid movement initiated by the production of oil from an adjacent field.
An alternative explanation for a sloping fluid contact is that a trap has been tilted after hydrocarbon invasion, and the contact has not moved.
A third possible cause of a tilted OWC may be a change in facies.
SEALS AND CAP ROCKS
For a trap to have integrity it must be overlain by an effective seal. Any rock may act as a seal as long as it is impermeable. Seals will commonly be porous, and may in fact be petroleum saturated, but they must not permit the vertical migration of petroleum from the trap. Shales are the commonest seals, but evaporites are the most effective. Shales are commonly porous, but because of their fine grain size have very high capillary forces that prevent fluid flow.
Shales may selectively trap oil, while permitting the upward migration of gas. Gas chimneys may sometimes be identified on seismic lines either by a velocity pull-down of the reflector on top of the reservoir, and/or by a loss in seismic character in the overlying reflectors. Indeed some petroleum accumulations are sometimes identified because of their gas-induced seismic anomalies.
CLASSIFICATION OF TRAPS