Page 103 - Petroleum Geology

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Fig. 4-4. Block diagram of petroleum in an anticlinal trap.

reservoir rock and under the cap rock because there are many anticlines in
which the geometry of the individual rock units changes with depth.
Entrapment may be in a single reservoir, or in multiple reservoirs in the
same anticline. A single reservoir accumulation is sometimes referred to as a
“pool”, but the terminology is not standard over the world. The general
term for an accumulation from which production has started is a field - oil
field or gas field, as the case may be.
The nature of the petroleum and its composition vary from one field to
another; and they may vary from one reservoir to another in the same field.
Variation between petroleums of different reservoirs in a single field is com-
monly from heavy oil with little or no gas near the surface, through
intermediate oils, to light oils with gas, gas and condensates at depth.
Within a reservoir, oil lies on water; and the interface - the oil/water
contact (O/WC or OWC) - is a horizontal or nearly horizontal surface. If
free gas is present when the reservoir is discovered, it lies on the oil and the
gadoil contact (G/OC or GOC) will be horizontal. Gas that segregates as a
result of changes in the physical state of the reservoir during production will
not necessarily have a horizontal interface with the oil because the accumula-
tion will depend on the effective permeability of the reservoir to gas, and this
may not be uniform.
The oil/water and gas/oil contacts of different reservoirs in the same field
are usually at different depths below datum. Occasionally the oil/water con-
tact of two or more reservoirs is found at the same depth below datum.
When this happens, it is argued that these reservoirs are connected in some
way, leading to a common plane of physical equilibrium. Gas/oil contacts