Page 104 - Petroleum Geology

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may also coincide, but they are more difficult to assess unless they can be
shown to be original gas/oil contacts that have not been affected by produc-
tion.
The quantity of petroleum, in terms of the volume of oil or gas in place in
the reservoir also varies from reservoir to reservoir. Reservoirs are rarely full,
in the sense that the petroleum column rarely extends down to the point at
which it would spill out of the trap (the spill points). They are never full in
the sense of occupying all the available pore space, because petroleum does
not displace all the water in the pore spaces. Irreducible water saturation of
20 to 40% of the pore space is quite common (this is the reservoir engineer’s
“connate water”). However, if the reservoir exists by virtue of cracks, joints
and fissures, the total volume of these may be very nearly filled with petro-
leum due to their large volume in relation to the enclosing surface area and
the consequent relative reduction in the water adsorbed to solid surfaces and
trapped in the interstices.
The volume of petroleum in a field, and its distribution in different re-
servoirs, is clearly important. The volume of oil or gas in place in a reservoir
is estimated by estimating the total volume of rock that contains oil or gas,
multiplying this by the best estimate of the mean porosity of the reservoir,
and then multiplying the product by the best estimate ,of the proportion of
oil in the pore spaces (the oil saturation). The volume of recoverable oil -
oil that is recoverable by present technology at present prices - is obtained
by multiplying the volume of oil in place by a recovery factor (usually
25-30%). The volumes of gas are given at standard temperature and pres-
sure, such as 15°C and 760 mm of mercury (the sta.ndard always being
stated). During production, these estimates are revised by reservoir engineers
on the basis of performance, new data, trends and theory.
The sizes of anticlines and anticlinal accumulations vary greatly. The
Middle East is famous for anticlines that extend for several hundreds of kilo-
metres and accumulations 100 km (60 miles) long, such as Kirkuk in Iraq.
We shall examine the size-distribution of oil fields in Chapter 7, but it is
worth noting here that 2% of the world’s petroleum fields contain about
80% of the known recoverable reserves.
The area occupied by a petroleum accumulation is important mainly for
its influence on the chances of finding it, but it also influences the eco-
nomics of producing it. A thin extensive reservoir is more expensive to
develop than a thicker and less extensive reservoir with the same porosity
and permeability, and the same volume of original oil in place.
The entrapment of petroleum by a fault (Fig. 4-5) involves: (a) an inclined
reservoir; (b) a cap rock; (c) a fault that forms an up-dip barrier across the
reservoir either by virtue of the fine-grained, relatively impermeable material
in the fault plane itself, or by virtue of such material being in juxtaposition
across the fault; and (d) some barrier in the reservoir along the fault, to
prevent lateral migration.