Page 20 - Petroleum and Gas Field Processing

the gas and water. Complete displacement of water by gas, or oil, never
occurred. Some salt water stayed with the gas and/or oil within the pore
spaces and as a film covering the surfaces of the rock grains; this water is
known as the connate water, and it may occupy from 10% up to 50% of
the pore volume.
The geologic structure in which petroleum has been trapped and has
accumulated, whether it was the source rock or the rock to which
petroleum has migrated, is called the petroleum reservoir.
In summary then, the formation of a petroleum reservoir involves
first the accumulation of the remains of land and sea life and their burial
in the mud and sedimentary materials of ancient seas. This is followed by
the decomposition of these remains under conditions that recombine the
hydrogen and carbon to form the petroleum mixtures. Finally, the formed
petroleum is either trapped within the porous source rock when a cap rock
exists or it migrates from the source rock to another capped (sealed)
structure.

1.3 TYPES OF PETROLEUM RESERVOIR

Petroleum reservoirs are generally classified according to their geologic
structure and their production (drive) mechanism.

1.3.1 Geologic Classification of Petroleum Reservoirs
Petroleum reservoirs exist in many different sizes and shapes of geologic
structures. It is usually convenient to classify the reservoirs according to
the conditions of their formation as follows:
1. Dome-Shaped and Anticline Reservoirs: These reservoirs are
formed by the folding of the rock layers as shown in Figure 1.
The dome is circular in outline, and the anticline is long and
narrow. Oil and/or gas moved or migrated upward through the
porous strata where it was trapped by the sealing cap rock and
the shape of the structure.
2. Faulted Reservoirs: These reservoirs are formed by shearing and
offsetting of the strata (faulting), as shown in Figure 2. The
movement of the nonporous rock opposite the porous formation
containing the oil/gas creates the sealing. The tilt of the
petroleum-bearing rock and the faulting trap the oil/gas in the
reservoir.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.