Page 41 - Synthetic Fuels Handbook

NATURAL GAS 29

through temperature effects occurred over geologic time (millennia) and shortening the
time to laboratory time and increasing the temperature to above and beyond the cracking
temperature (at which the chemistry changes) does not offer conclusive proof of high tem-
peratures (Speight, 2007a).
Nevertheless, at some point during or after the maturation process, the gas and crude oil
migrate from the source rock either upward or sideways or in both directions (subject to the
structure of the accompanying and overlying geological formations). Eventually, the gas
and crude oil became trapped in reservoirs that may be many miles from the source rock.
It is rare that the source rock and the reservoir were one and the same. Thus, a natural gas
field may have a series of layers of crude oil/gas and gas reservoirs in the subsurface. In
some instances, the natural gas and crude oil parted company leading to the occurrence of
reservoirs containing only gas (nonassociated gas).
Reservoirs generally comprise a geologic formation that is made up of layers of porous,
sedimentary rock, such as sandstone, in which the gas can collect. However, for retention
of the gas each trap must have an impermeable base rock and an impermeable cap rock to
prevent further movement of the gas. Such formations, known as reservoirs or traps (i.e.,
naturally occurring storage areas) vary in size and can retain varying amounts of gas.
There are a number of different types of these formations, but the most common is,
characteristically, a folded rock formation such as an anticline as occurs in many petro-
leum reservoirs (Fig. 2.1), that traps and holds natural gas. On the other hand, a reservoir
may be formed by a geologic fault that occurs when the normal sedimentary layers sort
of split vertically, so that impermeable rock shifts down to trap natural gas in the more
permeable limestone or sandstone layers. Essentially, the geologic formation which layers
impermeable rock over more porous, oil and gas rich sediment has the potential to form a
reservoir.

Ground

Gas cap

Oil layer
with dissolved gas
FIGURE 2.1 An anticlinal reservoir containing oil and (associated) gas.

Reservoirs vary in size from a few hundred meters to tens of kilometers across in plain,
and tens to hundreds of meters thick, with the gas trapped against an impermeable layer
similar to crude oil traps (Speight, 2007a). Some reservoirs may be only hundreds of feet