A Geologist's View
background information written by a geologist
'Until recently oil and gas
exploration was undertaken on rock formations such as
Jurassic,Triassic and Cretaceous. Older rock formations , such
as what you may find in South Africa, Namibia and Uganda (and
other countries) were always thought would never hold
hydrocarbons as they were too old. I believe that the older rock
formations for example Permian and Carboniferous could contain
hydrocarbons in sufficient quantities. My reasons for this are
Life forms started beyond the
formations of these rocks and there would have been large lakes
with life forms within the lakes. Surrounding the large lakes
there would be large mountains surrounding some of these lakes.
Over the course of several million
years, combining the land movement and erosion, over this time,
would eventually cover these lakes and lagoons and the water
life inside the lakes would be trapped. The process of erosion
continued causing the temperatures to rise, creating cooking
areas and, over time, releasing the hydrocarbons causing them to
migrate from the cooking area through the porous layers of rock
until they hit non-porous rock. Further earth movement would
then create the trap structures.
This is my theory and I do believe
that there would be oil and gas in these older rock formations,
and not necessarily as deep'.
As a petroleum geologist, one
wonders. Nature has a way of barring oil from obvious hiding
places and hoarding it in unsuspected corners.
At first glance the principles
of petroleum geology are simple:
1. petroleum liquids (oil) and
gases are combinations of hydrogen and carbon. Hydrocarbon
compounds are organic substances, not minerals.
2. Hydrocarbons are derived
from animal and plant matter of all geological ages, buried in
the sediments of ancient oceans. Therefore oil and gas are most
often found together with salt water in or in close association
with rocks of marine origin.
3. At first, the conversion of
organic remains into hydrocarbons is the result of bacterial
action. The main product is methane (marsh or coal-mine gas).
At burial depths of a few thousand meters and temperatures
between 50 degrees and 150 degrees both oil and gas are
generated by chemical reactions. Additional overburden leads to
the breakdown of any oil previously formed. The main product is
again methane, this time the result of excessive heat.
4. In order, to accumulate in
commercial quantities hydrocarbons need space, but they do not
occur in underground caves and rivers. Petroleum is produced
from the pores and minute fissures of host rocks such as
sandstones, limestone and dolomites.
5. The reservoir fluids, gas,
oil and water, follow the laws of gravity. If all three are
present into the same container, gas rises to the top, oil stays
in the middle and water rests at the bottom.
6. Since hydrocarbons continue
to rise through permeable rocks unless stopped from escaping to
the surface, one of the most important requirements for a
petroleum reservoir is an impermeable "roof". In order to form
a trap for oil or gas the roof, viewed from below, must be
The problem is that these
subsurface conditions cannot be observed or measured accurately
from above. Educated guessing plays a large part in petroleum
The most obvious petroleum
reservoir is a porous rock formation under a tight layer, both
folded into a done-shaped anticline (figure 1A). If the entire
sequence of sediments above the subsurface anticline has been
deformed in the same manner, the structure can be mapped at the
surface. So, is there a direct oil finding method after all?
No, because even if a structure has a surface expression, it may
lack the right combination of porous and dense rocks or it may
be filled with water.
A structure trap can be defined
as one whose roof has been shaped by local deformation, not only
folding but also by faulting (figure 1B).
Structural traps that can be
mapped at the surface are quickly found and the number of
undiscovered ones are rapidly demising. Oil finding has become
more complicated since it began a century ago.
A different kind of petroleum
reservoir owes its existence not to deformation, but to a change
or break in the porous formation. It can only be detected by
stratigraphy, the study of layered rocks and is therefore called
a stratigraphic trap.
PRIMARY STRATIGRAPHIC TRAPS
(Figure 2A) shows a product of
the environment in which the sediment in question was laid
down. Sandstone lenses of all shapes and sizes, for instance,
can be the result of wave and current action in ancient oceans,
or they can represent parts of buried deltas and river
channels. Another common type of primary stratigraphic trap was
created by local replacement of dense limestone by porous
Sandstone enclosed in shale and
dolomites enclosed in limestone are similar in many respects,
at least as far as the trapping mechanism is concerned.
Another important group of traps, the organic reefs, is somewhat
different. In the stratigraphic sense a reef is a mound-like or
layered rock structure built by corals and other marine
organisms. It can be large like the modern Great Barrier Reef
off the east cost of Australia, or small like the atoll of
Bikini. One of the best known fossil examples is the oil
reservoir of Leduc, Alberta. Traps of this nature fall in the
primary stratigraphic category but since they were built above
the surrounding sea floor, often to heights of several dozens of
meters, they have much in common with structural traps and as
exploration targets they are in a class by themselves (Figure
In contrast to primary
stratigraphic traps, secondary stratigraphic traps are a product
of developments that took place long after the deposition of the
reservoir rock. In the most important of these events, ancient
layers of sediments were lifted above sea level, exposed to
weathering and erosion for a few million years, and then
returned to equatic environment to be covered with new
sediments. The usual result of such "temporary" interruption of
the depositional process is an angular unconformity between the
older and younger sets of rocks. A reservoir rock sandwiched
between dense formation, truncated by erosion, and capped by a
younger dense layer can become a trap for migrating petroleum
It is probably clear by now,
that most stratigraphic traps could not exist without some
structural help, however subtle. Small, steep-sided "pinnacle"
reefs and small completely hydrocarbon filled porous lenses are
the only pools where structure has a negligible effect on the
fluid distribution. Widespread sands, on the other hand, can be
useless from the standpoint of the petroleum geologist if they
lie absolutely flat and horizontal. Only when such a sand has
been tilted or folded ever so slightly, can petroleum liquids
gather to form a pool. Even in the exploration for
stratigraphic traps structure is important.
book 'Mining Oil and Gas Explained.' 1982 Northern Miner Press