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A Geologist's View

 Some interesting 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 as follows.
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 concave.

The problem is that these subsurface conditions cannot be observed or measured accurately from above.  Educated guessing plays a large part in petroleum exploration.


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.




(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 dolomite. 

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 3).

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 (Figure 2B).

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. 

 from the book 'Mining Oil and Gas Explained.' 1982 Northern Miner Press Ltd Canada

J.P.Taylor. 6 Sunnyridge, Mold, Flintshire. North Wales  CH7 1RU
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