Origin, migration and accumulation of oil
If rocks in which oil might have formed in geological time are present in a given area, it may be possible that oil still exists in that area. The problem of the origin of crude oil in nature is therefore important for oil exploration.
Many theories exist as to the origin of oil or natural gas. The two most widely accepted theories are the organic and the inorganic.
It is now generally accepted that oil originated from the decomposition of aquatic, mainly marine, animals and plants buried under successive layers of mud and silt perhaps as much as 400-500 million years ago. There are some facts which point to the organic origin: (1) Many of the constituents and properties of petroleum could come only from an organic source. (2) Some petroleums are optically active, and, only petroleum derived from organic matter has this property. (3) Composition as highly complex and varied and of such heavy molecular weight as that of the petroleums is found only in organic matter.
An inorganic source of petroleum advocated by some scientists remains an alternative though somewhat speculative hypothesis.It states that hydrogen and carbon came together under great temperatures and pressure far below the earth’s surface and formed oil and gas. This then seeped through porous rocks to collect in various underground traps.
The primary requirement for the genesis of petroleum from organic material is an environment of shallow seas, in which the water is rich in animal and vegetable life from microscopic to large. The second requirement is that organisms, on dying, should sink to the bottom of the sea and be buried by mud from rivers. Conditions on the sea bottom must be such that no rapid decay of the dead organisms takes place by bacterial action; the oxygen content of the water must be small.
In the course of time mud and silt layers deposited on top of the potential source beds produce pressures and higher temperatures in these beds. At a burial depth of several thousand feet chemical processes, probably not dependent on bacterial activity, transform the soft parts of the organisms into oil and gas. Indications are that gas is preferentially generated at greater depths.
As the overburden pressure tends to compact the 'source rocks', oil and gas, probably together with some of the associated water, is squeezed out, provided adjoining formations are sufficiently permeable, that is, that they allow the passage of liquid and gas through the pores of the rock or through a system of fractures and cracks. So the rock fluids start to migrate, either upwards or sidewards or possibly downwards. There is evidence that oil has thus travelled over long distances, even dozens of miles.
In the past the path of migration must often have led to the surface, where the oil was washed away or its lighter components evaporated into the air. Seepages in all parts of the world are evidence of oil and gas still escaping from the subsoil today.
Sometimes migration is halted, for instance by a layer impervious to the passage of fluids. If oil is thus trapped in a porous formation and is no longer able to move, an oil accumulation forms. The porous formation provides storage capacity for the fluid in its pores or interstices, as a sponge holds water, and is called the 'reservoir rock'. The impervious layer that prevents further movement of fluid is the 'sealing formation', usually referred to as 'seal' or 'cap rock'. The seal must be shaped in such a way as to effectively trap the fluid in the reservoir. Various types of traps created by different geological phenomena are described below.
2.3. Answer the questions on the text:
1. What theories are most widely accepted by scientists? 2. What did oil originate from? 3. What is required for the genesis of petroleum? 4. What do mud and silt layers produce in the course of time? 5. Under what conditions are oil and gas squeezed out? 6. Where do rock fluids start to migrate? 7. Why is migration halted sometimes? 8. How does and oil accumulation form? 9. What is the “sealing formation”? 10. How must the seal be shaped?