Unit 8 origin, migration and accumulation of oil

Words to be remembered:

 

origin - происхождение         

migration – движение нефти или газа через породы, миграция   

accumulation – залежь, скопление             

decomposition – разложение, разрушение                  

marine - морской  

mud - грязь                                              

silt - ил  

constituent – составная часть                

shallow - мелкий                               

to sink - осаждаться   

rapid decay – быстрое разложение 

to take place – происходить, случаться

oxygen content - содержание кислорода

to deposit – откладываться, отлагаться

deposit – залежь      

source beds = source rocks – нефтематеринская порода   

to transform – преобразовывать 

associated water – попутная вода (добываемая вместе с нефтью) 

to squeeze out - выдавливать  

to seep - просачиваться           

adjoining – смежный, соседний, расположенный рядом  

layer = bed = formation – пласт, слой         

permeable - проницаемый 

pore - пора                                

fracture = crack – разлом, трещина  

evidence – признак, доказательство                       

impervious - непроницаемый   

trap – улавливать (о жидкости); ловушка           

capacity – вместимость 

interstices - пустоты         

reservoir rock – коллекторская порода 

seal = cap rock – покрывающая порода         

 

The problem of the origin of crude oil in nature is important for oil exploration.

Many theories exist about 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 layers of mud and silt perhaps as much as 400-500 million years ago.  

 An inorganic source of petroleum is also advocated by some scientists. It remains an alternative 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. 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 source beds produce pressures and higher temperatures in these beds. At the depth of several thousand feet chemical processes transform the soft parts of the organ­isms into oil and gas. There are indications that gas is 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, if adjoining forma­tions are 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.

Sometimes migration is halted, for instance by a layer impervious to the 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 usually referred to as 'seal' or 'cap rock'. 

Questions on the text:

 

  1) What two theories are most widely accepted by scientists? 2) What did oil originate from according to organic theory? 3) What does the inorganic theory of petroleum state? 4) What is required for the genesis of petroleum? 5) What do mud and silt layers produce in the course of time? 6) Under what conditions are oil and gas squeezed out? 7) Where do rock fluids start to migrate? 8) Why is migration halted sometimes? 9) How does an oil accumulation form? 10) What is the 'seal' or 'cap rock'?