15 - 2 Petroleum Accumulation by Kamran, Abdul Rehman, Bilal Aslam, Bilal Saleem

8/6/2019 15 - 2 Petroleum Accumulation by Kamran, Abdul Rehman, Bilal Aslam, Bilal Saleem

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y The hydrocarbons stored in rocks due to any reason iscalled as petroleum accumulation.


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Its Propertiesy Ifpetroleum is to be produced from the subsurface it must

accumulate in rocks that are porous and permeableenough for reasonable volumes ofpetroleum to beproduced at reasonable cost.

y Most reservoir rocks have permeabilities greater than 1millidarcy; some have permeabilities ofmany darcies (1darcy is 1012 m2).

y Most reservoir rocks are sediments such as sandstones,

conglomerates, and limestones with porosities typicallyranging from 10 to 30 per cent, the petroleum beingcontained in the pores ofthe sediments.

y Petroleum can also accumulate in fractures in limestones,or even within the source rocks themselves.


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Cont .y The petroleum typicallyfills 5090 per cent ofthe pore

space, the remainder ofthe pore space containingformation water trapped when the petroleum entered

the field.y In some accumulations the reservoir can be filled by

gas or both gas and oil may occupy the pore volume.

y Oil reservoirs can occur at any depth within the crust

but liquid oil accumulations are rarelyfound deeperthan 5 km, for the temperatures at greater depths maybe so high (more than 160 C) that any oil in thereservoir would be cracked to gas.


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y Petroleum accumulations are ephemeral features; thepetroleum often leaks through the seals with thepassage of time.

y It is thought that most ofthe petroleum generatedduring the history ofthe Earth has leaked away andhas been destroyed at the surface by bacterial actionand evaporation.

y Most of the world's existing large petroleumaccumulations were generated and migrated within

the past few tens ofmillions ofyears, and fewpetroleum accumulations have been found that havecontained petroleum for more than 100 million years.


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y In order for petroleum to accumulate in a trap, several

criteria must be satisfied. The crude oil and natural gasis trapped in a porous and permeable rock body: areservoir rock encased above, and sometimes below, byfine-grained rocks that are relatively impermeable to

crude oilthe seals.y Crude oil (and gas) is kept in the trap by the buoyancy

forces acting on it, which tend to move the oil to thesurface ofthe sedimentary basin but are prevented

from doing so by the seals. The trap must completelyseal an area ofreservoir rock; if it does not, nopetroleum accumulation will develop.


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y Most seals are fine-grained rocks such as shales ormudstones. The best seals are salt beds, which can

even trap gasfor long periods o

fgeological time.

y Lateral sealing is accomplished either by structuraldeformation ofthe reservoir rock and seal into aclosed anticline or by the reservoir rock changing

laterally into a seal rock such as a shale. Faulting of

thetrap might also place a shale next to a reservoir rockand thus seal the trap.


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y The porosity (volume ofpore spaces) andpermeability (capacityfor transmitting fluids) ofcarrier and reservoir beds are important factors in

the migration and accumulation of

oil. Mostpetroleum accumulations have been found inclastic reservoirs (sandstones and siltstones). Nextin number are the carbonate reservoirs (limestones

and dolomites). Accumulations of

petroleum alsooccur in shales and igneous and metamorphicrocks because ofporosity resulting fromfracturing, but such reservoirs are relatively rare.


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Times of Accumulationy The earliest time to judge when the oil and gas have

accumulated.

yIt depends upon the conditions on which migrationtook place. For this we study mainly two factors.

y 1). The Law ofGas.

y 2). Time Trap Formed.


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1). The Law of Gas.Ifthe temperature increase then

volume ofgas also increased andit is decreased when pressureincreased. But in the case ofpetroleum accumulation we canneglect the temperature effect .

As shown in the followingfigure.

We need minimum 143 atmpressure to inter the gas in trap.The capacity ofa trap to hold thegas is actually the function ofpressure.

y Figure 1:


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y The all gas which is accumulated is due to the pressurewhich is already developed because ofdepth and strata

over the trap. Ifthis pressure is not developed then theaccumulation was also impossible. The following figure(2)also shows that the over lying strata can develop thepressure if this strata removed then gas will escaped.


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Figure 2


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This diagram is showing the selective

trapping of oil and gas.y Stage 1: The oil and gas are above

the spill point, and both willcontinue to be trapped until thefree water is all displaced and oil

water contact (OWC) reachesthe spill point.

y Stage 2: Gas continues to betrapped, until gas oil contact(GOC) reaches the spill point.

while oil spills out and goes upon the dip.

y Stage 3: Gas has filled the trapand spills out up the dip. Oil bypasses the trap and goes up thedip.

Figure 3:


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This diagram is showing the series of traps.y At A trap 1 is in the state of

stage 2 (Fig. 3): oil is beingspilled out into trap 2. Traps 3and 4 are full ofSault water.

y In B, trap1 has spilled all its oilinto trap 2. Trap 2 is full ofoil,and the excess is passing up thedip into trap 3. Trap 1 is full ofgas.

y In C, trap 1 is unchanged, trap 2has a gas cap (the excess gascame from trap 1), trap 3 is fullofoil and trap 4 is still full of

water but will be the next to befilled with oil spilling out oftrap.

y Figure 4:


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2). Time Trap Formed.y The accumulation can not be occurred before the trap

isformed, if it is like that then we can find out the time ofaccumulation

yTrap can be formed in one geological episode or more thenone episode.

y This may be possible that as the reservoir rocks made,instantly the deposition occur and the trap formed or itmay also possible that more time required to form the trap.

y

Or it may also possible that whole of

the reservoir rockslife, just modification processes occur and no trap formed,so it means that there will no trap formed and till the trapnot formed, no first accumulation of the oil will occur.


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Trap 1 is the oldest andformed at a time whenthe shale deposit at the

edge ofsandstone bed.Trap 2 formed after faulting.

Trap 3 formed after theerosion and shaledeposited.

Trap 4 is the lens trap andmade in the sandstone.

Trap 5, 6 and 7 made afterfolding subsequently.

This diagrammatic section illustratingthe method of determining therelative time when a trap was formed.

Stratigraphic time planes are labeledfrom a to e.

Figure 5:


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15 - 2 Petroleum Accumulation by Kamran, Abdul Rehman, Bilal Aslam, Bilal Saleem