Petroleum: Formation and Occurrence of Petroleum!
Petroleum is a natural or mineral oil, which mainly occurs in well-defined areas. Now it has been proved that oil is formed by slow chemical and biochemical decomposition of the remains of organic matter entombed in sedimentary rocks. These rocks are marine or estuarine in origin.
This means that these organic remains, whether seaweeds, marine animal elements, or plant life such as diatoms, are entombed in the bottom sediments of shallow seas or estuaries under stagnant bottom, conditions which prevent complete decomposition before burial.
Subsequently, this organic material is transformed, perhaps by bacterial action and by age-long chemical change, into small globules of oil and gas. In course of geological time, as the original muds are being compacted into snales or marls, these globules, together with a much greater quantity of original water, are squeezed out and are lodged in any convenient porous layer, such as sand or porous limestone.
In this way a water-bearing porous layer may carry globules of oil and bubbles of gas. The next phase is the concentration of this oil into oilfields. This concentration is primarily brought about by the force of gravity.
Oil being lighter than water floats, and so the globules trickle up to the top of a given layer of sand, say 50 feet in thickness, until their further upward progress is checked by an impervious cover of rock, such as shale. If this rock layer were itself horizontal, further movement of the oil would then stop.
But this is seldom the case. In course of geological time, the layers of rock become tilted by processes of mountain building, and in front of the major mountain ranges of the world the stratified rocks are thrown into a series of folds or anticlines. These anticlines become oil-traps for oil and gas which float up within a porous layer from the adjacent troughs or synclines.
The oil thus concentrated along the summits of such an anticline is actually floating on water within its porous rock. If gas is present in greater Quantity than can be kept in solution in the oil, the excess gas fills the highest part of the anticline, forming a ‘gascap’. The most usual type of oil accumulation is in such anticlines.
Other types of oilfields are (i) where the oil is concentrated in the higher edge of a tilted sandstone wedge. Such an occurrence is called a stratigraphic trap or shoreline oilfield, (ii) where an intrusive body of salt gives a domed structure to the adjacent strata, (iii) where the sealing of a sandstone layer is caused by a fault.
The geologists have found that marine sedimentary deposits of Mesozoic and Tertiary times are highly favourable for the occurrence of petroleum. The search for oil has thus been, though not always, correlative with the search for these deposits. Workable deposits of petroleum are confined entirely to sedimentary rocks which have suitable structures.
Figure 9.4 illustrates the formation and occurrence of oil. Figure 9.4(A) shows how the warping and folding of rocks, connected with mountains building create anticlines, synclines and faults which trap oil-forming organic material. Figure 9.4(B) shows how oil and gas accumulate in the dome of the anticline where they are tapped by wells.
The oil in the rocks is tapped by sinking bore-holes. Oil usually spouts under the pressure of age or water underlying it. If spouting does not occur or ceases, pumping is resorted to until sufficient supplies of oil are given. When pumping has exhausted the well it is abandoned.
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