Chapter 17, Problem 1TC

To determine

The formation of oil and natural gas, its extraction from the seabed, and the reason why the physical characteristics of the surrounding rock are important.

Answer to Problem 1TC

When the bodies of planktons and masses of bacteria get accumulated and buried within the anoxic environment of certain basins, the anaerobic bacteria convert their tissues into relatively insoluble organic components. At a higher depth below the ocean floor surface, the hydrocarbons get altered further under the effect of high temperature and pressure. This altered material undergoes gradual cooking for about millions of years under thick sediment, and oil is gradually formed. Oil being less dense than their surrounding materials moves from the source rock to the surface through the porous overlying layers and then get accumulated in the reservoir rocks that hinder the further movement of the oil. Geologists take the help of the reflected sound waves from the subsurface structures to find the signature combination of depth, layered sediments, and reservoir structure before proceeding to drilling.

Explanation of Solution

Petroleum is virtually always related to marine sediments, signifying that the organic materials from which it developed were once marine. Planktonic creatures and masses of bacteria would be the most probable candidates. Their bodies were apparently collected in quiet basins with low supply of oxygen, and there were some bottom scavengers.

The anaerobic bacteria would convert the original tissues into simpler, comparatively insoluble organic compounds that were perhaps buried, probably initially by turbidity currents, then later by the constant dropping of sediments from the ocean above. The further alteration of the hydrocarbons by high pressures and temperatures should have occurred at substantial depth, possibly 2 kilometers or more below the surface of the ocean floor. Gradual cooking under this thick sedimentary layer for over millions of years marked the completion of the chemical changes that generated oil.

Compared to the surrounding sediments, oil is less dense. Therefore, it is able to migrate by means of the porous superimposing formations toward the surface from its source rock. It may be collected within the pore spaces of reservoir rocks as an impervious overlying layer that impedes further migration of the oil upward. During the probing for oil, the sound reflected from the subsurface structures is used by the geologists for searching the characteristic combination of layered sediments, depth, and reservoir structure prior to their drilling.