Hydrocarbons usually occur in specific types of underground rocks. All oil and gas fields have four general features known to geologists as sources, structure, reservoirs, and seals.
The sources are sedimentary rocks, principally shales. These rocks contain carbon from dead plants and animals which were deposited when the rocks were formed from sediments millions of years ago; for example, fine clay, rich in plankton, built up over eons at the bottom of an ancient sea. The organic remains gradually rot, and are buried and compressed by new sediments.
The underground heat and pressure, if conditions are right, change the carbon from the rocks into hydrocarbons. Oil tends to form in rocks which were deposited in the basins of ancient seas, with the carbon largely coming from marine plants and animals.
Gas, on the other hand, frequently comes from deposits which contain carbon largely from land plants such as trees and woody debris. Either way, suitable source rocks have to be present for hydrocarbons to form.
When hydrocarbons form, they gradually seep from the source rock and travel along the path of least resistance, usually up, to areas of lower pressure. Gas and oil are lighter than water, and tend to collect in relatively high pockets.
Since the rock layers hydrocarbons encounter on their way vary in shape and composition, and lie at odd angles to one another, the gas and oil often follows a winding migration path. Often the movement is through porous rocks such as sandstone formed from sand deposits, or through faults & other breaks in the rock.
Usually, one or more broad rock trends or traps help funnel the hydrocarbons into small areas. Common traps include salt domes (dome-like structures formed by the flowing of salt deposits under pressure), ancient reefs, and areas where folding and faulting of the rock layers create suitable pockets.
Petroleum geologists hope to find underground traps that contain hydrocarbons; to do this, they look at the types and combinations of rock layers there. Below is a very simple example of such a structure.
It is one thing for a potential trap to exist; it is another to be able to get at hydrocarbons which may be there. Coarse sandy sediments form porous rocks, such as sandstone, which act as reservoirs for underground hydrocarbons.
Because they are so porous, reservoir rocks allow hydrocarbons both to collect inside them, and to pass through so that the gas or oil can be extracted. Geologists and geophysicists can determine from seismic studies and exploratory drilling whether suitable reservoir rocks may be present.
Finally, there must be a seal a layer of rock above the reservoir rocks to prevent the hydrocarbons from moving further upward. Common seals include shales, carbonate rocks such as limestone, and evaporites (rocks such as salt and anhydrite, the source rock for gypsum).
Offshore hydrocarbons which do not end up trapped within rock formations continue moving up, and eventually vent in the ocean.
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