Petroleum is the name for all hydrocarbon related, naturally occurring materials, including oil, natural gas, and tar.  It is made up of hydrocarbon molecules (hydrogen combined with carbon).    Petroleum supplies almost half of our total energy requirements.  

Sedimentary rocks are the dominant type of host rock for oil deposits.  Oil deposits are generally formed in younger rocks, especially those which have not undergone metamorphic processes.  Heating is required to form oil, but if too much occurred (such as during a metamorphic event), the petroleum would get driven off.

Most petroleum resources are contained deep underground, therefore exploration relies heavily on drilling methods.   In addition to drilling, there are a variety of geophysical methods, including seismic, gravity, and magnetic.  Petroleum exploration utilizes several branches of geology, including paleontology, stratigraphy, sedimentation, and structure.  These are needed to obtain detailed observations of the drill core samples.  Details such as these make it possible to make stratigraphic correlations of rock units between different drill holes.  These rock units can then be traced in the subsurface and their geometrical distribution can be understood. 

Oil Formation

Accumulation of organic material capable of forming petroleum can occur in a number of different ways.  Ideal environments are those which accumulate abundant organic material in non-oxidizing conditions.  This can occur near the edge of larger sedimentary basins, behind barrier reefs, or in lagoon areas, where water circulation is more restricted.  This is possible in smaller water body seas that may be connected to a larger ocean, or it may be isolated.  If it is isolated, there is less circulation, and hence less oxygen.  This situation is good for forming oil, because less of the organic material gets oxidized.  The type of organic material which is deposited determines the outcome of the petroleum.  Land derived organic material tends to produce more natural gas than kerogen.  Marine derived organic material tends to produce more kerogen than natural gas. 

There are four requirements necessary to form an oil deposit:  1) there must be a source rock, 2) there must be a heating event, 3) there must be a reservoir rock, and 4) there must be a trapping mechanism.  The source rock must contain abundant organic matter.  The best source rocks are organic-rich shales, limestones and sandstones which contain 0.5 % to 5 % organic matter.  The organic material liquefies during the heating event, converting to hydrocarbons in the process.  The first hydrocarbons to form are called kerogen, which consists of carbon (69-80 %), Hydrogen (7-11 %), Nitrogen (1.25-2.5 %), Sulfur (1-8 %), and Oxygen (9-17 %).  Kerogen becomes crude oil when the oxygen and nitrogen are removed in some manner.

The fluid that forms initially is a mixture of oil and water, but these separate during the changes over time of the oil deposit.  Oil does not mix with water, is less dense than water, and seeks a higher position in the rocks.  This happens by a slow migration process where the oil moves through spaces of the rocks.  Gas also separates out of the oil.  Since the gas is a vapor with the lowest density, it seeks a highest possible position (Figure F19).

Figure 19:   Cross section showing separation of oil, gas and water due to immiscibility and density differences.

Rock Porosity

Porosity is the void space in the source rock.  It effects the quality of both the source rock and the reservoir rock.  This is the most important factor affecting the quality of a petroleum reservoir because it determines the maximum volume which is available for oil accumulation (Figure F20).  A rock with high porosity has a lot of open spaces.  Examples of good source rocks with high porosity include coarse-grained sandstone or limestone.  If the spaces are well connected (ie, lacking cement), then the oil can migrate.  The ability of the rocks to transmit a fluid, or the linear flow rate, is called “permeability”.  Good reservoir rocks have both good porosity and good permeability

Porosity can also be formed later, and is called “secondary” porosity.   One type of secondary porosity is that which develops by bleaching out (by groundwater) of the cements between the grains, creating more open void spaces.  Another type of secondary porosity is the result of stresses, which squeeze and shatter the rocks at depth.  This causes  fractures to develop, which also creates more voids.  The porosity is reported as a percentage of the total volume.  Porosity is measured by dividing the pore space volume by the total volume, and then multiplying by 100.   A good reservoir rock usually contains 15 to 25 % porosity. 

Oil Maturity

Oil can form in only a restricted temperature range (approximately 150 to 350 degrees F).  When a formation is heated to this temperature range, it is said to be “mature”.  In a normal gradient downward, the depth (pressure) where this condition is achieved corresponds to approximately 8,000 – 18,000 feet.  Immature sediments are those which have not been heated to this temperature range.

Oil Traps 

Traps are situations where a non-permeable boundary causes oil to pool.  The most important factors affecting the formation of a trap are the lithology, the geometry and structural setting of the rocks.  With regards to lithology, the grain size and porosity, of the rock are critical.  Trap rocks, or “cap” rocks, are those which form an impermeable layer which stops oil migration.  Examples of good trap rocks include shale and salt, because of their low porosity and fine grain size.  The trap rock must be situated above the reservoir rock, to stop the upward migration of the oil. 

Oil traps form in many different ways.  The simplest example is where layered sedimentary rocks (including a caprock overlying a source rock) are folded into an anticline (Figure F21A).  Anticlines were first conceived as the best place to look for oil by geologists about 150 years ago.  The arched up area under the caprock acts like a concave, or dome-shaped surface which traps the oil beneath it.  Other situations can also form arched sedimentary layers, such as the draping of sediments over a high point (Figure F21B), and the arching of beds caused by upward movement of a salt dome  (Figure F21C).   Faults can also cause a caprock to arrive in a position overlying a source rock (Figures F22).  Many of the oil deposits at Prudhoe Bay are formed in traps caused by unconformities, which are erosional surfaces in the rocks which formed when the rocks were uplifted and subsequently buried again (Figure F23).

Figure 21:   Anticlinal oil traps formed by A) tectonic uplift, B) sediment draping and C) intrusion of a salt plug.

Figure 22:   Structural oil trap caused by faulting.

Figure 23:   Prudhoe Bay Oil Fields developed below regional unconformities.