Petroleum System

The concept of the petroleum system (Magoon, 1987, 1988) provides geoscientists with a unifying model to

explain the distribution of hydrocarbons in the shallow subsurface (Earth’s crust). In simplest terms, the concept

emphasizes the genetic relationship between a generative petroleum source rock and the resulting entrapped

accumulations.

Basic elements of a petroleum system include the following:

> Source rock

> Migration path

> Reservoir rock

> Seal (top, bottom, lateral)

Cross-section of a generic petroleum system, including source and reservoir rocks, traps, and migration pathways.

(modified from Magoon, 1988)

Plan view of the petroleum system shown above; line of cross-section shown in purple.

(redrawn from Magoon, 1988)

In addition to these four basic components, a petroleum system by definition includes all the geologic processes

required to create these elements. Crucial factors of proven (i.e., economic) petroleum systems include:

> Organic richness/type and volume of generative source rock

> Adequate burial history to ensure proper time-temperature conditions for source rock maturation

> Timing of maturation and expulsion in relation to timing of trap formation

> Presence of migration pathway linking source and reservoir rocks

> Preservation of trapping conditions from time of entrapment to present day

> Relative efficiency of sealing layers

 

Not all petroleum systems are created equal. Certain petroleum systems such as those found on the

Arabian basin, the North Sea, and the Gulf of Mexico region among others, are considered “prolific”

petroleum systems based on the volumes of entrapped hydrocarbons present. Prolific petroleum

systems by definition contain numerous individual giant and supergiant oil and/or gas fields (> 500

million barrels of oil equivalent).

Petroleum systems may be identified according to three levels of certainty: known, hypothetical, and

speculative (Magoon, 1988). In a known system, a good geochemical match exists between the source

rock and accumulations; in the hypothetical case, a geochemical match is lacking but geochemical

evidence is sufficient to identify the source rock. In the case of a speculative petroleum system, the

presence of economic accumulations are lacking, but the existence of source rocks and oil/gas

accumulations are postulated on the basis of geologic or geophysical evidence.

In addition, petroleum systems may be classified according to type (I, II, III) and lithologic composition

(siliciclastic, carbonate) of source rock. Type I refers to relatively rare source rocks and oil shales with

disseminated kerogen that is rich in marine aliphatic structures (e.g., algae), containing a high initial ratio

of atomic hydrogen to carbon (>1.5) and a low ratio of oxygen to carbon (<0.1; Tissot and Welte, 1978).

Type II source rocks are more common marine shales derived from a mixture of phytoplankton, zooplankton,

and microorganisms (bacteria) that are deposited in reducing environments, containing relatively high H/C
and low O/C ratios. Type III refers to kerogen derived primarily from terrestrial higher plants, containing

elatively low initial H/C and high O/C ratios. Type III source rocks include coals and are generally gas-prone

(rare oil source rocks).

Maggon (1988) also notes the additional classification of “purebread” (if the petroleum system was

deposited in a geologic setting in which the structural framework did not change significantly during the

geologic life of the system) or “hybrid” (in which the petroleum system would not have formed if not for a

major structural reorganization during the formation of the system). I offer the suggestion that it also helpful

to note the number of second-order depositional sequences that contribute to the various components of a

given petroleum system. A petroleum system may be considered as “self-contained” if the source-reservoir-seal

triplet occurs within a single second-order supersequence or supersequence set (Sloss subsequence).

Purebred and self-contained petroleum systems are more likely to be prolific because of close juxtaposition

of major elements and simpler stratal geometries and migratory pathways.

Magoon (1988) presents a listing of petroleum systems of the United States and the United States Geologic

Survey (USGS, 1996; 2000) has recently assessed the remaining potential of petroleum systems worldwide.

References

Magoon, L.B., 1987, The petroleum system—A classification scheme for research, resource assessment,

and exploration (abstract); Amer. Assoc. Petrol. Geol. Bulletin, v. 71, p. 587.

Magoon, L.B., 1988, The petroleum system—A classification scheme for research, resource assessment,

and exploration, in Magoon, L.B. (ed.), Petroleum Systems of the United States; USGS Bulletin 1870,

p. 2-15.

Tissot, B.P. and D.H. Welte, Petroleum Formation and Occurrence; Berlin, Springer-Verlag, p. 142-146.

USGS, 1996, 1995 National assessment of United States oil and gas resources; USGS Digital Data

Series DDS-35 and DDS-36 (CD-ROM).

USGS, 2000, 1995 USGS World petroleum assessment 2000 – Description and results; USGS Digital

Data Series DDS-60 (CD-ROM).