Page 15 - The Petroleum System From Source to Trap

1. T11e Petroleum System 7

petroleum system. After the system is identified, the rest in any aspect, e.g., thickness. Regardless of whether this
of the investigation is devoted to determining the strati­ sandstone is penetrated when the prospect is drilled, the
graphic, geographic, and temporal extent of the existence of this eolian sandstone outside the play area is
petroleum system. The bigger the petroleum system, the still valid. In the same way, already discovered oil and
more likely it will have generated and accumulated gas fields as well as other noncommercial quantities of
commercial quantities of hydrocarbons. As indicated hydrocarbons that are genetically related can be mapped
earlier, the petroleum system defines a level of investiga­ as a petroleum system, which can then be projected into
tion that usually lies between that of a sedimentary basin the play area as hydrocarbon charge. This hydrocarbon
and a play. charge can then be evaluated with respect to the play or
The term oil system was first introduced by Dow (197 4) prospect.
and is based on the concept of oil-source rock correla­
tion. The term petroleum system was first used by
Perrodon (1980). Independently, Demaison (1984) PETROLEUM SYSTEM HISTORY
devised the generative basin, Meissner et al. (1984)
described their hydrocarbon machine, and Ulmishek (1986)
identified an independent petroliferous system. All of these Dow's Oil System
concepts are similar to Dow's oil system. Expanding
upon previous work, Magoon (1987, 1988, 1989a,b) The concept of an oil system was presented in 1972 at
attempted to formalize criteria for identifying, naming, the AAPG annual meeting in Denver (Dow, 1972) and
and determing the level of certainty for the petroleum was later published (Dow, 1974). The oil system, as Dow
system. This volume further refines the petroleum (1974, p. 1254) presented it, was based on oil-oil and
system concept and shows how the system is mapped oil-source rock correlations
and used to evaluate exploration opportunities (see later
sections). to develop an understanding of the distribution of the three
major oil types in the Williston basin ... , and where each type is
most likely to be found in the future. The focus of the paper is
on geology and interpretation of geochemical data, not on the
Play and Prospect Investigations presentation of new geochemical data.

Beyond sedimentary basin and petroleum system Dow (1974, p. 1254-1255) goes on to state that because
analysis, the remaining levels of investigation are play the source rocks are isolated by evaporites,
and prospect analysis. Prospects were first used by explo­
ration geologists to describe present-day structural or The distribution of oil in the basin therefore can be described in
stratigraphic features that could be mapped and drilled. terms of three major source-reservoir oil systems. Each system
A series of related prospects is a play. As information contains a source rock and a group of reservoir rocks and is
about petroleum geochemistry increased, the definition isolated from other oil systems by an evaporite seal.
of a play became broader. For example, Bois (1975)
defined a petroleum zone, which he considered to be He then names the oil systems. In Dow's (1974, p. 1261)
similar to a play (Bois et al., 1982), to include hydro­ summary section he states,
carbon mixtures of similar composition. More rigorous The model developed in the Williston basin depends on the
definitions of a play and a prospect have included a ability to (1) separate oils into genetic types, (2) relate each type
source rock as well as a migration path (White, 1980, to a specific source sequence, (3) understand the quantity of
1988; Bishop et al., 1983; Sluijk and Nederlof, 1984; organic matter and the degree of thermal maturation required
Dolton et al., 1987; Bird, 1988). The use of quantitative for generation and expulsion of oil in commercial quantities,
petroleum geochemistry (Mackenzie and Quigley, 1988) and (4) map the distribution of both vertical and horizontal
with play and prospect evaluation provides important migration pathways and seals. The most likely distribution of
volumetric information for economic analysis. each oil type in the subsurface can be mapped with the
Plays and prospects are defined more traditionally in foregoing approach. Plays then can be made in these high-grade
this volume, that is, to include present-day exploration areas where the chance of finding oil is greatest.
potential for undiscovered commercial oil and gas accu­
mulations (Table 1.1). The play is one or more prospects, Dow's (1974) paper is important for the following
and a prospect is a potential trap that must be evaluated reasons: (1) oil-source rock correlation was the keystone
to see if it contains commercial quantities of hydrocar­ to identifying the system; (2) the name included the
bons. The presence of reservoir rock, seal rock, trap source and reservoir rock separated by a hyphen; (3) the
volume, hydrocarbon charge, and timing are usually term play was used as a distinct concept; (4) in each oil
involved in this evaluation. For example, if the reservoir system description, a mass balance comparison was
rock in the play is eolian sandstone, then the distribution carried out on the theoretical amount of oil generated
and quality of this sandstone is mapped from outcrop and reserves (the calculations were left out of the paper);
and well control so that it can be projected into the play (5) the use of the term oil system excluded gas and
area using seismic information. The probability that this condensate; and (6) the criteria for applying this concept
eolian sandstone occurs in the play area can be evaluated beyond the Williston basin was only implied, not stated.