Page 20 - The Petroleum System From Source to Trap

12 Magoon and Dow

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400 300 200 1 0 0
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Rock .c. (I) �
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Unit - .... Q)
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(I) 0 Q)
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Thick
Fm. 1

2

Placer Fm.
George Sh.
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. .
Boar Ss. ::·.:�·>
Top gas window " _ _ - - - - - - - - - . . ...... ··.·.
\ Time oil generation Deer Sh. 3
- -
- - - - - - Elk Fm.
Critical Moment

1
Figure . 2. Burial history chart showing the critical moment (250 Ma) and the time of oil generation (260-240 Ma) for the ficti­
tious Deer-Boar(.) petroleum system. This information is used on the events chart (Figure . 5). Neogene (N) includes the
1
Quaternary here. All rock unit names used here are fictitious. Location used for burial history chart is shown on Figures . 3
1
1
and . 4. (Time scale from Palmer, 1983.)
chart. These two processes are followed by the preserva­ of certainty indicates the confidence for which a partic­
tion time, which takes place after the generation­ ular pod of active source rock has generated the hydro­
migration-accumulation of hydrocarbons occur, and is carbons in an accumulation. In a known petroleum
the time when hydrocarbons within that petroleum system, a good geochemical match exists between the
system are preserved, modified, or destroyed. When the active source rock and the oil or gas accumulations. In a
generation-migration-accumulation of the petroleum hypothetical petroleum system, geochemical information
system extends to the present day, there is no preserva­ identifies a source rock, but no geochemical match exists
tion time, and it would be expected that most of the between the source rock and the petroleum accumula­
petroleum is preserved and that comparatively little has tion. In a speculative petroleum system, the existence of
been biodegraded or destroyed. The last event is the either a source rock or petroleum is postulated entirely
critical moment as determined by the investigator from on the basis of geologic or geophysical evidence. At the
the burial history chart, and it shows the time repre­ end of the system's name, the level of certainty is
sented on the map and cross section. indicated by (!) for known, (.) for hypothetical, and (?) for
Table 1 . 3 shows all the discovered accumulations speculative (Table 1.4).
included in the petroleum system, provides a basis for
mass balance equations, and is a basis for ranking a Petroleum System Name
system.
The name of the petroleum system includes the source
Level of Certainty rock, followed by the name of the major reservoir rock,
and then the symbol expressing the level of certainty. For
A petroleum system can be identified at three levels of example, the Deer-Boar(.) is a hypothetical petroleum
certainty: known, hypothetical, or specutive. The level system consisting of the Devonian Deer Shale as the oil