Page 29 - The Petroleum System From Source to Trap

1. The Petroleum System 21

This example shows the use and limitations of oil-oil the present day, the source rock would presently be at
and oil-source rock correlations. First, if two oils are maximum burial depth; see Magoon, Chapter 22; Cole
identical, they may not necessarily be in the same and Drozd, Chapter 33, this volume.) Various articles
petroleum system even though the oil-source rock corre­ and computer programs exist to model petroleum gener­
lations indicate that they are from the same source rock. ation and migration; most use the burial history curve or
Second, if two oils are different, they can still be from the geohistory chart (Waples, Chapter 17, this volume). The
same source rock. For example, if the organic facies essential elements of the system should be shown on the
changes within a pod of active source rock, the oils may burial history chart.
be from the same petroleum system. Finally, to identify a Next, a map and cross section are drawn for the
petroleum system uniquely, the extent of hydrocarbon critical moment (Figures 1 . 3 and 1.4), and all accumula­
shows must be mapped relative to the pod of active tions are itemized (Table 1 . 3). The critical moment is
source rock. This example also shows why the distribu­ important because the geometry of the migration paths
tion as well as the quality, quantity, and thermal and traps are reconstructed for about the time the oil and
maturity of source rock should be mapped worldwide at gas accumulated. If the critical moment is prior to
the time of deposition (Klemme, Chapter 3, this volume). present-day, then the location of present-day traps on a
play or prospect map (not shown) can be compared to
Fictitious Example #2 the location of traps at the critical moment to determin if
oil and gas have remigrated. If the traps have shifted
The previous example identified and mapped the from the critical moment to the present day, the shifted
geographic and stratigraphic extent of seven petroleum trap or prospect would have to be charged with remi­
systems. The next example investigates and describes grated oil or gas. These maps can also be compared to
one petroleum system (see Peters and Cassa, Chapter 5, determine if physical or microbial alteration (or destruc­
this volume). tion) occurred during the preservation time. A table of
To demonstrate how the four figures and one table accumulations for this petroleum system indicates its
work together to characterize a petroleum system, we size and is the basis for further calculations and compar­
illustrate a petroleum system study with a specific isons carried out in the case studies.
example. Figures 1.2-1.5 and Table 1.3 depict a fictitious Last, a petroleum system events chart is constructed
petroleum system, the scrcalled Deer-Boar(.) petroleum to summarie the essential elements, processes, preserva­
system. The oil accumulations and shows prove the tion time, and critical moment (Figure 1 . 5). In our ficti­
existence of at least one system, but if there is more than tious example, the description is as follows. The Deer
one group of oils, there could be two or more systems Shale, a Devonian (390-380 Ma) source rock is buried by
present. To identify and name each system, oil and Devonian-Perrni (380-250 Ma) rocks to its maximum
source rock samples were collected and analyzed for the depth in the Late Permian (250 Ma). The process of
following reasons: (1) to establish oil families, (2) to generation-migration-accumulation of hydrocarbons
determine which family originated from which source occur during the Permian (260-240 Ma), and the critical
rock, (3) to map the quantity and type of organic matter moment is 250 Ma. These hydrocarbons accumulated
in the source rock, and (4) to map the thermal maturity of under the George Shale (300-286 Ma) and in the Boar
the source rock. On a map, we have shown the present­ Sandstone, reservoirs of Pennsylvanian age (315-300 Ma)
day location of each oil accumulation attributed to each that formed into traps during the Late
group (Table 1 . 3) and have indicated the pod of active Pennsylvanian-Early Permian (290-270 Ma). The preser­
source rock. Much of this information is included in vation time is 240 m.y. (Figures . 3 and 1.4).
1
Figures 1.3 and 1.4. The events chart can be viewed as a team organiza­
In our fictitious example, sufficient information was tional tool. For example, geologic time is studied by the
collected to identify and map the thermal maturity of the paleontologist and stratigrapher, the reservoir by the
Deer Shale as the most likely source for the oil accumula­ petrophysicist and stratigrapher, and trap formation by
tions in the Boar Sandstone (Peters and Cassa, Chapter 5, the structural geologist and geophysicist.
Figure 5.12, this volume). However, because of a lack of These four figures and table are simplified to make
thermally mature samples of the Deer Formation, an important points about a single petroleum system Each
oil-source rock correlation was inconclusive (see Lewan, figure could be drawn to include additional information
Chapter 11, this volume, for solution). Therefore, our unique to a particular petroleum system. Once a
level of certainty is hypothetical because we are unable to petroleum system is named, mapped, and described, it
demonstrate geochemically that the oil originated from can be analyzed in many ways. For example, this volume
the Deer Shale. However, geographic and stratigraphic contains case studies that describe petroleum migration
evidence is sufficient to assign a name and level of from the pod of active source rock to a trap.
certainty-the Deer-Boar(.) petroleum system.
At the location of the most thermally mature source
rock, a burial history chart has to be made to determine SUMMARY
the critical moment (Figure 1 . 2). The critical moment, in
this case, is when the source rock is at maximum burial Sedimentary basins, petroleum systems, plays, and
depth and is near the time when most hydrocarbons prospects can be viewed as separate levels of petroleum
migrated into primary traps. (If the critical moment were investigations, all of which are needed to better under-