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Defining a
Petroleum  System
Figure/Table Captions
 Figure 1. Example of a name of a petroleum
system.
 Figure 2. Criteria for selecting a reservoir rock
name.
 Figure 3. Burial history chart
showing the critical moment (250 Ma) and the time of oil generation
(260-240 Ma) for the fictitious Deer-Boar(.) petroleum system. All rock
unit names used here are fictitious. From Magoon and Dow (1994).
 Figure 4. Map showing the
geographic extent of the fictitious Deer-Boar(.) petroleum system at the
critical moment (250 Ma). Thermally immature source rock is outside the
oil window. The pod of active source rock lies within the oil and gas
windows. From Magoon and Dow (1994).
 Figure 5. Geologic cross-section
showing the stratigraphic extent of the fictitious Deer-Boar(.)
petroleum system at the critical moment (250 Ma). Thermally immature
source rock lies updip of the oil window. The pod of active source rock
is downdip of the oil window. From Magoon and Dow (1994).
 Figure 6. The events chart showing
the relationship between the essential elements and processes as well as
the preservation time and critical moment for the fictitious
Deer-Boar(.) petroleum system. From Magoon and Dow (1994).
 Table 1. Oil and Gas Fields in the
Fictitious Deer-Boar(.) Petroleum System, or the Accumulations Related
to One Pod of Active Source Rock. From Magoon and Dow (1994).
The Petroleum System Concept
The petroleum system is a unifying concept that
encompasses all of the disparate elements and processes of petroleum
geology. Practical application of petroleum systems can be used in
exploration, resource evaluation, and research. This article discusses
its application to petroleum exploration.
A petroleum system encompasses a pod of active
source rock and all genetically related oil and gas accumulations. It
includes all the geologic elements and processes that are essential if
an oil and gas accumulation is to exist.
Petroleum
describes a compound that includes high
concentrations of any of the following substances:
·
Thermal and biological
hydrocarbon gas found in conventional reservoirs as well as in gas
hydrates, tight reservoirs, fractured shale, and coal
·
Condensates
·
Crude oils
·
Natural bitumen in reservoirs,
generally in siliciclastic and carbonate rocks
System
describes the interdependent elements and processes that form the
functional unit that creates hydrocarbon accumulations. The essential
elements of a petroleum system include the following:
·
Source rock
·
Reservoir rock
·
Seal rock
·
Overburden rock
Petroleum systems have two processes:
·
Trap formation
·
Generation-migration-accumulation of hydrocarbons
These essential elements and processes must be
correctly placed in time and space so that organic matter included in a
source rock can be converted into a petroleum accumulation. A petroleum
system exists wherever all these essential elements and processes are
known to occur or are thought to have a reasonable chance or probability
to occur.
A petroleum system investigation identifies,
names, determines the level of certainty, and maps the geographic,
stratigraphic, and temporal extent of a petroleum system. The
investigation includes certain components:
·
Petroleum-petroleum geochemical
correlation
·
Petroleum-source rock
geochemical correlation
·
Burial history chart
·
Petroleum system map
·
Petroleum system cross-section
·
Events chart
·
Table of hydrocarbon
accumulations
·
Determination of
generation-accumulation efficiency
Identifying a Petroleum System
Before a petroleum system can be investigated, it
must be identified as being present.
To identify a petroleum system, the
explorationist must find some petroleum. Any quantity of petroleum, no
matter how small, is proof of a petroleum system. An oil or gas seep, a
show of oil or gas in a well, or an oil or gas accumulation demonstrates
the presence of a petroleum system. The steps required to identify a
petroleum system are:
·
Find some indication of the presence of petroleum.
·
Determine the size of the petroleum system by the
following series of steps:
a.
Group genetically related occurrences of petroleum by using geochemical
characteristics and stratigraphic occurrences.
b.
Identify the source using petroleum-source rock
correlations.
c.
Locate the general area of the pod of active source rock
responsible for the genetically related petroleum occurrences.
d.
Make a table of accumulations to determine the amount of
hydrocarbons in the petroleum system and which reservoir rock contains
the most petroleum.
·
Name the petroleum system.
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Naming a Petroleum System
A unique designation or name is important to
identify a person, place, item, or idea. As geologists, we name rock
units, fossils, uplifts, and basins. The name for a specific petroleum
system separates it from other petroleum systems and other geologic
names. The name of a petroleum system contains several parts:
1.
The source rock in the pod of active source rock
2.
The name of the reservoir rock that contains the largest volume
of in-place petroleum
3.
The symbol expressing the level of certainty
An example of a petroleum system name and its
parts is shown in Figure 1. Figure 2 shows how a reservoir rock name is
selected.
A petroleum system can be identified at three
levels of certainty: known, hypothetical, and speculative. The level of
certainty indicates the confidence for which a particular pod of mature
source rock has generated the hydrocarbons in an accumulation. At the
end of the system's name, the level of certainty is indicated by (!) for
known, (.) for hypothetical, and (?) for speculative.
The following indicates how the level of
certainty is determined:
Known (!)--A positive oil-source rock or gas-source rock
correlation
Hypothetical (.)--In the absence of a positive petroleum-source
rock correlation, geochemical evidence
Speculative (?)--Geological or geophysical evidence
Geographic, Stratigraphic, and Temporal
Extent
Petroleum systems are limited by time and space.
Each system can be described in terms of its own unique temporal and
spatial elements and processes.
Temporal aspects
A petroleum system has three important temporal
aspects:
(1)
Age
(2)
Critical moment
(3)
Preservation time
The
age of a system is the time required for the process of
generation-migration-accumulation of hydrocarbons.
The
critical moment is the time that best depicts the
generation-migration-accumulation of hydrocarbons in a petroleum system.
A map and cross-section drawn at the critical moment best show the
geographic and stratigraphic extent of the system. The burial history
chart (Figure 3) shows the critical moment and the essential elements
for the fictitious Deer-Boar(.) petroleum system.
The preservation time of the petroleum
system begins immediately after the generation-migration-accumulation
process occurs and extends to the present day. It encompasses any
changes to the petroleum accumulations during this period. During the
preservation time, remigration, physical or biological degradation, or
complete destruction of the petroleum may take place. During the
preservation time, remigrated (tertiary migration) petroleum can
accumulate in reservoir rocks deposited after the petroleum system
formed. If insignificant tectonic activity occurs during the
preservation time, accumulations remain in their original position.
Remigration happens during the preservation time only if folding,
faulting, uplift, or erosion occurs. If all accumulations are destroyed
during preservation time, then the evidence that a petroleum system
existed is absent. An incomplete or just completed petroleum system
lacks a preservation time.
Spatial Aspects
Each petroleum system can be defined spatially by
its geographic and stratigraphic extent.
The geographic extent of a petroleum
system is determined at the critical moment. It is defined by a line
that circumscribes the pod of active source rock and all oil and gas
seeps, shows, and accumulations originating from that pod. Figure 4
shows the geographic extent of the fictitious Deer-Boar(.) petroleum
system.
The stratigraphic extent of a petroleum
system is the span of lithological units which encompasses the essential
elements within the geographic extent of a petroleum system. The
stratigraphic extent can be displayed on the burial history chart and
cross-section drawn at the critical moment. The cross-section in Figure
5 shows the stratigraphic extent of the fictitious Deer-Boar(.)
petroleum system at the critical moment.
Events Chart
An events chart (Figure 6) shows the temporal
relation of the essential elements and processes of a petroleum system.
It also shows the preservation time and the critical moment for the
system. An events chart can be used to compare the times that the
processes occurred with the times that the elements formed.
A petroleum system events chart shows time on one
axis and the essential elements and processes on the other. The time
required for the generation-migration-accumulation process is the same
as the age of the system. The chart also shows the preservation time and
critical moment for the system. The events chart for the fictitious
Deer-Boar(.) petroleum system is shown in Figure
6.
The events chart is arranged according to
increasing difficulty. For example, mapping and dating the essential
elements of a petroleum system are usually easier than mapping and
determining the time over which the processes took place. Because the
petroleum system deals only with discovered accumulations, there is no
question that the elements and processes worked correctly to make oil
and gas fields. Later, however, the events chart is transformed into a
risk chart to better evaluate a play or prospect.
Size of a Petroleum System
The size of a petroleum system includes the total
volume of all recoverable hydrocarbons that originated from a single pod
of active source rock. This total volume is used to compare against
other petroleum systems and to determine the generation-accumulation
efficiency.
The discovered hydrocarbons include shows, seeps,
and accumulations of oil and gas. The size of a petroleum system is
determined using a table such as the following for the fields in the
Deer-Boar(.) system, with reserves of approximately 1.2 billion barrels
(bbl) (Table 1).
Generation-accumulation efficiency is the ratio
(expressed as a percentage) of the total volume of trapped (in-place)
petroleum in the petroleum system to the total volume of petroleum
generated from the pod of active source rock.
Mapping a Petroleum System
A petroleum system is mapped by showing the
geographic, stratigraphic, and temporal extent of the system.
The geographic extent is the area over
which the petroleum system is known to occur. It is defined in map view
by a line on the earth's surface that circumscribes the pod of active
source rock as well as all the known petroleum shows, seeps, and
accumulations that originated from that pod. The geographic extent is
outlined to correspond to the time of the critical moment. It is similar
to the known extent, or known geographic extent.
The stratigraphic extent of a petroleum
system is the span of lithological units which encompasses the essential
elements within the geographic extent of a petroleum system. The
stratigraphic extent can be displayed on the burial history chart and
cross-section drawn at the critical moment. The stratigraphic extent is
from below the pod of active source rock or the petroleum of the
discovered accumulations in the system, whichever is deeper, to the top
of the overburden rock.
The temporal extent of the petroleum
system is shown on the events chart and includes the age of the
essential elements and processes, the preservation time, and the
critical moment. By displaying together the time over which these
separate events took place, the relation between forming and charging
the traps containing the accumulations is easily evaluated.
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Examples of Two
Petroleum Systems
To better understand how a petroleum system is
mapped and described, two examples are presented: the Mandal-Ekofisk(!)
and the Ellesmerian(!) petroleum systems (from Cornford, 1994, and Bird,
1994, respectively). The petroleum in the former system migrated across
stratigraphic units (or vertically) into many accumulations, whereas the
latter migrated along stratigraphic units (or laterally) into a few
accumulations. Both oil systems are multibillion barrels in size. These
two examples illustrate many of the concepts and principles discussed
above.
Mandal-Ekofisk(!) Petroleum System
Introduction
The Mandal-Ekofisk(!) petroleum system in the
Central Graben of the North Sea contains 21.4 billion bbl of oil and
39.4 trillion ft3 of gas in 39 fields (Cornford, 1994). The
age of the reservoir rock ranges from Devonian to Tertiary age with
about 85% of the petroleum in rock adjacent to the Cretaceous-Tertiary
boundary, specifically the Ekofisk Formation of Late Cretaceous age.
Based on geochemical evidence, the Upper Jurassic (Kimmeridgian) to
Lower Cretaceous source rock is the Mandal Formation. A positive
oil-source rock correlation indicates a known system.
Figure/Table Captions
 Figure 7. Burial history chart of the Mandal
source rock (after Cornford, 1994).
 Figure 8. Present-day map of Mandal-Ekofisk (!)
petroleum system (after Cornford, 1994). A-A’—line of cross-section in
Figure 9.
 Figure 9. Cross-section showing
hydrocarbon habitats in the Central Graben, illustrated with speculative
migration pathways (heavy arrows). Unlike the Viking and Witch Ground grabens to the north, migration is mostly vertical through fractures
produced by halokinesis. (After Cornford, 1994). Line of section shown
in Figure 8.
 Figure 10. Example of oil-source
rock correlation. Sterane molecular weight distributions for named oils
and early mature to mature Mandal Formation source rock extracts. (From
Cornford, 1994).
 Figure 11. Events chart showing
the timing of essential elements and processes within the Mandal-Ekofisk(!)
petroleum system (after Cornford, 1994).
 Table 2. Volumes of in-place
resources for the fields of the Mandal-Ekofisk(!) petroleum system
(after Cornford, 1994).
Geologic Setting
This petroleum system formed in sedimentary rocks
deposited in a failed rift system in the North Sea between Great
Britain, Norway, and Denmark. The prerift rocks are mostly underburden
rocks and are not involved in this petroleum system except as reservoir
rocks for a minor amount of petroleum. The synrift sedimentary section
contains the source rock. The reservoir rock, seal rock, and overburden
rock were deposited during the postrift period of sedimentation.
Burial History Chart
To determine more accurately when the Mandal
source rock was actively generating petroleum, a burial history chart (Figure
7) was constructed. Based on this and other charts, peak
generation of petroleum occurred at about 30 Ma, which was selected as
the critical moment.
Petroleum System Map
The petroleum system map in Figure 8 shows the
pod of active source rock and the oil and gas accumulations that were
charged by this same pod of active source rock; all are within the
geographic or known extent of the system. Most accumulations for the
Mandal-Ekofisk(!) overly the active source rock, and the gas/condensate
fields overlie the most mature source rock.
Petroleum System Cross-section
The petroleum system cross-section in Figure 9
shows migration pathways and the spatial relation of the active source
rock to the reservoir rocks. This section trends longitudinally along
the Central Graben and shows the vertical migration path from the active
source rock through the Cretaceous rocks and horizontally along the
basal Paleogene reservoir rocks until it accumulates in various traps.
The underburden rock is pre-Late Jurassic in age and is not involved in
the petroleum system except as minor reservoir rocks and where the
Permian salt (Zechstein Group) creates diapirs that form petroleum traps
and migration paths in fractured chalk.
Oil-Source Rock Correlation
The oil-source rock correlation is a
multiparameter geochemical approach; biological markers are one
parameter. Biological marker analysis by Mackenzie et al. (1983) and
Hughes et al. (1985) from reservoirs in the Greater Ekofisk, Forties,
Montrose, and Argyll fields shows that these oils originated from the
Mandal Formation source rock, as illustrated in Figure
10.
Petroleum System Events Chart
An events chart indicates when the essential
elements and processes took place to form a petroleum system, the
critical moment, and the preservation time. In Figure
11, the source
rock is the Upper Jurassic to Lower Cretaceous Mandal Formation, which
was deposited as the rift formed. Most overburden rock of Cretaceous to
Cenozoic age was deposited after the rift formed. The seal rock ranges
from Permian to Neogene and consists of halite, shale, and chalk. Based
on volume of petroleum, the Permian to Jurassic reservoir rocks are
least important; the most important reservoir rocks are Late Cretaceous
to early Paleogene in age. Most traps were created as the rift formed
and filled through structural movement and halokenesis. Petroleum
generation-migration-accumulation occurred from just over 100 Ma to the
present day. The critical moment, or peak generation, is at 30 Ma.
Size of Petroleum System
The size of the Mandal-Ekofisk(!) petroleum
system, as shown in Table 2, is determined by the total volume of
in-place hydrocarbons that originated from the pod of active Mandal
source rock. The in-place hydrocarbons are determined from the
recoverable hydrocarbons and, where possible, surface deposits, seeps,
and shows.
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Ellesmerian(!)
Petroleum System
Introduction
The Ellesmerian(!) petroleum system of the North
Slope, Alaska, contains approximately 77 billion bbl of oil equivalent
(Bird, 1994). The age of the reservoir rock ranges from Mississippian to
early Tertiary. Total organic carbon and assumed hydrogen indices from
the marine shale source rocks indicate the mass of petroleum generated
to be approximately 8 trillion barrels of oil (Bird, 1994). These
estimates indicate about 1% of the generated hydrocarbons are contained
in known traps. More importantly, the U.S. Geological Survey estimates
another 1% is trapped in undiscovered accumulations in the Ellesmerian(!)
petroleum system (Bird, 1994).
Figure/Table Captions
 Figure 12. Geographic extent of Ellesmerian (!)
petroleum system (from Bird, 1994).
 Figure 13. Map showing thermal maturity of the
Shublik Formation and Kingak Shale, two main source rocks of the
Ellesmerian (!) petroleum system (from Bird, 1994).
 Figure 14. Generalized cross-section of North
Slope, showing elements of the Ellesmerian (!) petroleum system.
Location in Figure 12.
 Figure 15. Burial history chart of Inigok 1 Well,
with isotherms (from Bird, 1994).
 Figure 16. Left: Biomarker analysis from the main
reservoir rock of Prudhoe Bay field in comparison to that from Shublik
Formation, Kingak Shale, and Pebble shale unit (from Seifert et al.,
1980). Right: 13C values are similar in the reservoir and
Shublik Formation and Kingak Shale (right) (Sedivy et al., 1987).
 Figure 17. Events chart for the Ellesmerian(!)
petroleum system (from Bird, 1994).
 Table 3. Hydrocarbon resources in the Ellesmerian(!)
petroleum system (from Bird).
Geologic Setting
The North Slope evolved from a passive
continental margin to a foredeep during the Jurassic. Prior to the
Jurassic, Paleozoic and Mesozoic strata were deposited on a passive
continental margin. They consist of Carboniferous platform carbonate
rocks and Permian to Jurassic shelf to basinal siliciclastic rocks. The
passive margin converted to a foredeep during the Jurassic and
Cretaceous when it collided with an ocean island arc. The foredeep began
to fill with sediments in the Middle Jurassic and continues to do so.
The foredeep basin fill consists of orogenic
sedimentary materials eroded from the nearby ancestral Brooks Range that
were deposited as a northeasterly prograding wedge of nonmarine, shallow
marine, basin-slope, and basin conglomerates, sandstones, and mudstones.
Petroleum System Map
Figure 12 shows the areal extent of the
Ellesmerian(!) petroleum system. The limit is determined by the extent
of the contiguous active source rock and the related petroleum
accumulations.
Petroleum System Maturity Map
Figure 13 shows the thermal maturity of the two
main Ellesmerian(!) petroleum system source rocks, the Shublik Formation
and the Kingak Shale. Note that Ellesmerian(!) petroleum system traps
(shown in Figure 12) are mostly located above immature source rocks.
Petroleum System Cross-Section
The cross-section of the Ellesmerian(!) petroleum
system (Figure 14) shows major structural-stratigraphic elements, the
occurrence of oil fields, elevation of selected vitrinite reflectance
values, and reflectance isograds.
Burial History Chart
Analysis of the burial history chart of the
Inigok 1 well (Figure 15) and other burial history charts indicates peak
petroleum generation (the critical moment) probably occurred in Late
Cretaceous time (approximately 75 Ma) in the western North Slope and in
early Tertiary time (approximately 50 Ma) in the central and eastern
part of the North Slope. Also, note the large increase in the rate of
sedimentation during the Early Cretaceous.
Oil-Source Rock Correlation
Biological marker analysis (Figure
16, left), from the main reservoir rock, Sadlerochit Group, of Prudhoe
Bay field, shows that the oil originated from the Shublik Formation, the
Kingak Shale, and the Hue Shale. Carbon isotopic composition comparisons
(Figure 16, right) indicate that Shublik and Kingak share similar
13C values with oil from the Prudhoe Bay field, whereas the Hue
Shale does not.
Petroleum System Events Chart
The events chart (Figure
17) for the Ellesmerian(!)
petroleum system indicates when its elements and processes occurred. The
cross-hatched pattern shows the estimated time of the tilting of the
Barrow Arch, which resulted in remigration of petroleum from older to
younger (early Tertiary) reservoir rocks.
Size of Petroleum System
The size of the Ellesmerian(!) petroleum system,
shown in Table 3,
is determined by the total volume of in-place
petroleum that originated from the pod of active Ellesmerian(!)
petroleum system source rock. The in-place petroleum is determined from
the recoverable petroleum and, where possible, surface deposits, seeps,
and shows. In Table 3, trap type A is structural, B is stratigraphic,
and C is combination.
Applying
the Petroleum System Concept: Basin, System, Play, and Prospect
Words frequently have more than one meaning;
nomenclature in this discipline of petroleum geology is no exception. To
more clearly separate the petroleum system from the sedimentary basin
and the play and prospect, the meaning of these words needs to be
clarified with respect to each other and the petroleum province.
Table/Figure Captions
 Table 4. Items to
be compared in evaluation of sedimentary basin, petroleum system, play,
and prospect.
 Figure 18. Petroleum system events chart (left)
and complementary play-prospect risk chart (from Magoon, 1995).
 Figure 19.
Present-day map of petroleum system, with complementary play or
prospect.
 Figure 20. Plot of prospects and drilled wells
(100% becoming fields) versus time (upper); plot of reserves versus time
(lower).
 Figure 21. Cross-section of Papers Wash and South
Papers Wash fields, San Juan Basin ( From Vincelette and Chittum, 1981).
 Figure 22. Left: Preferred prospect if mature
source rock is directly under the reservoir. Right: Preferred prospect
if mature source rock is downdip from prospects. From Barker, 1992.
 Figure 23. Cross-section of a vertically drained
petroleum system (from Demaison and Huizinga, 1994).
 Figure 24. Cross-section of a
laterally drained
petroleum system (from Demaison and Huizinga, 1994).
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Petroleum Province
Petroleum province,
a geographic term, is an area where petroleum occurs in commercial
quantities. Basin is sometimes used geographically to mean
petroleum province, such as the Williston Basin or Paris Basin. The
Zagros fold belt could be a structural province or a petroleum province,
not a basin.
A map showing differential thickness of
sedimentary rocks is used to determine basins (thick), uplifts (thin),
and fold belts (folded). These features are properly named provinces; if
they contain petroleum, they are called petroleum provinces. The use of
"basin" in this context is improper; it is also inconsistent with the
petroleum system concept described below, which defines "basin" as the
area into which sedimentary rocks are deposited.
Sedimentary Basin
A sedimentary basin is a depression filled
with sedimentary rocks. The presence of sedimentary rocks is proof that
a basin existed.
The depression, formed by any tectonic process,
is lined by basement rock, which can be igneous, metamorphic,
and/or sedimentary rock. The basin fill includes the rock matter,
organic matter, and water deposited in this depression. In certain
cases, such as with coal and some carbonate deposits, the sedimentary
material is formed in situ. The essential elements of a petroleum system
are deposited in sedimentary basins. Frequently, one or more overlapping
sedimentary basins are responsible for the essential elements of a
petroleum system. Traps are formed by tectonic processes that act on
sedimentary rocks. However, the moment petroleum is generated,
biologically or thermally, a petroleum system is formed.
Petroleum System
The petroleum system includes the pod of
active source rock, the natural distribution network, and the
genetically related discovered petroleum occurrences. Presence of
petroleum is proof that a system exists.
The pod of active source rock is part of the
petroleum system because it is the provenance of these related petroleum
occurrences. The distribution network is the migration paths to
discovered accumulations, seeps, and shows.
In contrast to the play and prospect, which
address undiscovered commercial accumulations, the petroleum system
includes only the discovered petroleum occurrences. If an exploratory
well encounters any type or amount of petroleum, that petroleum is part
of a petroleum system.
Play and Prospect
The play and prospect are used by the
explorationist to present a geologic argument to justify drilling for
undiscovered, commercial petroleum accumulations.
The play consists of one or more
geologically related prospects, and a prospect is a potential
trap that must be evaluated by drilling to determine whether it contains
commercial quantities of petroleum. Once drilling is complete, the term
"prospect" is dropped; the site becomes either a dry hole or a producing
field.
The presence of a petroleum charge, a suitable
trap, and whether the trap formed before it was charged are usually
involved in this evaluation.
These terms are compared in Table
4.
Relationship of Play to Petroleum System
In a play, the petroleum accumulations are
commercial and undiscovered. In a petroleum system, the petroleum
occurrences are already discovered (Magoon, 1995). Other differences are
listed in Table
4. Usually, a play is predicated without any particular
petroleum system in mind. However, when a play is based on a particular
petroleum system, it is called a complementary play.
The petroleum system concept is used two ways in
exploration. By mapping a petroleum system, an explorationist learns new
play concepts to add new oil or gas fields to the petroleum system. This
relation is shown in the following equation:
PStotal
= PSpartial + CP1 + CP2 + CP3
where:
PStotal = petroleum system with all accumulations discovered
PSpartial = petroleum system with only some of the
accumulations discovered
CP1, ... = the complementary play (prospect) concepts used to
find the remaining undiscovered commercial accumulations in the
petroleum system
The petroleum system is also used as an analog to
another less-explored petroleum system. For this approach to work, the
explorationist must have a series of petroleum system case studies
available for comparison.
Applying
the Petroleum System Concept: Reducing Exploration Risk
In exploration, the general question is Where
can we find substantial quantities of hydrocarbons that are economical
to produce? To solve this problem, exploration geologists find and
evaluate a prospect. In addition to helping evaluate petroleum charge,
trap, and timing, the petroleum system concept can help in the
exploration process by determining exploration intensity and assessing
risk.
Play
A play is one or more prospects that may define a
profitable accumulation of undiscovered petroleum. Traditionally, a play
is developed and evaluated without any particular petroleum system in
mind. For example, if a prospect (play) is identified near a series of
oil fields in anticlinal traps, it could be argued--using geophysics and
geochemistry--that the prospect is an anticlinal trap charged with the
same oil.
Three independent variables--petroleum charge
(fluids), trap (sedimentary rocks), and timing (time)--are usually
evaluated. Petroleum charge is the volume and characteristics of
the oil and gas available to the trap, if it exists. The trap
includes the reservoir and seal rocks and the trapping geometry formed
by the reservoir-seal interface. Timing is whether the trap
formed before the petroleum charge entered the trap.
Each independent variable has equal weight
because if any variable is absent (0), the prospect is a failure; if all
variables are present (1.0), the prospect is a commercial success.
Therefore, each independent variable can be evaluated on a scale of zero
to one (0-1.0). Exploration risk is determined by multiplying the three
variables: charge, trap, and timing.
Within each independent variable, a series of
subevents (which are also independent) must be evaluated. For example,
if a trap is to be evaluated, the reservoir rock must be mapped
carefully and its properties predicted using geologic principals. A
similar procedure is carried out for the seal and trapping geometry.
These subevents must be reduced to a single number between 0 and 1.0
that represents the independent variable, the trap. The subevents that
contribute to petroleum charge and timing should also be evaluated in a
similar manner.
A practical way to carry out this exercise is to
first map the petroleum system so the knowledge about the system can be
used to evaluate the complementary play.
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Complementary Play
The
complementary play evaluates the exploration risk for finding
undiscovered hydrocarbons associated with a particular petroleum system.
First,
the petroleum system case study is completed. As the case study
develops, an idea(s) or play(s) that involves this petroleum system will
occur to the investigator. This play complements the petroleum system
because it could add hydrocarbons (if discovered) to the system.
The
events chart in Figure 18 shows how the risk chart for the complementary
play (prospect) is related to the petroleum system vis-a-vis the three
independent variables--trap, petroleum charge, and timing.
The experience acquired while executing the
petroleum system case study provides the measure of difficulty in
mapping and determining the age of the essential elements and, more
importantly, for the two processes--trap formation and
generation-migration-accumulation of petroleum. Obviously, there is no
risk or uncertainty related to the discovered accumulations in the
petroleum system, but there are varying levels of difficulty in the
reconstruction of events that caused these accumulations. This measure
of difficulty can be incorporated into the risk chart.
For example, geologic and geophysical information
for the producing fields indicates the traps are easily mapped and the
time of formation is narrowly constrained. However, this same type of
information over the geographic extent of the petroleum system indicates
these types of traps have all been tested successfully and the only
prospects left are ones that are more difficult to map and date; hence,
their relative risk increases.
Using the risk chart in this manner allows the
investigator and prospect evaluator an opportunity to separate what is
known on the events chart from what is unknown on the risk chart for the
prospect.
Assigning Risk
A petroleum system map can be used to evaluate
the time and volume of hydrocarbon charge or to assign risk to a
complementary play or prospect by using its position relative to the
geographic extent of the system.
Least to Most Risk
Using Figure 19 and stipulating that the
complementary play is on the migration path for this petroleum system, a
play located within or outside the geographic extent of the system has
the following level of risk:
1. Least risk; accumulations surround the trap.
2. Some risk; accumulations located on three sides.
3. Riskier; accumulations located on only one side.
4. Most risk; accumulations distant from prospect.
Studies of the reservoir rock and seal rock as
well as trap formation are needed to evaluate migration paths and traps.
Exploration Intensity
In a petroleum province, drilling density usually
indicates how intensively an area has been explored. Though this is a
relative measure, a petroleum province having one exploratory well every
square kilometer is well explored compared with a province that has one
well every 100 km2. Exploration intensity by province ranges
from lightly to moderately to heavily explored. However, in a petroleum
province with overlapping petroleum systems, the shallowest petroleum
system may be heavily explored compared with the deeper petroleum
systems. To determine level of exploration, each petroleum system in the
province of interest should be mapped and the size and location of the
commercial accumulations compared with the dry exploratory wells. The
dry-hole ratio or success ratio determines exploration intensity and
success.
The graphs in Figure 20 conceptually summarize
the exploration process relative to time. The top graph shows that a
frontier petroleum province or petroleum system starts with only
prospects (1.00 or 100%); with time, some or all (shown here) of those
prospects become oil (gas) fields. The bottom graph shows that the
highest percentage of the cumulative petroleum reserves are found early
in the exploration process. The quicker we determine the size and extent
of a petroleum system, the more likely we will be able to decide whether
to continue drilling exploratory wells.
Applying
the Petroleum System Concept: Examples
Linking the elements (source, reservoir, seal,
and overburden) to the processes of petroleum geology (trap formation
and hydrocarbon generation-migration-accumulation) is an effective
exploration approach. Mapping and studying a petroleum system helps
explorationists predict which traps will contain petroleum and which
will not. It also helps them focus on that part of a province that will
most likely contain accumulations. Below are some examples of how the
petroleum system concept can be applied to petroleum exploration at
local and regional levels.
Local Example
Consider the cross-section in Figure
21, from the
Papers Wash field from the San Juan Basin, New Mexico. The cross-section
shows that three separate prospects (traps) were tested (drilled). The
deepest trap was filled to the spill point with oil, the middle trap was
partially filled, and the shallowest trap was empty. This arrangement
suggests that oil mi grated to the traps from a mature source rock
downdip to the north by filling the traps in sequence.
If these three prospects in Figure 21 had not
been tested, which would we drill first? With an understanding of the
petroleum system that charged these prospects, we could be more
confident in recommending which prospect to drill first. If we knew that
mature source rock was located directly under the reservoir, then we
would expect all traps to be filled an equal amount (Figure
22, left).
Conversely, if we knew that the source was mature downdip to the north,
then we would drill the deepest prospect, not the middle or shallowest
prospect to the south (Figure 22, right).
Regional-Scale Applications
Petroleum system studies may serve as analogs for
undocumented petroleum systems in prospective petroleum provinces.
Because a petroleum system study describes both elements and processes,
we can use them as look-alike and work-alike analogs. Petroleum systems
also can be classified in different ways according to our needs--an
example of applying a petroleum system classification scheme to
petroleum exploration.
Vertically drained
petroleum systems
Demaison and Huizinga (1994) divide petroleum
systems into vertically and laterally drained. An earlier section of
this chapter describes the Mandal-Ekofisk(!) petroleum system, which is
a vertically drained system. Vertically drained systems are generally
found in rifts, deltas, wrenches, and overthrust provinces where
migration is controlled by faults and fractures. Faults and fractures
limit the size of the fetch area available to traps, so a number of
small- and medium-sized accumulations abound.
Vertically drained systems (Figure
23) have the
following characteristics (Demaison and Huizinga, 1994):
·
Accumulations occur above or
near to the pod of active source rock.
·
Lateral migration distances are
short.
·
Multiple, stacked accumulations
usually contain the same genetic oil.
·
Surface seepages are common in
supercharged systems.
·
The largest accumulations are
seldom found early in the drilling history; instead, many medium to
small accumulations are found.
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Laterally Drained
Petroleum Systems
According to Demaison and Huizinga (1994),
laterally drained petroleum systems have a laterally continuous seal
overlying a laterally continuous reservoir. This reservoir/seal couplet
is generally contained within a long, uninterrupted ramp. Provinces with
these systems have low to moderate structural deformation. Tectonic
stability is critical for maintaining seal integrity. Laterally drained
systems are most commonly found in foredeep and cratonic sag basins.
Plunging low-amplitude arches are necessary for connecting traps to the
pod of active source rock. The Ellesmerian(!) petroleum system is an
example of a laterally drained system.
Laterally drained systems (Figure
24) have the
following characteristics:
·
Oil accumulations generally
occur in thermally immature strata located far from the pod of active
source rock.
·
Accumulations containing oil
that migrated long distances on average account for 50% of the entrapped
oil.
·
A single reservoir of the same
age as the active source rock contains most of the entrapped oil and
gas.
·
In supercharged systems, large
deposits of heavy oil often occur in thermally immature strata near the
eroded margin (geographic extent) of the petroleum system.
·
The largest accumulation is
usually found early in the drilling history of the system. After that,
mostly small accumulations are found (J. Armentrout, personal
communication, 1997).
The cross-section (Figure
24) is an example of a
laterally drained petroleum system, patterned after the Eastern
Venezuelan Basin.
References
Barker, C., 1992, The role of
source rock studies in petroleum exploration, in K.S. Johnson and
B.J. Cardott, eds., Source Rocks in the Southern Midcontinent, 1990
Symposium: Oklahoma Geological Survey Circular 93, p. 3-20.
Bird, K.J., 1994, Ellesmerian(!)
petroleum system, North Slope, Alaska, USA, in L.B. Magoon and
W.G. Dow, eds., The Petroleum System--From Source to Trap: AAPG Memoir
60, p. 339-358.
Cornford, C., 1994, The
Mandal-Ekofisk(!) petroleum system in the Central Graben of the North
Sea, in L.B. Magoon and W.G. Dow, eds., The Petroleum
System--From Source to Trap: AAPG Memoir 60, p. 537-571.
Demaison, G., and B.J. Huizinga,
1994, Genetic classification of petroleum systems using three factors:
charge, migration, and entrapment, in L.B. Magoon and W.G. Dow,
eds., The Petroleum System--From Source to Trap: AAPG Memoir 60, p.
73-89.
Hughes, W.B., A.G. Holba, D.E.
Miller, and J.S. Richardson, 1985, Geochemistry of the greater Ekofisk
crude oils, in B.M. Thomas et al., eds., Petroleum Geochemistry
in the Exploration of the Norwegian Shelf: London, Graham and Trotman,
p. 75 -92.
Mackenzie, A.S., J.R. Maxwell,
and M.L. Coleman, 1983, Biological marker and isotope studies of North
Sea crude oils and sediments: Proceedings of the 11th World Petroleum
Congress, London, Section PD1(4), p. 45-56.
Magoon, L.B., 1995, The play
that complements the petroleum system--a new exploration equation: Oil &
Gas Journal, vol. 93, no. 40, p. 85-87.
Magoon, L.B., and W.G. Dow,
1994, The petroleum system, in L.B. Magoon and W.G. Dow, eds.,
The Petroleum System--From Source to Trap: AAPG Memoir 60, p. 3-24.
Sedivy, R.A., I.E. Penfield,
H.I. Halpern, R.J. Drozd, G.A. Cole, and R. Burwood, 1987, Investigation
of source rock-crude oil relationships in the northern Alaska
hydrocarbon habitat, in I. Tailleur and P. Weimer, eds., Alaskan
North Slope Geology: Pacific Section SEPM Book 50, p. 169-179.
Seifert, W.K., J.M. Moldowan,
and R.W. Jones, 1980, Application of biological marker chemistry to
petroleum exploration: Proceedings of the 10th World Petroleum Congress,
Bucharest, p. 425-440.
Vincelette, R.R., and W.E.
Chittum, 1981, Exploration for oil accumulations in Entrada Sandstone,
San Juan basin, New Mexico: AAPG Bulletin, vol. 65, p. 2546-2570.
Authors
Leslie B. Magoon
 Leslie B. Magoon graduated from the University of
Oregon in Eugene in 1966 with an M.S. degree in geology. Presently, he
is a senior research geologist with the U.S. Geological Survey, Menlo
Park, California. Prior to that he was with Shell Oil Company for 8
years as an exploration geologist. Over the last 32 years, he has been
involved in petroleum geology with emphasis on geochemistry in the Rocky
Mountain states, California, Alaska, Colombia, and Malaysia. He has
numerous publications on the geology and geochemistry of petroleum
provinces in Alaska, the Cook Inlet-Alaska Peninsula, and the North
Slope. For the last 15 years he has devoted much of his time to
developing and presenting the petroleum system. From 1990-1991, he was
an AAPG Distinguished Lecturer. At the 1996 AAPG Annual meeting, Magoon
and W.G. Dow, as coeditors, received the R.H. Dott, Sr., Memorial Award
for AAPG Memoir 60, The Petroleum System--From Source to Trap.
Edward A. Beaumont
 Edward A. (Ted) Beaumont is an independent petroleum geologist from
Tulsa, Oklahoma. He holds a BS in geology from the University of New
Mexico and an MS in geology from the University of Kansas. Currently, he
is generating drilling prospects in Texas, Oklahoma, and the Rocky
Mountains. His previous professional experience was as a sedimentologist
in basin analysis with Cities Service Oil Company and as Science
Director for AAPG. Ted is coeditor of the Treatise of Petroleum Geology.
He has lectured on creative exploration techniques in the U.S., China,
and Australia and has received the Distinguished Service Award and Award
of Special Recognition from AAPG.
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