Petroleum system and the play concept

Quick refresher

An accumulation needs all the elements at once: a mature source rock (one heated enough to make oil and gas), a migration path, a reservoir, a seal and a trap. If any one of them is missing, there can be no accumulation.
The elements are rocks. The processes are what happens to them over time: burial, maturation, generation, migration, accumulation and preservation.
The critical moment is the time when most of the hydrocarbons are generated and migrate. Traps and seals must exist by this moment.
A petroleum system describes what has already happened to the hydrocarbons of one generation area (the “kitchen”, where the source rock makes hydrocarbons). A play is an exploration model for one stratigraphic level, and it also includes accumulations that have not been discovered yet.
Play → lead (a possible trap, with little data) → prospect (a mapped trap, ready to drill).
The geological chance of success is the product of the chances of the elements (the chances multiplied together). The risk that is shared by the whole play is removed by the first discovery.

Key relationships

P_g = \prod_i P_i

geological chance of success when the elements are independent

P_g = P_\text{play}\cdot P_\text{loc}

shared risk of the play and local risk of the prospect

Common pitfalls

Treating a structure on a seismic section as a ready accumulation: the trap is only one of the elements.
Forgetting about time: a trap that formed after migration may stay empty.
Mixing up the petroleum system (what happened) and the play (where to look).
Thinking that several prospects in one untested play are independent chances.

After this lesson you can

list the elements and processes of a petroleum system and explain what happens if any of them is missing
explain why the order of events matters, and find the critical moment on an events chart
tell a petroleum system from a play, and a play from a lead and a prospect
read a play fairway map (a map of the area where a play can work), built by laying the maps of the separate elements over each other
estimate the geological chance of success of a prospect and understand why the first well in a new play is especially valuable

The petroleum system: what an accumulation needs

The story of every accumulation begins with organic matter: remains of plankton, algae and land plants, buried together with fine sediments. This is how a source rock forms. It is usually a shale (a clay-rich rock) or a mixed clay–carbonate rock. The weight of the overburden that piles up above it presses the source rock down. The source rock sinks deeper and warms up, and its organic matter slowly turns into oil and gas. The part of the basin where the source rock is in the generation zone (hot enough to make hydrocarbons) is called the generation kitchen, or simply the “kitchen”.

The hydrocarbons that form leave the source rock (primary migration). They are lighter than water, so they rise along permeable layers (layers that let fluids through) and faults (secondary migration). They keep moving until they meet a trap. A trap is a part of a reservoir (a porous rock that can hold fluids) covered by a seal (a rock that fluids can hardly pass through). Under the seal there is nowhere further up to go. There the hydrocarbons collect and form an accumulation. If there is no trap on the way, the hydrocarbons spread out in the rocks or come out at the surface.

SchematicA petroleum system in section. The source rock in the deeper part of the basin enters the oil window (the zone where it is hot enough to make oil). The oil migrates up the dip of the reservoir. It fills an anticlinal trap (a trap in an arch-shaped fold) up to its spill point (the level at which oil begins to leak out of it). Then it goes on to a pinch-out trap (where the reservoir layer thins out to nothing). The sketch is not to scale.

Elements and processes of a petroleum system

Element or process: Source rock
What it is: a fine-grained rock rich in organic matter, which makes hydrocarbons when it is heated
What can go wrong: there is little organic matter, or it is of the wrong type; the rock has not sunk into the generation zone, or it is “overcooked” (overmature)

Element or process: Reservoir
What it is: a porous and permeable rock that holds the accumulation (Porosity, Rock permeability)
What can go wrong: there is no reservoir, or it is thin, or too tight (not permeable enough) to give up the oil

Element or process: Seal
What it is: a rock of low permeability and high capillary entry pressure (the pressure needed to push oil into its pores): shales, salt, anhydrite
What can go wrong: faults cut the seal, or it is too thin, or it cannot hold a tall column of hydrocarbons

Element or process: Overburden
What it is: the rock above the source rock: it buries the source rock to the generation zone and helps to form traps
What can go wrong: burial is too small, so the source rock stays immature

Element or process: Trap formation
What it is: folds, faults, pinching out and replacement of layers (a layer turns into another rock sideways), reefs: a geometry from which hydrocarbons have nowhere to go
What can go wrong: the trap formed after migration, or it has no closure (its shape does not close)

Element or process: Generation, migration, accumulation
What it is: ripening of the organic matter, release of the hydrocarbons and their movement to the traps
What can go wrong: the migration paths lead the hydrocarbons past the trap

Element or process: Preservation
What it is: the accumulation is not destroyed after it forms
What can go wrong: late faults, uplift and erosion, biodegradation (bacteria eating the oil), tilting of the layers so that hydrocarbons spill out of the trap

Element or process	What it is	What can go wrong
Source rock	a fine-grained rock rich in organic matter, which makes hydrocarbons when it is heated	there is little organic matter, or it is of the wrong type; the rock has not sunk into the generation zone, or it is “overcooked” (overmature)
Reservoir	a porous and permeable rock that holds the accumulation (Porosity, Rock permeability)	there is no reservoir, or it is thin, or too tight (not permeable enough) to give up the oil
Seal	a rock of low permeability and high capillary entry pressure (the pressure needed to push oil into its pores): shales, salt, anhydrite	faults cut the seal, or it is too thin, or it cannot hold a tall column of hydrocarbons
Overburden	the rock above the source rock: it buries the source rock to the generation zone and helps to form traps	burial is too small, so the source rock stays immature
Trap formation	folds, faults, pinching out and replacement of layers (a layer turns into another rock sideways), reefs: a geometry from which hydrocarbons have nowhere to go	the trap formed after migration, or it has no closure (its shape does not close)
Generation, migration, accumulation	ripening of the organic matter, release of the hydrocarbons and their movement to the traps	the migration paths lead the hydrocarbons past the trap
Preservation	the accumulation is not destroyed after it forms	late faults, uplift and erosion, biodegradation (bacteria eating the oil), tilting of the layers so that hydrocarbons spill out of the trap

More on this: Source rocks, maturation and migration, Reservoir and seal, Traps and timing.

Time decides: the critical moment and the events chart

A source rock does not start to generate oil right after it is deposited. It starts when it has sunk deep enough and warmed up. The rate of generation depends strongly on temperature. So most of the hydrocarbons form and migrate within a limited interval of geological time. The moment that best describes the generation, migration and accumulation of most of the hydrocarbons of the system is called the critical moment.

From this comes the main rule of timing: by the critical moment, the traps and seals must already exist. A trap that formed later will fill only if generation is still going on. It can also fill if hydrocarbons flow into it from old accumulations that were destroyed (remigration, or repeated migration). A handy tool for this check is the events chart. It shows on one time scale when the elements were deposited, when the traps formed, when generation took place and how long preservation lasts.

Worked exampleWhen the source rock enters the generation zone

Given (assumed data)

Time of deposition of the source rock: 150 million years ago
Rate of burial: 40 m per million years, constant
Temperature at the surface (the basin floor): 10 °C
Geothermal gradient (how fast temperature rises with depth): 30 °C/km
Assumed interval of main oil generation: from 100 to 130 °C

Solution

The depth at which the temperature reaches 100 °C is $z = (100 - 10)/30 = 3.0$ km, that is 3000 m.
The time to sink to this depth is $3000/40 = 75$ million years. So the main generation starts about 75 million years ago.
For 130 °C, $z = 4.0$ km, and the time to sink is 100 million years. This is about 50 million years ago.
The middle of the generation interval is about 62.5 million years ago. This is our estimate of the critical moment.

Answer: The main oil generation in this example runs from about 75 to 50 million years ago, and the critical moment is about 62.5 million years ago.

What it means. A trap that formed, for example, 90 million years ago is ready to receive oil by this time. A trap that appeared 40 million years ago is too late. The main oil has already passed it by. Only later batches of hydrocarbons can fill it (with further burial, mostly gas), or hydrocarbons from remigration. The real generation temperatures depend on the type of organic matter and on how fast the rock is heated, and burial is rarely steady. These questions are covered in the lesson Source rocks, maturation and migration. The data in this example are assumed for illustration.

SchematicEvents chart for the petroleum system of the example: the elements, the formation of two traps, the interval of generation and migration, the critical moment and the time of preservation. Trap A formed before the critical moment, and trap B after it. The dates are assumed for illustration.

The play: an exploration model

A petroleum system answers the question “what happened?”. It tells us where the hydrocarbons of the known accumulations, shows (traces of oil or gas) and surface seeps (places where oil comes out at the surface) came from. An explorer needs a different question: “where do we look next?”. This is what the play is for. It is a model that explains why accumulations could exist at one stratigraphic level in one part of the basin. There is a reservoir and a regional seal. Hydrocarbons reach the place from the kitchen. And traps of a certain type exist. We also use the word “play” for the group of known and expected accumulations that share such a common geological “formula”: a similar reservoir, seal, source, time of migration, trap type and fluid type.

Petroleum system and play

: Question
Petroleum system: what has happened to the hydrocarbons
Play: where to find new accumulations

: Defined by
Petroleum system: one kitchen of an active source rock and all the hydrocarbons linked to it
Play: a stratigraphic level and a common set of elements: reservoir, seal, source, type of trap

: Includes
Petroleum system: known accumulations, shows, surface seeps
Play: discovered accumulations, prospects and leads, including untested ones

: Link
Petroleum system: one system can feed several plays at different levels
Play: one play can receive hydrocarbons from several systems

	Petroleum system	Play
Question	what has happened to the hydrocarbons	where to find new accumulations
Defined by	one kitchen of an active source rock and all the hydrocarbons linked to it	a stratigraphic level and a common set of elements: reservoir, seal, source, type of trap
Includes	known accumulations, shows, surface seeps	discovered accumulations, prospects and leads, including untested ones
Link	one system can feed several plays at different levels	one play can receive hydrocarbons from several systems

A play is mapped. We use outcrops (places where the rock comes to the surface), wells, seismic data and regional geological reports. From them we build a separate map for each element: where there is a reservoir, where the regional seal survives, and where the hydrocarbons from the kitchen could have reached. Laying these maps over each other picks out the play fairway: the area where all the elements come together. Outside it at least one element is missing, and it makes no sense to look there for traps of this play.

SchematicA play fairway as the overlap of the maps of the elements: the spread of the reservoir, the regional seal and the area that hydrocarbons from the kitchen reach. Inside the fairway are leads, prospects and a discovery. A dry well outside it is explained by the absence of a seal. The sketch is not to scale.

Play, lead, prospect

The search works like a funnel: from the basin to the plays, and from the plays to single traps. The fairway of a play may cover hundreds or thousands of square kilometres, and a single trap is many times smaller. The further down the funnel, the more detailed the data and the more precise the estimate.

Play: An exploration model of a level and an area: a common reservoir, seal, source and type of trap.
Lead: A possible trap seen in sparse data (for example, on a regional grid of two-dimensional seismic lines). Its outline and volume are uncertain. It is too early to drill, and more study is needed.
Prospect: A mapped trap for which the volumes and the chance of success have been estimated. We can choose the place for an exploration well.
Discovery: A prospect in which a well has found hydrocarbons. Next the accumulation is appraised, which means that its size and quality are measured (Volumetric estimation of hydrocarbons in place).

For a lead to become a prospect, we must show four things. First, the trap has a porous and permeable reservoir, with a reasonable chance. Second, the trap is older than the main migration of the hydrocarbons. Third, it lies on the migration paths. Fourth, the possible volume is big enough to pay for the well.

From idea to exploration well

1
Regional data
Outcrops, drilled wells (logs, cuttings, core), regional seismic data, reports of geological surveys and contractors, and information about the work of neighbours. Service companies sometimes shoot seismic data in advance at their own cost and sell it to many companies (a multiclient survey).
2
Play maps
Maps of the elements, play fairways and the first leads. Choice of promising areas.
3
Access to the subsurface
Getting an area by licence or contract (Licences and contracts for subsoil use). Companies keep their play models secret, so that competitors do not see their evaluation. Well data and seismic data stay confidential for the period set by law, and sometimes companies swap them.
4
Detailed work
Detailed seismic surveys, often three-dimensional, turn leads into prospects, with trap maps, volumes and chances of success (Seismic interpretation).
5
Drilling and updating the model
The result of an exploration well, success or failure, refines the play model and the chances for the other prospects.

Geological chance of success

Before drilling we cannot know for sure what a prospect contains. So we give each element a chance (a probability) that it is present and works. The elements are the source and migration, the reservoir, the seal, the trap and the timing. Success needs all the elements at once. If they are independent, the chances multiply.

Geological chance of success of a prospect

P_g = \prod_{i=1}^{k} P_i = P_1\,P_2\cdots P_k

where:

$P_g$ geological chance of success: the well will find moveable hydrocarbons (hydrocarbons that can flow), fraction
$P_i$ chance that element $i$ (source and migration, reservoir, seal, trap, timing) is present and works, fraction
$k$ number of elements assessed, —

Units: dimensionless

Holds when

The elements are independent. If they depend on each other (for example, a reservoir and a seal from one depositional environment), we must not multiply them. We use conditional probabilities instead (the chance of one element given that another is present).
Each $P_i$ is a subjective estimate. We calibrate it against drilling statistics in similar plays.
Geological success is not the same as economic success: what is found may not be enough to pay for development.

The product is never greater than the smallest of the chances: a prospect is no more reliable than its weakest element. Five elements of 0.8 give only $0.8^5 \approx 0.33$ .

Some elements are common to the whole play: does the kitchen work, and is there a regional seal? Others belong to a particular trap: the accuracy of the closure map, and the quality of the reservoir at this spot. So we split the chance of success into the common play risk and the local risk of the prospect.

Common play risk and local prospect risk

P_g = P_\text{play}\cdot P_\text{loc}

where:

$P_\text{play}$ chance that the play as a whole works (the common elements), fraction
$P_\text{loc}$ chance of success of this prospect, given that the play works, fraction

Units: dimensionless

Holds when

The common and local elements are split so that each element is counted once.
$P_\text{loc}$ is a conditional probability: it is estimated on the assumption that the play works.

For the prospects of one play, the factor $P_\text{play}$ is shared. If the play does not work, all its prospects fail at once. After the first discovery in the play, $P_\text{play}$ for the neighbouring prospects becomes close to one.

Worked exampleChance of success of a prospect in an untested play

Given (assumed data)

Source and migration (common to the play): 0.7
Regional seal (common to the play): 0.6
Reservoir in the trap: 0.8
Trap (closure): 0.7
Timing: 0.9

Solution

The geological chance of success is $P_g = 0.7\cdot0.6\cdot0.8\cdot0.7\cdot0.9 = 0.212$ .
The common play risk is $P_\text{play} = 0.7\cdot0.6 = 0.42$ . The local risk is $P_\text{loc} = 0.8\cdot0.7\cdot0.9 = 0.504$ . Their product is the same 0.212.
Suppose the first well in the play makes a discovery. For a neighbouring prospect with the same local elements, $P_g \approx P_\text{loc} = 0.504$ . This is 2.4 times higher.

Answer: The chance of success of the first prospect is about 21%. After a discovery in the play, the chance of a neighbouring prospect rises to about 50%.

What it means. The first well in a new play tests not only its own trap but the whole play. So its value as information is higher than that of the wells that follow. For the same reason, drilling three prospects of one untested play does not lower the risk as much as drilling prospects in different plays. If there is no source or no seal, all three fail. These chances then enter the economic evaluation of the project (Expected monetary value and preference theory). The data in this example are assumed for illustration.

Terms and symbols

Petroleum system: A kitchen of active source rock, all the hydrocarbons linked to it, and the geological elements and processes needed to form accumulations.
Play: A group of known and expected accumulations and traps at one stratigraphic level, with a common reservoir, seal, source and type of trap.
Lead: A possible trap that has not been studied well enough to drill.
Prospect: A mapped trap with estimated volumes and chance of success, ready to drill.
Generation kitchen: The area where the source rock is in the zone of hydrocarbon generation.
Critical moment: The time that best describes the generation, migration and accumulation of most of the hydrocarbons of the system.
Events chart: A diagram on the geological time scale. It shows the deposition of the elements, the formation of traps, generation and migration, and preservation.
Play fairway: The area where the maps show all the elements of the play together.
Geological chance of success $P_g$: The chance that a well will find moveable hydrocarbons.

Common mistakes and misconceptions

Wrong

“There is an anticline on the seismic section, so there is an accumulation.”

In fact

A trap is necessary but not enough. We also need a mature source, a migration path, a reservoir, a seal and the right timing.

Wrong

“There is a rich source rock nearby, so the trap will be filled.”

In fact

The rock must reach maturity, the hydrocarbons must find a path to the trap, and the trap must exist by the time of migration and survive afterwards.

Wrong

“Five prospects in one new play are five independent chances.”

In fact

They share the play risk. If there is no source or no regional seal, all of them fail. Real diversification (spreading the risk) means drilling in different plays.

Wrong

“A petroleum system and a play are the same thing.”

In fact

A petroleum system is an idea about origin: what has already happened to the hydrocarbons of a kitchen. A play is an exploration model of one level, and it also includes accumulations that have not been discovered yet.

Wrong

“A dry well has proved that there is no play.”

In fact

First we find out which element failed. A local cause, such as an inaccurate trap map or no reservoir at the spot, does not disprove the play.

In depth: advanced detail

Limits of a petroleum system, levels of study and level of certainty

The geographic limits of a petroleum system are drawn around the kitchen of the active source rock and all the known accumulations, shows and seeps of hydrocarbons that come from it. We establish the link by geochemical correlation (comparison) of the oils with extracts from the source rock. The stratigraphic limits cover the sequence from the source rock to the highest accumulation. A system is usually named after its source rock and its main reservoir.

Level of certainty in a petroleum system

Level: Known
Basis: geochemistry has proved the link between the oil or gas in the accumulations and a particular source rock
How it is used: a basis for assessing plays where the source is no longer a risk

Level: Hypothetical
Basis: the source rock has been studied, but its link to the accumulations has not been proved
How it is used: the source risk is moderate, and geochemistry is needed

Level: Speculative
Basis: the existence of the source rock is inferred from geological arguments alone
How it is used: the source is the main risk of the play

Level	Basis	How it is used
Known	geochemistry has proved the link between the oil or gas in the accumulations and a particular source rock	a basis for assessing plays where the source is no longer a risk
Hypothetical	the source rock has been studied, but its link to the accumulations has not been proved	the source risk is moderate, and geochemistry is needed
Speculative	the existence of the source rock is inferred from geological arguments alone	the source is the main risk of the play

Study goes on at four levels: the sedimentary basin (where there is a sequence of sediments), the petroleum system (what has already happened to the hydrocarbons), the play (where to look for new accumulations of one type) and the prospect (where to drill). The first two levels are mostly geological. The last two are economic, because a play and a prospect have estimates of volumes, chances and cost.

Connections to other topics

Source rocks, maturation and migration — how organic matter turns into oil and gas, and how hydrocarbons migrate.
Reservoir and seal — what makes a rock a reservoir or a seal, and how much oil a seal can hold.
Traps and timing — types of traps, and why the time of their formation decides the fate of an accumulation.
Geological risk and chance of success — how the chances of the elements are estimated, and how their dependence is taken into account.
Seismic interpretation — how traps are mapped from seismic data and leads are turned into prospects.
Volumetric estimation of hydrocarbons in place — how the volume of hydrocarbons in a mapped trap is estimated.
Expected monetary value and preference theory — how the chance of success enters the economic evaluation of an exploration well.

Check yourself

1
A trap formed 20 million years after the end of the main generation and migration of oil in the area. What should we expect most likely?
1. A.The trap is empty, or it holds only late batches of hydrocarbons and the products of remigration
2. B.The trap is full of oil, and time does not matter
3. C.The trap is certainly full of gas
4. D.The trap is full of formation water, so it pays to drill it
Show the answer
Correct answer: A. The trap is empty, or it holds only late batches of hydrocarbons and the products of remigration
The main mass of oil passed by while there was no trap. Only late generation (with continuing burial, more often gas) or hydrocarbons from destroyed old accumulations can fill it. So the timing of the trap relative to the critical moment is a separate element of risk.
2
How does a lead differ from a prospect?
1. A.A lead is always larger in area than a play
2. B.A lead is a discovered accumulation, and a prospect is an undiscovered one
3. C.A lead is a possible trap that has not been studied well enough to drill; a prospect has been mapped, with volumes and a chance of success estimated for it
4. D.No difference, they are synonyms
Show the answer
Correct answer: C. A lead is a possible trap that has not been studied well enough to drill; a prospect has been mapped, with volumes and a chance of success estimated for it
A lead is the stage at which a trap is only guessed at in sparse data. After detailed work, if it is confirmed and the estimates are favourable, the lead becomes a prospect ready to drill.
3
For a prospect these chances are estimated: source and migration 0.8, reservoir 0.7, seal 0.6, trap 0.8, timing 0.9. What is the geological chance of success, and what should we study first?
Show the answer
$P_g = 0.8\cdot0.7\cdot0.6\cdot0.8\cdot0.9 \approx 0.24$ . The weakest element is the seal (0.6). Improving the estimate for the seal changes the final chance the most. So the extra study goes there (for example, analysis of the thickness and continuity of the seal from seismic data and wells).
4
Why does drilling three prospects in one untested play lower the company's risk less than drilling three prospects in different plays?
Show the answer
The prospects of one play share a risk: does the source work, and is there a regional seal? If the play does not work, all three wells fail, so the outcomes depend strongly on each other. The prospects of different plays do not share elements, so their failures are independent, and the portfolio is really diversified.
5
How does a petroleum system differ from a play?
Show the answer
A petroleum system is a genetic concept (it is about origin): a kitchen of active source rock and all the hydrocarbons linked to it. It describes what has already happened. A play is an exploration model: a group of known and expected accumulations at one stratigraphic level, with common elements. It answers the question of where to look. One system can feed several plays, and one play can receive hydrocarbons from several systems.
6
What is the critical moment of a petroleum system?
1. A.The time when the seal formed
2. B.The moment when the first accumulation was discovered
3. C.The time that best describes the generation, migration and accumulation of most of the hydrocarbons
4. D.The time when the source rock was deposited
Show the answer
Correct answer: C. The time that best describes the generation, migration and accumulation of most of the hydrocarbons
The critical moment ties processes and elements together in time. Traps and seals must exist by that time, and after it the accumulations must survive. We find it by modelling the burial and heating of the source rock.

Updated: 26 October 2026