AUTHOR

Harry Doust � Vrije Universiteit, DeBoelelaan 1085, 1081 HV Amsterdam, Nether-lands; harry.doust@falw.vu.nl

Harry Doust is a retired explorationist, currentlyserving as professor of regional and petroleumgeology at the Vrije University of Amsterdam,where he teaches Master of Science classes and

GEOHORIZONS

The exploration play: What dowe mean by it?Harry Doust

supervises students. His main research interestlies in the study of whether and under whatconditions standard phases in the evolution ofpetroliferous sedimentary basins can be rec-ognized. From this basis, he investigates therelationship between basin evolution and theoccurrence of common petroleum system andplay types. Before his retirement, Harry spentmore than 30 yr working with Shell Internationalin and on many parts of the world, particularlyin the evaluation of new ventures. He has pub-lished widely on regional explorationevaluations.

ACKNOWLEDGEMENTS

The thoughts expressed articulate several as-pirations that I felt during the years conductingnew-venture evaluations, and they are verypersonal. Nevertheless, these thoughts wouldhave been presented with much less claritywithout the help of two of the reviewers, towhom I am very grateful: Raymond Sorensen,for providing me with important historicalperspectives on the history of the play concept,and Peter Rose, for a large number of thought-provoking and insightful comments.The AAPG Editor thanks the following reviewersfor their work on this paper: Michael J. Di Marco,Peter R. Rose, and Raymond P. Sorenson.

ABSTRACT

Among the oil and gas exploration community, the “play” hasan almost mythical status—the successful play is the thing ofwhich legends aremade, and playmakers are regarded as heroesof the industry. But what is the play exactly and why do weneed it? Curiously, considering the long period it has been indaily use, it has never really been unambiguously defined and,as a result, it has come to be used differently for differentpurposes. Although the term is in common use, in practice, itsimprecision commonly leads us to simply ignore its significanceas a concept in our rush to concentrate on prospect definition.These questions arise: Do we really need the play, and can wegain by defining it more precisely? I strongly believe that theplay concept is such a valuable one that it should be central toexploration decision making; clustering petroleum accumula-tions into natural families helps us manage the risks inherent innew and existing venture evaluation. However, I believe thatplays can best help us in this way if they comprise meaningfulnatural groups that we can use both for reliable analog com-parison and in meaningful statistical analysis. In this review, Ipropose a three-tier hierarchic framework for play definitionbased on (1) the petroleum charge system, (2) the reservoir-seal formation pair or lithofacies, and (3) the trap type. Thesetiers can be related to the geodynamic, sedimentary, and tec-tonic events that drive phases in basin evolution, thus placingthe concept directly in its geologic context.

Copyright ©2010. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received October 12, 2009; provisional acceptance January 12, 2010; revised manuscriptreceived March 4, 2010; final acceptance June 30, 2010.DOI:10.1306/06301009168

AAPG Bulletin, v. 94, no. 11 (November 2010), pp. 1657– 1672 1657

WHAT IS A PLAY?

In the context of oil and gas exploration, the word“play” has been around for almost a century atleast. Precisely when andwhere it was bornmay beobscured in the mists of exploration history, butfor true-blood petroleumexplorers, it has long beena concept dear to the heart. Similar to many prac-tical concepts, the play appears to have arisen spon-taneously and to have evolved informally with thedevelopment of the petroleum industry. In the1960s, its use was stimulated by the emergence oftwo important concepts that prompted many ex-plorationists to recognize the value of clusteringsimilar fields and drilling opportunities into groups:the first was related to the extensive application ofreservoir depositional models, the second to under-standing of the vital link between the presence ofmature source rocks and the generation and mi-gration of petroleum.

In this review, I look at the term’s meaning andusage and the cause of its popularity with playersof the exploration game.

For many, the word may conjure up the ex-citement generated by finding a new petroleum ac-cumulation and then following it up successfully,much as our ancestorswould haveworked aminerallode. It raises images of the heroic geologist chasinga trend, being a daring explorer, andmaking a name(and perhaps earning a fortune; see Rose, 2000). Ibelieve thismay provide a clue to one of its greatestassets, namely that for each explorer, it has a verypersonal implication. Ask any petroleum geologistwhat he or she understands by the word and youwill probably, within limits, get an individual defi-nition based on his or her aspirations or experience.

Putting it in other words, the play answers to avery human need; it is a popular and useful con-cept precisely because of its flexibility. Although ageneral agreement exists that the play describesgroups of accumulations and prospects that re-semble each other closely geologically, sharing sim-ilar source, reservoir, seal, and trap conditions, aninherent lack of precision in this exists, whichmeansthat we all can use the termmore or less to serve ourindividual needs. To quote Magoon (1995, p. 86),“Depending on the objective of the explorationist,

1658 Geohorizons

the play concept… can have any degree of geologicsimilarity.” This flexibility is both its strength andits weakness—it almost certainly has contributed toits longevity and widespread use, but it also meansthat a significant scope for limiting its value throughmisunderstandings is present. The play concept isused in day-to-day exploration strategy planning,risk management, and acreage evaluation, wherepredictions of play development are commonly in-voked in support of venture proposals. It is alsowidely used in near-field–type prospect analysis, somany good reasons exist to ensure that the conceptworks well.

WHY DO WE NEED PLAYS?

The play represents the geologist’s attempt to rec-ognize patterns in petroleum occurrence that canhelp him or her predict the results of future ex-ploration. By recognizing groups of known petro-leum accumulations in which the parameters thatwe consider essential to success (source, reservoir,and seal rock character and formation, plus trapconfiguration) are shared, the geologist develops acontext for learning from previous experience andapplying it to areas or prospects currently underconsideration for investment or drilling. It formsthe context for building on success or recognizingfailure. The concept is vital for this reason, and itcontributes to strategic decisionmaking, especiallyin the following areas:

• Identification of where and to what objectivefuture exploration activity should be directed,that is, which areas or trends are worth follow-ing up and are likely to become core future pro-ductive areas;

• Management of the risks associatedwith drillingmapped prospects by grouping them into fam-ilies and comparing themwith successful analogfields, thus improving the success rate;

• Prediction of numbers of future discoveries andtheir possible volumes using successful analogfields or statistical techniques;

• Helping estimate the potential value of exploringin areas or for particular prospect types;

• Identification of the technologies needed to ex-plore for particular types of prospects, as well asthose needed to maximize the commerciality ofdiscoveries through field development;

• Deciding when a type of prospect is no longerworth pursuing or when an exploration ventureshould be terminated.

The implication of these points is that playsinstead of individual prospects should form thebasis of exploration strategy. After all, for mostexplorers, the most important decisions concernthe areas and trends to investigate rather than ofwhich individual prospects to drill (Snedden et al.,2003). This point has been repeatedly emphasizedby Rose (2001), among others, and seems to beparticularly relevant at present. Recent years have,in the view of many, seen an overemphasis on theprospect as the focus of explorationplanning, thanksto increasing attention being placed on high-quality(especially three-dimensional) seismic data in re-fining subsurface structure and directly predictingpore fill, thereby reducing the risks associatedwithdrilling prospects. For several years now, the pre-cision with which seismic data allows imaging oftrap geometries has meant that other contributingparameters less amenable to precisionmapping haveassumed something of a secondary role, and thishas probably meant that the play concept has beenused less effectively than it could be.

ATTEMPTS AT DEFINITION

The word “play” appears in several AAPG Bulletinarticles from the 1930s, perhaps the oldest beingthat of Lahee (1933). His discussion of geologi-cally distinct wildcat plays from northeast Texassuggests that the concept was at that time alreadybeing used inmuch the sameway as we understandit today. By the end of the decade, the concept wasbeing widely adopted in annual regional reviews inthe Bulletin (Sorensen, 2009, personal communi-cation), but the first systematic mathematical anal-ysis of plays was conducted by the Canadian Geo-logical Survey only in the early 1970s. It was laterformulated by Baker et al. (1986) as “a group of

prospects as well as oil and/or gas fields, all havingsimilar geologic origins—a family of geologicallysimilar traps” (Rose, 2001, p. 58). Many authorshave attempted to define the play, however, as theexamples below indicate. They give a flavor of thevariety of concepts and usage envisaged:

• Bois (1975, p. 87) referred to a petroleum zone(close to many definitions of play) as “A contin-uous portion of sedimentary volume which con-tains pools showing the following characteris-tics: (1) reservoirs within the same productivesequence occur throughout the zone, (2) hydro-carbons are of the same chemical composition,(3) traps are of the same type.”

• Miller (1982, p. 55): “A practical meaningfulplanning unit around which an integrated explo-ration campaign can be constructed. A play hasgeographic and stratigraphic units and is con-fined to a formation or group of closely relatedformations on the basis of lithology, depositionalenvironment, or structural history.”

• Petroconsultants, 1992, personal communica-tion: “An association of proven and prospectivehydrocarbon accumulations at a specific strati-graphic level, which share similar geologic con-ditions of entrapment of hydrocarbons such aslithology and depositional environment of res-ervoir and seal, and trap type.”

• Magoon and Dow (1994, p. 7): “Prospects… de-scribe present-day structural or stratigraphic fea-tures that could be mapped and drilled. A seriesof related prospects is a play.” Magoon (1995)qualified this: “In this report, I define a ‘play’ asone or more geologically related prospects.”

• Rose (2001, p. 60): “A family of geologicallyrelated fields, prospects and leads, all of sim-ilar origin and charged from common petroleumsource beds.”

• Allen and Allen (2005, p. 407): “A family ofundrilled prospects and discovered pools of pe-troleum that are believed to share a commongross reservoir, regional top-seal and petroleumcharge system.”They also suggest that “plays areessentially reservoir defined.”

• Gluyas andSwarbrick (2008, p. 169):Apetroleumplay “comprises a seal and reservoir combination

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coupled to a mature source rock” and a “playfairway” represents an area where a play can beexpected to work and that can be mapped.

Despite the differences of emphasis, a basic con-sensus clearly exists that plays are groups of relatedhydrocarbon accumulations and/or prospects, char-acterized by combinations of similar geologic param-eters suchas charge type, reservoir-seal couple,withorwithout trap style, essential to the processes of pe-troleum generation, migration, accumulation, and re-tention. Plays should have a clear geographic distri-bution that can be defined by polygons on the groundand/or be confined to limited stratigraphic intervals.

However, different authors tend to group ac-cumulations at different levels depending on theirobjective. In an excellent review of the petroleumgeology of the North Sea province, for example,Spencer et al. (1996, p. 347) allocated all of the fieldsfrom the multiple levels and trap types present into10 hydrocarbon plays. They noted that their “ap-plication of (the play) concept is subjective,” andthey admitted that “many of the 10 plays may begroups of plays,” but that for the purposes of theirconcise overview, this was a convenient division.

If plays are to be used as strategic elements ofexploration, they must have the potential to be-come commercial ventures, and in this respect, theconcept is a mixture of the commercial and thescientific. Magoon andDow (1994) illustrated thisneatly by highlighting “four levels of petroleuminvestigation,” and contrasting the sedimentary ba-

1660 Geohorizons

sin and petroleum system levels, which are purelyscientific concepts, with the play and prospectlevels inwhich economics become very important(Figure 1). After reviewing several definitions,Magoon and Sanchez (1995, p. 1733) noted that theunderlying objective of play definition “is to find un-discovered petroleum accumulations at a profit.”

RELATIONSHIP TO THE PETROLEUM SYSTEM

Magoon and Dow (1994, p. 10) formalized thedescription of where and how petroleum accumu-lations occur (hitherto commonly referred to as hy-drocarbon habitat) by incorporating it into a scien-tific concept—the petroleum system. They definedthe latter as “a natural system that encompasses apod of active source rock and all related oil and gasand which includes all the geologic elements andprocesses that are essential if a hydrocarbon accu-mulation is to exist” (Figure 2). It has proven to bea very popular context for understanding the dis-tribution of petroleum accumulations and predict-ing where and under what conditions future dis-coveries could be expected.

Although if we include the commercial aspect,the play will not, strictly speaking, be a scientificconcept, in practice plays form the families of ac-cumulations within a petroleum system. In the ex-ample petroleum system described by Magoon andDow (1994), the fictitious Deer-Boar(.) petroleumsystem, the fields identified share the same sourceand reservoir formations, but at least four trappingconfigurations, or families of petroleum accumula-tions can be recognized within it. Thus, the follow-ing could be distinguished as plays within this pe-troleum system (Figure 2): (1) dip-closed anticlinalfields (Raven and Big Oil fields), (2) hanging-wallfault-closure fields (Just, Owens, andHardy fields),(3) pinch-out fields at the updip edge of the res-ervoir (Marginal and Lucky fields), and (4) thrustedanticlinal fields in the fold belt (Teapot field)—thisplay also seems to lie in a different reservoir facies.

Eachof these represents a distinct play, inwhichthe source rock, reservoir or seal, and trap type areshared. Each of these plays is assumed to representcommercial or potentially commercial ventures. In

Figure 1. Magoon and Dow’s “four levels of petroleum in-vestigation,” which contrasts the sedimentary basin and petro-leum system levels—purely scientific concepts—with the play andprospect levels, in which economic issues become very important.From Magoon and Dow, 1994, used with permission of AAPG.

Figure 2. Example petroleum system cited by Magoon and Dow, 1994, the fictitious Deer-Boar(.) petroleum system. Illustrated here are a map, geologic section, and events chartassociated with this petroleum system. From Magoon and Dow, 1994, used with permission of AAPG. Four distinct plays can be recognized within this petroleum system, three of whichshare the same reservoir; dip-closed anticlinal fields, hanging-wall fault closed fields, reservoir pinch-out fields, and a thrusted anticline field (see text for discussion).

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practice, this may be interpreted to mean that oilor gas flow and/or production on test should havebeen demonstrated.

Magoon and Sanchez (1995) proposed that byconsidering already discovered accumulations suchas producing fields as integral elements of the pe-troleum system, it may be easier for explorationmanagement to separate the risks associated withknowledge of the petroleum system itself fromthose of drilling new prospects. Later, Magoon andBeaumont (1999, p. 3–26) described plays as “oneor more geologically related prospects” that may“define a profitable accumulation of undiscoveredpetroleum,” and they refer to the evaluation of theassociated exploration risk as the “complementaryplay.” This approach differs from the manner theconcept is frequently used in the industry, wherefields, prospects, and leads are commonly all in-cluded in the definition (see, for instance, some ofthe definitions previously quoted and especially,Rose, 2001).

This is a very important issue and goes to theheart of how the play is used in practice. The ap-proachofMagoonandBeaumont (1999) implies thata petroleum play would comprise two categories:

1. Families of commercial or potentially commer-cial accumulations that have been identified bydrilling (e.g., oil or gas fields) and that form partof the petroleum system.These provenplays canbe used in exploration as analogs and in analysisof risk and uncertainty.

2. Groups of undrilled prospects within the bound-aries of a petroleum system, sufficiently similarto an existing play that, following drilling (andassuming success), can be demonstrated to be-long to the same proven play. These comple-mentary plays (sensu Magoon and Beaumont,1999)would form the objectives for exploration.

Although I believe that this approach clarifiessome of the ambiguity around the definition ofplays, practical explorationists are likely to find, aswas noted by Rose (2001), that this separation ismore semantic than real and hinders the applica-tion of analogs and models. This is likely to be thecase, for instance, where undrilled prospects can

1662 Geohorizons

conditionally, but with confidence, be included inthe same category as proven plays, clearly havingshared characteristics.Whether undrilled prospectsand proven fields are grouped together as commonelements of a play, or whether they are kept asseparate categories (as previously discussed) willprobably depend on one’s perspective. Exploration-ists involved in active acreage evaluation will prob-ably prefer the former, whereas those conductingresearch or resource estimates are more likely toretain a distinction (see discussion in Rose, 2001).

In summary, the play concept can be accom-modatedwithin the petroleum system and actuallycan describe all of the commercial and potentiallycommercial accumulations as well as the prospectswithin it, grouped into families with shared geo-logic parameters. Assuming that plays are definedin a manner that approximates to natural popula-tions (or at least the commercially significant sub-sections of natural populations), the concept canbeboth a scientific and economic one and, as such, itcan provide a basis for geologically based estimatesof petroleum volumes yet to be found.

CURRENT USAGE

The play concept is commonly used to address twomain functions in exploration, both of which in-volve prediction and risk evaluation: (1) By group-ing already discovered accumulations into plays,predictions of the presence, distribution, and likelyperformance of remaining exploration opportunities(including the possible numbers of future discov-eries) be they areas, trends, or individual prospectscan be made, while (2) estimates of the possiblevolumes of petroleum resources thatmay be foundin the future in particular basins, provinces, levels,sedimentary facies, or trends can be attempted.

The Presence, Distribution, and LikelyPerformance of Exploration Opportunities

This function typically addresses planning and day-to-day ormedium-term investment decisions, such asthe chance of success and cost of drilling a prospector series of prospects, whether to invest long-term

in a particular basin or whether to bid for a par-ticular exploration opportunity. To help these de-cisions, active explorers identify the various typesor families of prospects present or thought to bepresent and estimate the geologic chance that es-sential parameters such as charge and reservoir arepresent and active. The chance factors applied arethen typically calibrated against proven accumula-tions belonging to the same plays. Detailed studyof the latter helps identify the reasons for success orotherwise in comparable drilled situations, so playdefinition should be based on detailed study ofexisting fields, accumulations, and failed wells andbe as comprehensive and unambiguous as possible.This type of analysis probably represents one of themost common day-to-day applications of the con-cept. It is important here to remember that theappropriateness and reliability of analogs used willdepend on howwell known andwell described theparticular play is in that basin or province—if so farunproven in a particular area (although provenelsewhere in the basin), a significant chance that theplay will fail is present.

Commonly, the issues associated with evaluat-ing prospects for drilling are presented on play sum-marymaps, whichWhite (1988, p. 944) noted “showthe superimposed boundaries for all geologic con-trols on oil and gas occurrence.” White added thatit is only at play level that all geologic hydrocarboncontrol factors can be properly mapped and in-terpreted, hereby emphasizing the importance ofplay analysis in exploration. Play fairway mapping(Gluyas and Swarbrick, 2008) is a similar means ofidentifying where a play can be expected to work:Within the fairway, areas of different risk can bedesignated by the construction of “common risk-segment” maps, whereas outside the fairway, thereservoir-seal couple may not work or no chargemay be possible.

A good example of a methodology for evalu-ating the chance of success associated with a play isgiven in Allen and Allen (2005): A play map isconstructed and used to identify areas of equalchance caused, for instance, by variations in charge,structural development, or effective reservoir. Playsthat have been demonstrated to be commercialin a particular area carry no risk in that area by

definition—the essential parameters are all present,although not necessarily all of equal quality. In thesecases, the chances of drilling a dry hole are limitedto factors specific to the particular prospect or lo-cality, such as individual fault-seal characteristicsand local access to timely charge.

Some authors consider the geometry of the trapto be an individual characteristic of each field orprospect, and they exclude the trap from play defi-nition, as is evident from some of the definitionspreviously quoted. Although I agree that, in detail,each individual trap-forming structure has charac-teristics specific to it, several distinct categories arecommonly recognized at a more general level. Agood summary of the types of traps that can prop-erly be used in play definition is given by Biddle andWielchowsky (1994). In the end, I believe that traptype or style is just as important in distinguishingdifferent families of petroleum accumulations andprospects and grouping them into plays for analogand risking purposes as are charge and reservoir.

Petroleum Resource Evaluations

Because plays form meaningful and convenientdivisions in which to cluster the resource base ofpetroleum systems and sedimentary basins, theyare widely used by planning groups in governmentinstitutions and industries for the probabilistic pre-diction of long-term undiscovered resource vol-umes. Such organizations, for example, the U.S.Geological Survey, make use of analysis of statis-tical techniques such as creaming curves and field-size distribution charts in their predictions. For this,an essential requirement is a sufficiently large dataset to be statisticallymeaningful, and to achieve this,larger units than plays (as previously defined) areused. The U.S. Geological Survey (Klett et al.,2000, under sectionGlossary, p. 6); Schmoker andKlett, 2000) uses assessment units or volumes ofrock comprising parts or all of total petroleum sys-tems. They define plays in much the same way asthose previously cited:

“Play: A set of known or postulated oil and gasaccumulations sharing similar geologic, geographic,and temporal properties, such as source rock, mi-gration pathway, timing, trapping mechanism, and

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hydrocarbon type,”but note that “a play differs froman assessment unit; an assessment unit can includeone or more plays.” Geologic analysis of both isused in probabilistic statistical estimation of un-discovered oil and gas resources: This makes it es-sential that plays are sufficiently precisely definedthat they are comparable both within the assess-ment unit and in different basins.

Ehrenberg et al. (2008) reported that a similarapproach was used by the Minerals ManagementService the United States. For their 2003 resourceassessment of the offshore Gulf of Mexico, theygrouped plays into larger units, also called plays de-fined on the basis of reservoir rock stratigraphy.Their classification recognized the following playtypes: Progradational, aggradational, aggradational-progradational, structural, retrogradational, fan 1(shelf edge), fan 2 (basin floor), and upper Pleisto-cene fan fold belt. Note that these plays are definedby somewhat mixed criteria, but several other au-thors also use a mixture of categories to define theirplays. An example is Montgomery (2005), who,reporting on the U.S. Geological Survey resourceassessment of the Arctic National Wildlife Refuge,identified 10 plays, grouped under the “topset, tur-bidite, thin-skinned thrust belt and wedge plays.”This division was presumably found to be conve-nient for this particular assessment, but I believe thatit makes wider comparison of plays more difficult.In a series of other articles describing plays in theUnited States,Montgomery seems to have used theterm to describe a prospective fairway (Montgomeryet al., 1999) and as a new potentially prospectivelithofacies (a submarine fan system) within a pro-ductive formation (Montgomery, 1997).

Creaming curve and field-size distribution anal-ysis at a play level are typically conducted on alreadydiscovered fields. They are used to identify the po-tential volume scope of future discoveries in thatplay and to indicate the potential size class intowhich such new discoveries may fall (Rose, 2001).They thus have a direct impact on predicting thevolumes and field sizes anticipated to be presentin complementary plays (Magoon and Beaumont,1999), although statistical analysis of the latter isincreasingly used to gain insight into likely meanfield sizes, for instance.

1664 Geohorizons

Snedden et al. (2003, p. 6) reviewed the perfor-mance of several plays from a sequence-stratigraphicperspective using creaming curves and used themto demonstrate how jumps in the plots can be cor-related with the exploration of new plays. Theirplays comprise thick stratigraphic intervals or trends,including theMiocene trend in theGulf ofMexico,the Middle Jurassic play of the North Sea, and thePliocene–Miocene play of the Kutei Basin of In-donesia. They recognized that for their purposes,the play represents “an assemblage of several plays,each with its unique characteristics of reservoir,entrapment etc.,” but that staying at a fairly generallevel was necessary to meet the criteria (sufficientsamples) for statistical analysis.

Reviewing these, it seems that three approachesto the division of plays are currently in use to ad-dress the various, commonly pragmatic, require-ments of the explorer:

1. Stratigraphic definition of plays (e.g., Oxfordian,middle Miocene), where the age of the mainreservoir determines the play. Structure and sub-divisionswithin the reservoirmaydefine subplays.

2. Facies definition of plays in which the sedimen-tary facies (lithology or depositional environmentof the reservoir) determines the play. Subplaysare commonly defined by stratigraphy (forma-tion) or type of structure.

3. Structural definition of plays, whereby the trap-ping geometry (commonly structural style) de-termines the play. In this case, stratigraphy orfacies define subplays.

Although these categories of play have beenmade for commendable purposes, it doesmean that

1. The term “play” is commonly used very infor-mally, and this implies that no structuredmeansto compare plays exists other than rather locally.Better definition of plays would help us learnhow they are developed in different situationsworldwide and make it easier to understand bet-ter what is crucial to their success.

2. No guarantee exists that plays as previously de-fined represent natural populations that can besubjected to statistical analysis.

I suggest that by standardizing how we use theterm, we could compare and contrast plays withmore precision, enlarge the potential to makemeaningful use of a wider selection of analogs, andgain a better understanding of the uncertaintiesinvolved in exploring undrilled prospects.

A RECOMMENDED APPROACH

As has been previously shown, current usage of theterm “play” is often informal and varies according todifferent authors’ objectives. To make the best useof the concept, however, wewould probably like tosee that plays as defined by ourselves and others,especially in publications, can be directly comparedand used without modification. To achieve this, Istrongly feel that, individually,within organizationsand industry-wide, geoscientists should endeavorto develop and adhere tomore strict playdescription.

I believe that this can be achieved through theuse of a more standardized hierarchic system forplay definition. At the broadest scale, the presenceand development of plays can most easily be com-pared and used for predictive purposes by identify-ing their place in basin history and/or the tectono-stratigraphic context in which they occur (Doust,2003). Particular play types commonly characterizeparticular phases in basin history; for example, tiltedfault block plays are most common in the preriftto synrift phases of basin history (see, for instance,Spencer et al., 1996), whereas specific reservoirlithofacies were shown to be associated with dif-ferent stages in basin evolution in Southeast Asianbasins by Doust and Sumner (2007).

Within this context, Doust (2003) recom-mended that plays should be defined by threecharacteristics at three levels corresponding to dif-ferent essential parameters. These levels span dif-ferent scales from more widespread to more localand can potentially give an ordered and unambig-uous context for their definition. The first two levelscorrespond in many ways to the manner in whichpetroleum systems are identified and named, thatis, by source rock and reservoir rock (Magoon andBeaumont, 1999).

1. The first and most widespread level can be de-fined by the petroleum charge system that theplay belongs to, that is, the formation fromwhichthe oil or gas has been generated or is presumedto have been generated and the migration mech-anism and timing that allow it to reach its res-ervoir destination (Magoon and Sanchez, 1995).This is commonly the most widespread param-eter in a part of a basin or area and, forming thebasis for petroleum system definition, is likely tobe shared between several different plays.

2. The second level, commonly less widespreadthan the first, would be the reservoir in whichthe accumulation occurs or is thought to occur.This can commonly be defined by the formationname, assuming that a discrete formation can beidentified. In some areas, particularly in deltaicsequences, where facies variations are rapid anddifferent lithologies are interbedded with eachother, the reservoir lithofacies may be preferredas a descriptor that defines and allows compar-ison between plays. Such an approach was usedby Doust and Sumner (2007) to describe litho-facies play types in Southeast Asian Tertiarybasins. They recognized alluvial fan, lacustrinedelta, lacustrine turbidite, and volcaniclastic res-ervoir lithofacies as characteristic of plays of theearly synrift of many of the basins. Doust andSumner (2007) then conducted statistical anal-yses on the number of fields, oil versus gas mixand volumes of oil and gas associated with thevarious lithofacies. Reservoir plays specific toeach of the basins in Southeast Asia are locallyidentified by the basin-specific formation name(e.g., Talang Akar Formation), but these mayinclude a variety of lithofacies. Reservoirs willnormally be overlain by a regional seal, so a playdefinition could comprise its reservoir/seal pairif the seal is considered to form a critical param-eter. We could, for instance, apply the descrip-tor Ekofisk/Rogaland (for plays in the CentralGraben of theNorth Sea) or Shu’aiba/NahrUmr(for plays in the Cretaceous of the Middle East).Where seals are intraformational, the reservoirformation would normally suffice, for example,Agbada (for plays in the paralic sequence of theNiger Delta).

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3. The third level, commonly the most restrictedgeographically, comprises the trap type, whichnormally forms the most specific element of aplay. Trap geometries may be limited by struc-ture or stratigraphic changes or by a combinationof the two. In some areas, the trap domain maybe geographically larger than the individual res-ervoir development, for instance, where narrowdeep-water channel sand bodies cross anticlinalstructures as in theNelson field in theNorth Sea(Underhill, 2003). In such cases, one could con-sider reversing the second and third tiers.

Using these criteria, plays should be defined bythe single feature thatmost characterizes the familyof fields and prospects, be it formation or facies orstructure type (trap dependence). In the end, playsshould be defined by the number of elements thatthey sharewith other plays (thesemight be first andsecond levels), plus one or more specific char-acteristics, which are critical to the existence of theaccumulations (third level). The widely used term“combination trap” tends to obscure this. Althoughstructure and stratigraphy both commonly con-tribute to the extent of an accumulation, for playdefinition, one should try to identify the parameterthat is dominant, the one that provides the mainreason for the existence of the field. The commer-cial element in plays implies that “producibility”must be considered as an important parameter.

Having proven plays with few or even only onerepresentative accumulation(s) in a particular ba-sin is possible. To make use of them as analogs, wewill wish to apply their play characteristics else-where, perhaps on a different continent in a se-quence of different ages. This emphasizes the needfor amore standard description.However,wemightlike to group less common accumulations into lessprecisely defined plays. A play with one or twofields or prospects is not very useful.

A MEANS TO SYSTEMATICALLYDEFINE PLAYS

In the previous section, a hierarchic framework fora standard play description has been proposed and

1666 Geohorizons

outlined. Certain characteristics of each of the es-sential parameters can, in my view, be regarded asdistinct enough to contribute to the definition ofseparate plays, and this can facilitate direct com-parison from basin to basin and from province toprovince, vastly increasing the analog value of provenplays. It is valuable to note the phase of basin evo-lution that the play occurs in, but I do not believethat this should form part of the play descriptorbecause structural trap types can be common toseveral basin phases. Plays will be locally definedby the formation names of their contributing pa-rameters, but if we are to use themmore widely asanalogs, more geologically comparable criteria areneeded.

Charge

At the broadest level, distinctly different types ofpetroleum charge arise from different source rocktypes deposited in different depositional settings. Isuggest that the following categories can be com-monly identified and would represent distinct ele-ments in play description:

• Type I source rock (primarily algal), deposited in(1) lacustrine, freshwater lake environments and(2) deeper marine and basinal environments.

• Type II source rock (primarily bacterial), depos-ited in (1) lagoonal environments, (2) restrictedmarine shelf environments, and (3) outer ma-rine shelf and slope environments.

• Type III source rock (primarily terrestrial), de-posited in (1) lowland onshore swamps (coaland coaly shale) and (2) marine shelf and deepbasin environments into which organic materialhas been introduced.

Reservoirs

At the second level, the play is commonly definedby the reservoir type. Clearly, the formation nameas such cannot form the basis for a standard clas-sification for analog use, although it should alwaysbe referred to. The following depositional faciesare commonly associated with reservoir litholo-gies or groups of lithologies and can be used to

distinguish themost important separate plays fromthis perspective:

• Siliciclastic sediments of nonmarine origin, de-posited in (1) alluvial to fluvial environments,(2) eolian or related desert environments, and(3) lacustrine or lake environments.

• Siliciclastic sediments deposited in various coast-al to shallow-water environments, including(1) coastal barrier and estuarine environments,(2) deltaic channel environments, and (3) shal-low subtidal and open shelf environments.

• Siliciclastic sediments deposited in various deeperwater environments, including (1) slope-channeland fan environments and (2) basinal channeland abyssal fans.

• Carbonate sediments, deposited in (1) coastallagoons, tidal flats, and shallow shoal environ-ments; (2) carbonate shelf environments withpatch reefs; (3) reefoid buildups (platform andpinnacle); and (4) slope or basinal carbonates.

Many other reservoir facies contribute to res-ervoirs (e.g., volcanic tuffs) and, where distinct,should be added to the list described above.

The character of the seal may be an importantelement in play description and can be commonlydescribed in terms of character and lithology incombination with the reservoir.

• Seal character: Regional, for example, basinwide,or intraformational, where multiple reservoirsand seals are interbedded.

• Seal lithology: Distinct categories include (in de-scending order of quality) salt, anhydrite, organicshale, silty shale, mudstone (carbonate), chert.

Trap Dependence

At the third level, distinct trap geometries can bedefined. Three main categories are commonly dis-tinguished; for a complete discussion, consult Biddleand Wielchowsky (1994).

• Dip-closed structures, including (1) compres-sion folds and anticlines; (2) sedimentary roll-over structures; and (3) drape or compaction

anticlines over diapirs and deeper structures,including salt (whichmay be considered to forma separate category of drape structure).

• Fault-closed structures in which the sealing mech-anism, which may rely on juxtaposition, fault-smear, or cataclasis, plays an important part,including (1) fault-tilt footwall and horst blocks;(2) simple hanging-wall closures; (3) inversionand reverse fault structures; and (4) thrustedanticlines, thin-skinned or basement-rooted.

• Closures associatedwith changes in stratigraphy,including (1) facies change and/or depositionalpinch-out; (2) buried depositional relief such asreefs, dunes, and turbidite bodies; and (3) trapsrelated to unconformities including truncation,relief, and onlap.

Again, other trap types exist (e.g., piercement)and should be distinguished as separate trap typesas appropriate.

To illustrate how such a classification could beused, I have grouped some of the accumulations inthe northern North Sea petroleum province intoproven plays using the described criteria, placed intheir basin context and defined as follows:

1. Charge formation, noting the phase in basinevolution it belongs to, and the type of petro-leum system it represents. In this case, the sourcerock is the Kimmeridgian Draupne Formation,belonging to a deep-marine type II petroleumsystem, in a synrift cycle of basin evolution.

2. Various reservoir formations or levels chargedfrom this source. They range from Devonian toOligocene in age and, thanks to the migrationpaths permitted by the rift structure, belong tothe prerift, synrift, and postrift basin cycles ofbasin evolution.

3. Trap types represented at each reservoir level.These range from faulted tilt blocks in the pre-rift sequence to more stratigraphic type plays inthe postrift.

Each distinct structural type at each reservoirlevel is populated with the fields in that category,and the play is then named after the most char-acteristic or largest accumulation (Figure 3). Thus,

Doust 1667

1668Geohorizons

Figu

re3.

GraphicrepresentationoftheDraupne-Brent(!)petro

leum

syste

minthenorth

ernNo

rthSea,illu

stratingthehierarchicmeans

recommendedplay

forp

laydefinition.The

singleactivesource

inthesynriftsequence

chargesseveralreservoirs

inprerift(pre–LateJurassic),synrift(LateJurassic),and

postrift(EarlyCretaceous–Tertiary)basin

cycles,andat

each

ofthesereservoirlevels,thefieldscan

beclassified

intoplays,dependingon

thetraptype

(indicatedhereby

carto

onso

fthe

structure).Themostimportantplay

inthispetro

leum

syste

minvolve

sfoot-wallfault-dipclo

suresintheMiddleJurassicBrentFormation.Thesuggested“typefield”ineach

play

isindicatedinred.

one of the most important plays in this area couldbe described as in Table 1.

Table 2 includes back-up information. Thisinformation can be readily compiled in EXCELformat and, linked to a field database, used for an-alog purposes. Allen and Allen (2005, p. 407) of-fered the following example as a brief descriptionof a play: “Mid-Jurassic submarine fan sandstonereservoirs in Late Jurassic fault blocks, sealed byLowerCretaceousmarinemudstones, and chargedduring the early Tertiary from Upper Jurassic ma-rine source rocks.” This covers much the sameground as the previous proposal but, being moredescriptive, can be less easily applied for analogpurposes.

This hierarchic approach has the flexibility toallow plays to be grouped in a systematic and struc-tured way to satisfy different requirements. For sta-tistical purposes, for instance, plays can be groupedat the reservoir formation or facies level, whichwould be comparable to the stratigraphic definitionas used by Snedden et al. (2003) and Ehrenberget al. (2008). They can be further integrated to the“assessment unit” or petroleum system (Klett et al.,2000) as needed.

THE PLAY AS AN ECONOMIC ANDCOMMERCIAL UNIT

Tobe of value to the explorer, the play should haveboth a geologic as well as an economic implica-tion. Ideally, it should thus be based on families ofexisting fields and potentially commercial discov-eries, which can be used as analogs to guide theeconomic evaluation of undrilled prospects thatseem to belong to the same play. This is probablythe most widespread use of the concept—not onlyis analysis of the chance of success in individualprospects facilitated, but whole play-based strate-gies can be established and monitored. Many oil orgas indications or occurrences of petroleum, how-ever, are currently noncommercial and do not havean economic status. Although some of these shouldprobably not, strictly speaking, be considered asplays, many of them will have the potential to be-comecommercial in the future, for instance, through

Doust 1669

developments in technology, so they should not beignored. Such accumulations commonly point tothe existence of a working petroleum system and acombination of factors which, under more favor-able circumstances, may result in a commercialaccumulation. A separate category of proven playshould be introduced for these currently noncom-mercial plays (see below).

Categories of plays are commonly definedwithreference to their status in the basin, province, orpetroleum system in which they occur. The fol-lowing categories may be recognized:

1. Speculative plays, where the play may not yethave been identified in a particular area but, ei-ther conceptually or by analogy with similar sit-uations in other similar basins, it is consideredthat it may be present. This category will typi-cally be associatedwith frontier provinceswheredata are sparse, will depend on the identificationof appropriate analogs, and will carry a corre-spondingly low chance of success. The tectono-stratigraphic stage in basin evolution provides avaluable context here.

2. Unproven plays, where a play may have beenidentified in an area and the concept shown towork, but as yet it has not yet been shown bydrilling to be commercial. This categorymay alsobe associated either with frontier areas or withan underexplored play in a producing area. Thisplay will have a greater chance of success, andsuccessful analogs from elsewhere will be usedto estimate its possible value.

1670 Geohorizons

3. Emerging plays, which have been shown to becommercial, include productive fields and, pos-sibly, potentially commercial prospects.Althoughthe play is proven, each prospect will carry sev-eral risks specific to the prospect. Identificationof these will rely on detailed knowledge of pro-ductive fields.

4. Mature plays, proven plays that have had a longperiod of production, representing a fairway ortrend that is in an advanced stage of explora-tion and development. The number of undrilledprospects, or complementary play (Magoon andSanchez, 1995) opportunities will typically besmall.

The first two represent exploration plays, thelast two are proven plays.

In recent years, the play concept has been in-creasingly used to categorize investment oppor-tunities other than purely geologic ones. The termis sometimes applied to opportunities representedby new technologies such as secondary or tertiaryrecovery in existing fields, whereas in other cases,the commercial aspect is dominant. Thus, for in-stance, Leonard and Ozkaynak (2000) classifiedelements in Shell’s high-level strategic themes,Deepwater, Exploration and Production (E&P)Gas,E&P Oil Focus, and Former Soviet Union (FSU)/Russia as plays. Each play then comprised severalproject elements in the Shell portfolio of globalexploration and production opportunities. Clearly,the use here is fundamentally different to that dis-cussed hitherto in this article.

SUMMARY

I have reviewed the use of the term “play” in currentuse in the industry. Although used very commonly,

Table 2. Details of Brent Play Parameters

Charge

Reservoir Level Trap Type Type Field Other Fields

Synrift marine type II source,Upper Jurassic (Kimmeridgian)

Deltaic channel and barriersands, Middle JurassicBrent Group

Eroded and slumped tiltedfootwall fault-dip closurebelow Cretaceous shale seal

Brent

Visund, Ninian, etc.

Table 1. Definition of the Brent Play

Play Name

Basin/Area Petroleum System Trap Type

Brent

Viking Graben Draupne-Brent(!) Footwall fault-dip

its loose definition results in a range of interpreta-tions that can lead tomisunderstandings, potentiallyaffecting strategic decision making. I also believethat the different ways it is usedmake it difficult tocompare like-with-like and identify either appro-priate analogs for comparison or suitable groupingsfor statistical analysis based on natural populations.

I suggest that amore disciplined approachwouldbenefit explorationists by making it possible to ac-cess and share a wider sample of unambiguouslydefined plays. Consistently described proven accu-mulations could provide us with a greater popula-tion of more appropriate potential analogs for un-drilled prospects or exploration plays.

The definition of plays can be best achievedwith a hierarchic identification at three levels. Theselevels, from more general to more specific, com-prise (1) the petroleum source formation or charge,(2) the reservoir formation or lithofacies in whichthe petroleum accumulations occur, and (3) thetrap type or geometry of the field. The first of theselevels relates directly to the petroleum system,whereas the reservoir or seal and trap type will bemore specific to the play. In areas where few ac-cumulations are in an individual play, meaningfulstatistical analyses may have to be conducted athigher levels.

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