Province, basin, system, play, and prospect

Series Exploring for Oil and Gas Traps Treatise in Petroleum Geology Traps, trap types, and the petroleum system Petroleum systems Leslie B. Magoon, Edward A. Beaumont Web page AAPG Store

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.

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 the table below

Item to be Compared Sedimentary Basin Petroleum System Play Prospect
Investigation Sedimentary rocks Petroleum Traps Trap
Economics None None Essential Essential
Geologic time Time of deposition Critical moment Present day Present day
Existence Absolute Absolute Conditional Conditional
Cost Very low Low High Very high
Analysis Basin System Play Prospect
Modeling Basin System Play Prospect

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.[1] Other differences are listed in the table above. 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:

${\mbox{PS}}_{{{\rm {total}}}}={\mbox{PS}}_{{{\rm {partial}}}}+{\mbox{CP}}_{{1}}+{\mbox{CP}}_{{2}}+{\mbox{CP}}_{{3}}$

where:

• PStotal = petroleum system with all accumulations discovered
• PSpartial = petroleum system with only some of the accumulations discovered
• CPcasing pressure1, … = 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.