Sedimentary basins, petroleum systems, plays, and prospects can be viewed as separate levels of investigation, all of which are needed to better understand the genesis and habitat of hydrocarbons. Sedimentary basin investigations emphasize the stratigraphic sequence and structural style of sedimentary rocks. Petroleum system studies describe the genetic relationship between a pod of active source rock and the resulting oil and gas accumulations. Investigations of plays describe the present-day geologic similarity of a series of present-day traps, and studies of prospects describe the individual present-day trap. Except for the petroleum system, these terms are widely used by petroleum geologists. The procedure to identify, characterize, name, and determine its level of certainty is discussed.

A petroleum system encompasses a pod of active source rock and all related oil and gas and includes all the essential elements and processes needed for oil and gas accumulations to exist. The essential elements are the source rock, reservoir rock, seal rock, and overburden rock, and the processes include trap formation and the generation-migration-accumulation of petroleum. All essential elements must be placed in time and space such that the processes required to form a petroleum accumulation can occur.

The petroleum system has a stratigraphic, geographic, and temporal extent. Its name combines the names of the source rock and the major reservoir rock and also expresses a level of certainty— known, hypothetical, or speculative. Four figures and a table that best depict the geographic, stratigraphic, and temporal evolution of a petroleum system include a burial history chart to establish the age and critical moment of the system, a map and a cross section drawn at the critical moment, an events chart to summarize the formation of the petroleum system, and a table of related accumulations. The petroleum system can be used as an effective model to investigate discovered hydrocarbon accumulations.