The petroleum system cross section (Fig. 2B; Table I), drawn at the critical moment, or time when most of the hydrocarbons were generated, shows the geographic and stratigraphic extent of the petroleum system and how each rock unit functions within the system to distribute the oil and gas. Stratigraphically, the petroleum system includes a petroleum source rock, reservoir rock, seal rock, and overburden rock. This cross section is in contrast to structural or stratigraphic cross sections.
The presence of adequate overburden rock in the correct geometry provides (1) the burial needed to thermally mature the source rock, (2) the up dip vector needed for oil and gas to migrate to shallower depths, (3) the burial depth variations needed to form traps for petroleum accumulations, and (4) the burial depth of accumulations that allow for the preservation or biodegradation of oil. If the history of the petroleum system is to be correctly modeled, the age, thickness, and erosional history of the overburden rock is required. The cross section, drawn to represent the end of the Paleozoic (250 Ma), shows the geometry or structural style of the essential elements at the time of hydrocarbon accumulation, or critical moment, and best depicts the stratigraphic extent of the system (Fig. 2B).