Our genetic classification of petroleum systems is founded on a simple working nomenclature that consists of combining qualifiers from each of the following three categories: (1) charge factor (supercharged, normally charged, or undercharged), (2) migration drainage style (vertically drained or laterally drained), and (3) entrapment style (high impedance or low impedance).
The charge factor is estimated on the basis of the richness and volumetrics of mature source rocks. The source potential index (SPI), which combines source-rock richness and thickness into a single parameter, is a convenient shortcut for comparing the petroleum potential of diverse source rocks containing dissimilar kerogen types and for rapidly estimating regional charging capacity. In extensively explored basins, a positive correlation exists between the magnitude of SPI and basin-wide petroleum reserves.
The migration drainage style is determined from the structural and stratigraphic framework of a basin. Vertical-migration drainage, which occurs mainly through faults and fracture systems breaching a seal, is characteristic of petroleum systems contained within rift basins, deltaic sequences, salt-dome provinces, wrench basins, and fold-and-thrust belts. In contrast, lateral-migration drainage is dominant wherever stratigraphically continuous seal-reservoir “doublets” extend over a very large area in a tectonically stable province (e.g., commonly foreland or intracratonic platform basins). Recognition of the dominant migration style helps to predict the location of zones of petroleum occurrence in relation to the “hydrocarbon kitchens.”
The entrapment style, which is also dependent on the structural framework and the presence and effectiveness of seals, describes the degree of resistance (i.e., impedance) working against dispersion of the petroleum charge.
Application of these working concepts should help to significantly reduce geologic risk, particularly in new ventures-type exploration.