General statement

Within a sedimentary basin or thrustbelt, petroleum that is generated within a volume of active source rock migrates out of the hydrocarbon kitchen and through the basin along a commonly complex network of carrier beds, faults and fracture arrays. In most instances this carrier network disperses the petroleum to isolated cul de sacs within the sedimentary section or to the ground surface where it forms seeps (Macgregor, 1993). Less commonly, the petroleum is directed towards structural or stratigraphic features that retard its upward migration. Such features are traps, the focus of hydrocarbon exploration and development. Only some of the traps hold a sufficient volume of petroleum to warrant commercial exploitation. At each stage in the expulsion and migration of the petroleum some fraction of the original petroleum generated in the kitchen is dissipated (Fig. 19.1) so that the portion that eventually resides in commercial traps can be quite small. Biteau et al. (2003) refer to the efficiency ratio of hydrocarbons accumulated (HCA) to hydrocarbons generated (HCG) as the petroleum system yield (PSY). Different petroleum systems in the same basin or thrustbelt may have different PSY as in the Magdalena basin of Colombia, a thrustbelt setting, where the Villeta-Caballos (!) and Villeta-Monserrate (!) have reported PSYs of 6.0% and 4.9%, respectively (Biteau et al., 2003). In most thrustbelts much lower efficiency ratios should be expected. The expulsion efficiency of a source rock helps to determine not only the PSY of the petroleum system, but also the composition of accumulated petroleum.