Constraints on petroleum migration are inseparable from constraints on petroleum and natural gas generation. Oil generation depends on the quantity, type, quality, and carbonization level of organic matter. A minimum quantity of mature oil-like bitumen is needed in a unit volume of source bed before expulsion can begin. To generate this bitumen, the thermal history of the source bed must satisfy certain kinetic requirements.

The preferential direction of fluid movement within a source bed during the initial phase of migration is along the bedding surfaces where organic matter tends to be concentrated. The fluids move along surfaces of weakness such as varves, laminae, lenses, and partings.

Expulsion is expedited by (1) surface-active agents that reduce interfacial tension and improve oil-mobility; (2) volume reduction of kerogen resulting from loss of volatiles during generation; and (3) temporary permeability increases along bedding surfaces and microfractures opened by the substantial pressure buildup accompanying fluid generation.

Expulsion requires overpressuring within a sealed source system. Overpressuring is a natural consequence of generation and is due primarily to the appreciable volumetric increase of generated fluids compared to the original organic matter; this increase more than compensates for the reduced kerogen volume.

Overpressuring can cause microfractures and can locally open existing fractures and fault surfaces to fluids. Microfractures interconnect the near-horizontal migration avenues, permitting upward and/or downward fluid migration through the source system to carrier beds and reservoirs.

The effects of overpressuring are transient. After a migration surge reduces pent-up pressure, the parted surfaces along the bedding, the microfractures and faults reseal, permitting another pressure build-up and pulsed release. This process recurs until the generation cycle has declined to a level where pressures do not increase sufficiently to open the surfaces of weakness in the system. Thus, the expulsion mechanism is an ineluctable adjunct of generation.

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