Observations in deep sedimentary basins around the world confirm the theory of gravity-induced cross-formational flow of groundwater. Thus, geologically mature basins are hydraulically continuous environments in which the relief of the water table, commonly a subdued replica of the land surface, generates interdependent systems of groundwater flow with patterns modified by permeability differences. In these systems, meteoric waters infiltrate and move downward in upland-recharge areas, migrate laterally under regions of medium elevations, and are discharged in topographic depressions. Where flow systems meet or part, relatively stagnant zones develop.

The theory is advanced that in geologically mature basins, gravity-induced cross-formational flow is the principal agent in the transport and accumulation of hydrocarbons. The mechanism becomes operative after compaction of sediments and the concomitant primary migration cease and subaerial

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topographic relief develops. Hydrocarbons from source or carrier beds then are moved toward discharge foci of converging flow systems and may become entrapped en route where hydrogeologic conditions are favorable. Accordingly, deposits are expected and observed to be associated preferentially with ascending limbs and stagnant zones of flow systems and hence to be characterized by relative potentiometric minima, downward increase in hydraulic heads possibly reaching artesian conditions, reduced or zero lateral hydraulic gradients, and relatively high salinity of groundwater. Continuous flow of meteoric waters imports hydrocarbons into such traps until the "spillpoint" is reached. The excess becomes source material for new accumulations. However, a change in surface topography entails d layed readjustment of the flow pattern and redistribution of petroleum. As some hydrocarbons can remain in place, deposits also may occur in discharge and stagnant regions of relict flow systems.

Although its consequences may be different, the basic principles of the present theory are not incompatible with current concepts of petroleum migration and accumulation. Rather, they complement the history of hydrodynamic evolution and petroleum migration of large sedimentary basin by revealing a hitherto unrecognized, important developmental stage.

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