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Within the geologic setting of the Gulf Coast the regional movement seaward of groundwater has a significant effect on some phases of oil and gas movement and concentration. Evidence indicates that the beginning of hydrocarbon formation comes fairly soon after deposition of the sediments.

The effects of groundwater on oil or gas accumulation may be separated into two categories: (1) accumulation in shallow, relatively unconsolidated sediments with a high water content, and (2) accumulation at greater depths where shale is at least moderately compacted and where fluid movement is mainly through sandstone, siltstone or permeable carbonate rocks. Evidence of the effects of groundwater in the first case must come mainly from studies of modern sediments and from groundwater hydrology and geochemistry. In the second case, the evidence rests on data from all phases of subsurface geology and from the history as well as the distribution of hydrocarbon reservoirs, both structural and stratigraphic.

An increased understanding of both of these categories of groundwater effects on oil or gas concentration should result in a concomitant increase in exploration efficiency and success.

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