Oil and gas are produced from Tertiary sandstone reservoirs in deltaic and related depositional systems in the United States Gulf Coast, Niger, and MacKenzie-Beaufort basins. In each area there is an orderly, predictable interrelationship of sedimentation, stratigraphy, depositional environment, and structure, with the characteristics, ages, and distribution of producing trends.

In comparing and contrasting the three areas, it is apparent that they have many aspects in common, resulting from the fact that they are relatively young, subsiding paralic basins on "Atlantic-type" margins. They contain thick accumulations of deltaic sediments that have prograded in regressive basin-filling sequences as the basins subsided. Therefore each has a vertical gross lithologic sequence that has shale at the base, overlain by interbedded sandstones and shales, overlain by massive sandstones. The vertical sequence is repeated laterally from basin to land. In each basin the stratigraphic units thicken basinward across a series of normal, listric, down-to-the-basin syndepositional faults, with which are associated "rollover" anticlines that form traps. Trapping associated with diapiric structures is also characteristic.

Significant differences are related to the different geologic settings and geologic histories of the basins. For example, the presence of salt in the Gulf Coast basin has resulted in a wide variety of salt-dome-related trapping mechanisms in addition to the shale diapirs and rollover anticlines common to all three areas. Pre-Tertiary tectonic settings, different in each case, control basin configurations and affect structural trends. Vertical and lateral differences in depositional systems and sequences, and in sandstone geometries, result from variations in ratios of rates of sedimentation to rates of subsidence that are, in part, tectonically controlled.

The framework for the occurrence of oil and gas is well understood in the maturely explored and intensively studied Gulf Coast Tertiary basin. Concepts developed there can be applied to developing the less-explored Niger basin and to exploring the frontier Mackenzie-Beaufort basin.

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