ABSTRACT

Oil and gas accumulations are closely related to the environment of deposition of the containing rocks. Rapid sedimentation in deltas where organic production was great appears to have been optimum for petroleum generation and preservation in terrigenous clastics. The fact that many large accumulations are in such deposits means that there has been little migration, lateral or vertical, of oil or gas, and that the source material was in the sands and adjacent silt and shale. It is believed that this source material was converted into petroleum soon after burial of the sediments, and that it was concentrated mostly in lenticular sands which were deposited in distributary channels and in the delta fringe areas. Later developement of "structures" in many of the deltaic areas caused only slight adjustment of the oil and gas, and most of the accumulations would be where they originally accumulated even if there had been no local uplifts.

Examples of petroleum accumulations in deltaic deposits are described and illustrated. It is pointed out that porous strata deposited in many nondeltaic environments almost never contain oil or gas even where the beds form "traps" on local uplifts.

Stratigraphic analyses should be made of each potentially productive formation in all sedimentary basins as a basis for assessing the oil and gas possibilities. Recognition of the environmental control of hydrocarbon occurrence can guide exploration to the most favorable areas.