ABSTRACT

Oil and gas accumulations occur within or adjacent to sedimentary rocks which had abundant organic matter developed and preserved, therefore conditions during deposition determined whether hydrocarbons are present. Some other factors which are necessary for petroleum accumulations are porous rocks, such as quartzose sand, porous calcarenites, limestone reefs, fractured shale and anhydrite; and sealing rocks, such as shale, salt, and less permeable parts of the reservoirs. Structural closure is important in concentrating the hydrocarbons in most of the discovered fields because tectonically active areas were usually also the sites of depositional environments which were the most favorable for the preservation of abundant organic matter. However, there are numerous porous strata in all major sedimentary basins which are sealed and structurally closed but which have no petroleum potential because of unfavorable depositional histories.

Large deltas which were constructed in rapidly subsiding segments of oceans or large interior sea margins had all the requirements for oil and gas occurrence: plentiful organic remains; rapid deposition which preserved much of the organic matter; relatively slow compaction which allowed the hydrocarbons to escape from the fine-grained sediments and move into the more porous sediment bodies; many porous sands; abundant clay and silt to seal the sands laterally and vertically; and syndepositional development of local "highs". No other environment in the silicate clastic province had all of these favorable conditions.

Petroleum accumulations in carbonate rocks resulted from rapid deposition of organic-rich lime sediments in a fast subsiding area with shallow restricted marine environments that had periodic influx of fine-grained terrigenous sediments. Porous calcarenites or oolites which formed on shallow, open-sea platforms, or limestone reefs which grew far from shore and were never covered with terrigenous mud, had little organic material preserved to form petroleum.

Numberous undiscovered oil and gas accumulations are in stratigraphic and stratigraphic-structural traps in the favorable facies of terrigenous and carbonate formations. The basins or parts of the basins without the controlling factors should not be explored.