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Deltas deposited during the Miocene, Pliocene, and Pleistocene in the vicinity of the present-day Mississippi River delta illustrate a closed system for the generation and entrapment of hydrocarbons. During those times various processes contributed to this closed system of deltas.
1. Vast quantities of sediments were derived from erosion of igneous, metamorphic, and sedimentary rocks of the high continental interior. These were transported gulfward and deposited in major deltaic masses by one or more large rivers ancestral to the Mississippi River.
2. Rapid burial in the deltas preserved stream-transported organic matter and related nutrients. The sediments also were enriched by abundant marine organic matter concentrated in the delta area by prevailing winds and ocean currents. Abundant raw materials and reducing conditions necessary for the generation and preservation of hydrocarbons were present.
3. Frequent shifting of distributaries resulted in the deposition of discontinuous lenticular bodies of sand and clay, a situation favorable for retention of locally generated hydrocarbons. In places, rapid deposition of large volumes of clay produced thick sections of under-compacted shales that are troublesome to drill.
4. Contemporaneous deformation, in the form of growth faults and folds, modified existing lenticular bodies and created structures which have trapped major quantities of oil and gas.
5. Differential distribution of the weight of the deltaic sediments, coupled with regional tilting, caused upward flowage of deeply buried Mesozoic salt beds. The resulting piercement salt domes or deep salt anticlines have contributed to the formation of other hydrocarbon traps.
In the closed system the hydrocarbons were formed almost in situ and impounded by early migration in structural and stratigraphic traps. Orogenic movements were not needed to form structural traps as these were formed by large-scale slippage, slumping, and salt movement within the deltaic masses. Thus, all the elements in this closed system worked together in a positive direction to form the giant oil and gas fields that are being exploited today. Many more remain to be found. Therefore future exploration for hydrocarbons in the Gulf Coast will be like its history--exciting, productive, and profitable.
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