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Miocene sands have produced, through 1967, approximately 6.76 billion bbl of oil and 35 Tcf of gas in about 650 fields in coastal and offshore Louisiana and Texas. More than 80% of the production has come from fields in the Louisiana segment, where all 19 giant Miocene oil fields are located.
The Miocene sediments of coastal and offshore Louisiana and Texas form a seaward-dipping and thickening wedge of interbedded marginal-marine sandstone and shallow-marine shale with maximum thickness at any locality of about 25,000 ft. A composite section in the Gulf Coast geosyncline is at least 45,000 ft thick.
Rapid sedimentation in large deltas, where there were prolific organic production and accumulation and where the organic material was preserved by rapid burial, made possible the many large Miocene petroleum accumulations. These favorable conditions were confined mainly to the Louisiana coastal and adjacent offshore areas that subsided at a faster rate than the coastal interdeltaic regions on the east and west, and confined the Miocene Mississippi River to that part of the northern Gulf basin. Downward movement along faults that bound the deeply buried salt-filled grabens also took place as the major deltas prograded, causing diapiric structures and "rollover" anticlines to form where organic-rich deltaic mud, silt, and sand were deposited.
The tectonic-sedimentation history of the Gulf Coast Miocene clearly demonstrates the close relation between depositional environments and petroleum occurrence.
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