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ABSTRACT

In the northern Gulf Coast offshore region, USA, some 10 billion barrels oil and 130 trillion cubic feel gas have been discovered. Most gas and all oil analyzed are of thermal origin. All oils analyzed have a common chemical origin. Mesozoic (Cretaceous/Jurassic) and/or Paleocene marine shale and carbonate are the most likely common source beds. Reservoir rocks of Miocene to Pleistocene ages, however, are thermally immature. Thus, major vertical migration processes are involved in the development of all offshore oil and gas fields. Where the potential source beds are thermally mature today, a one-stage migration history has occurred. This applies to deep-water areas in the Gulf and to areas near the present coast line. Where Pre-Miocene potential source beds today lie deeper than about ?35,000 feet, source beds most likely are over-mature today. Here, oil and gas migrated first into Mesozoic to Miocene reservoirs: subsequently these primary traps leaked partially. Their oil and gas escaped into overlying late Miocene to Pleistocene thermally immature reservoirs: a two-stage vertical migration process has occurred.

Acceptance of these observations will lead to a major new play exploring the numerous mapped but undrilled structural anomalies present in Miocene strata on the Texas and Louisiana shallow-water shelves. Reservoirs in this new play will be Miocene sandstones, hydrocarbons are obtained from pre-Miocene source beds, traps were formed in pre-Miocene, Miocene and post-Miocene times. The play is decisively structural at this time; stratigraphic parameters will be established as new data on Miocene sedimentology are obtained. Critical vertical and lateral migration pathways can here mapped today using gravity, magnetic and seismic data.

This approach leads to early definition of regional structure and of cross fault trends, the most likely vertical migration paths for oil and gas. Subsequently, more detailed structural definitions and wildcat drilling will follow in smaller lower-risk areas. The acreage situation in the region is mobile and positive. Any discovery of oil and gas in Shallow waters of the Gulf Coast shelf will have a major economic advantage over discoveries of the same or larger Held size but located in deep waters off the shelf.

Vertical two-stage migration as major geological process is present also in other parts of the world. Huge oil and gas reserves have already been found in such areas, even prior to full recognition of the underlying geological processes. Examples are readily available from Brazil, NW-Europe, Trinidad, and East Venezuela.

Two-stage oil and gas migration already has become a practical and economically important exploration concept of worldwide application. On the Texas and Louisiana shelves an additional 10 billion barrels oil and 130 trillion cubic feet gas could be discovered in this play.

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