The Magdalena River flows northward across the Colombian Andes traversing a series of en echelon, sediment-filled structural depressions. The Magdalena basins resist easy classification in that, until the late Miocene, they have been parts of much more extensive basins: an extensional, back-arc basin during the Triassic-Jurassic; a pericratonic trough during the Cretaceous and early Tertiary; the inner margin of a broad, east-facing foreland trough during the mid-Tertiary; and more recently an array of intermontaine or "successor" basins. The geologic character of the Magdalena basins is tied intimately to that of the bordering Central and Eastern Cordilleras. Since 1918, there has been nearly continuous exploration activity in the Magdalena basins resulting in the discov ry of more than 2.6 billion barrels of oil (GBO) and 2.7 trillion cubic feet (tcf) of gas--more than half of the total oil and about a third of the total gas reserves of the country. As of the end of 1989, the daily production from the basins averaged 143,432 bbl of oil and 182.8 mcf of gas.

The abundant hydrocarbon resources of the Magdalena basins are based on the presence of a thick, organic-rich limestone and shale succession (La Luna or Villeta) deposited in an extensive pericratonic trough along the northwest margin of the Guyana shield during the Cretaceous. In the south, nearer the paleogeographic margin of the trough, shallow-marine sandstones (Caballos and Monserrate) bounding the Cretaceous marine megacycle are the prime reservoirs. To the north, nearer the axis of the trough, Cretaceous sandstone reservoirs are absent and production is almost exclusively from mid-Tertiary molasse deposits. The Magdalena basins contain a wide variety of structural and stratigraphic traps, most developed during or prior to peak of maturation of the Cretaceous source beds. Recent discoveries of

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giant oil accumulations, such as the San Francisco field, were made in large, hanging-wall anticlines previously considered breached and unproductive. The testing of deeper reservoirs and new structural concepts during the 1980s have resulted in many important discoveries. From the standpoint of hydrocarbon exploration and exploitation, the Magdalena basins are not yet "mature." The potential for additional major discoveries is excellent and it is certain that with improved production techniques current estimates of remaining ultimately recoverable reserves in the producing fields will be revised upward.