ABSTRACT: Hydrocarbon Habitats of the Middle and Upper Magdalena Valleys, Colombia

Steven Schamel

The Magdalena River flows northward across the Colombian Andes, traversing a series of en echelon, sediment-filled structural depressions. Until the late Miocene, these basins were 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 middle Tertiary; and, more recently, an array of intermontane of successor basins. Exploration activity since 1918 has resulted in the discovery of more than 2.6 billion bbl of oil and 2.7 tcf gas--more than one-half of the total oil and about one-third of the total gas reserves of the country.

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 periocratonic trough along the northwestern margin of the Guyana shield during the Cretaceous. In the south, nearer the paleogeographic margin of the trough, shallow marine sands (Caballos and Monserrate) bounding the Cretaceous marine megacycle are the prime reservoirs. To the north, nearer the axis of the trough, Cretaceous sand reservoirs are absent and production is almost exclusively from middle Tertiary molasse deposits. A wide variety of structural and stratigraphic traps developed during or prior to peak of maturation of the Cretaceous source beds. Recent discoveries of 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 has 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 with improved production techniques, current estimates of remaining ultimately recoverable reserves will be revised upward.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990