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SCHAMEL, STEVEN, Earth Sciences and Resources Institute, University of South Carolina, Columbia, SC

ABSTRACT: Structural Setting for Hydrocarbons in the Middle and Upper Magdalena Basins, Colombia

The structures hosting hydrocarbons in the Middle and Upper Magdalena Basins vary systematically in style and age along the length of this prolific Andean petroleum province. The striking variations in trap style and age are the result of the multiphase, but diachronous, evolution of the Northern Andes -- a structurally complex "orogenic float." The Magdalena Basins form a series of en echelon, sediment-filled structural depressions, the origins of which are intimately tied to structuring of the bordering Central and Eastern Cordilleras. Until the late Miocene they had been parts of much more extensive sub-Andean basins: an extensional, back-arc basin during the Triassic-Jurassic; a pericratonic trough during the Cretaceous-early Tertiary; the inner margin of a broad, east-facing fore and trough during the mid-Tertiary; and finally an array of relatively small, intermontane "successor" basins.

The separate "successor" basins reflect principally the separate frontal thrust systems forming the eastern margin of the Central

Cordillera during the mid-Tertiary. The frontal thrust systems were arranged in an en echelon pattern younging southward. It was against this irregular thrust front of the Central Cordillera that the westward-directed thrusts of the Eastern Cordillera were molded in the late Miocene-Pliocene, resulting in widespread lateral ramping and displacement transfer within the basement-rooted thrust sheets. The anticlinal traps in the Neiva subbasin are pre-Miocene and east-vergent, in the Girardot subbasin they are west-vergent and both pre- and post-Miocene; and in the Middle Magdalena Basin they are west-vergent and post-Miocene.

The interplay of structuring and syn-orogenic sedimentation has given rise to a multiplicity of scenarios for burial and maturation/migration of hydrocarbons from the underlying, organic-rich Cretaceous source rocks. Depending on location within the Magdalena Basins, entry into the oil generative window was as early as early Paleogene or as late as Recent. An understanding of the regional contrasts in structural trap style and age and in the timing of hydrocarbon generation/migration can aid the discovery of the remaining hydrocarbon resources of the Magdalena Basins.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.