Hydrocarbon Structural Traps above Inverted, Mesozoic Normal Faults in the Putumayo Foreland Basin, Southern Colombia
The Putumayo foreland basin (PFB) formed by Paleocene to recent eastward overthrusting of the Colombian Andes onto the basement of the South American craton. The thicker part of the PFB is adjacent to thrust faults along the Andean mountain front and contains up to 3 km of Paleocene to recent clastic sedimentary rocks of non-marine origin. The distal eastward edge of the basin thins to zero across a large flexural arch formed in the Paleozoic basement of South America. The PFB is obliquely intersected by the 50-km-wide, north-northeast-trending Caquetá basement arch that parallels the structural grain of the Paleozoic orogeny and disrupts the deeper, elongate axis of the Andean foreland basin. The Caquetá arch is responsible for structural and stratigraphic separation between the PFB and its northeastward projection, the Llanos foreland basin of northern Colombia. Instead, the PFB represents the direct northward continuation of the Oriente foreland basin of Ecuador and the Marañon foreland basin of Peru with which PFB shares structural and stratigraphic affinities. In this study, we used 3000 km of seismic data tied to 16 wells over an area of 28,000 km2 to reconstruct the main periods of subsidence in the PFB, the controls on its major source and reservoir rocks, and the timing and origin of major structural traps. Source rocks include organic rich shale of the late Cretaceous Caballos and Villeta formations. Reservoirs include the Villeta and Pepino formations of Late Cretaceous-Eocene age that were deposited during the early period of foreland basin formation. The most distinctive trapping characteristic of the PFB is the presence of 10 known oil fields including the Orito field with past production of 230 million barrels of oil, which are found in anticlines formed above inverted, north-south-striking normal faults of Mesozoic age. This belt of inverted-related structures is even more pronounced in the southward extension of the PFB into the Oriente and Marañon basins where more than 22 north-south-trending fields are found in inverted structures. Seismic data from the PFB show that separation on inverted normal faults is small (~40 m) with episodes of inversion occurring during the Eocene or during a later Pliocene to recent phase with folds expressed at the ground surface. Unfortunately, there are no deep wells to verify the presence of rift fill and perhaps enhanced source potential in the syn-rift wedges.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California