DEAN, J. S., Advantage Resources, Denver, CO, J. T. WILSON and G. F. MAINZER, Anderman-Smith, Denver, CO, F. ESCOBAR and G. AGUIRRE, ENAP, Punta Arenas, Chile
ABSTRACT: Preliminary Assessment of the Lago Mercedes Discovery, Magallanes Basin, Chile
The Lago Mercedes #1 well, spudded January 17, 1991, was positioned to test a seismically defined structural culmination located along a blind thrust near the deep foreland axis of the western Magallanes basin. This fault, which defines the leading edge of Andean-related thrust detachment in the region, is responsible for a trap geometry that is genetically related to, but fundamentally different from the numerous unrooted Tertiary folds in the area. It was felt that the folding associated with this Lago Mercedes thrust could produce a trap whose timing better correspond with source rock maturation and plumbing as well as fracture enhancement of potential reservoirs.
Although the Lower Cretaceous Springhill Formation comprised the primary target, it was anticipated that the geometry of the fold allowed for the possibility of several fractured intervals in the hanging wall, including volcaniclastic rocks of the underlying Jurassic Tobifera "basement" sequence, recently found to be productive elsewhere along the eastern platform of the basin.
During drilling of the well, encouraging gas and condensate shows were encountered in numerous horizons. The most surprising of these later proved to be a Permo-Triassic granodiorite underlying the Tobifera. Although relatively widespread on outcrop, this represents the first time a pre-rift intrusive body has been penetrated in the subsurface. All of the hydrocarbon-bearing intervals exhibit minimal matrix porosity but varying degrees of fracturing.
Subsequent testing of the well yielded combined flow rates of in excess of 12 MMCFD of rich gas and 1140 BPD of 52 A.P.I. condensate. The most prolific zone corresponds to an intensely fractured and partially weathered interval in the uppermost portion of the intrusive. Additional testing is planned prior to any estimate of recoverable reserves. Nevertheless, this unique accumulation underscores the possibility for nonconventional reservoirs throughout the lightly explored Sub-Andean basin trend, particularly fold-thrust belts which have the potential to "create" reservoirs and trap geometry simultaneously.
AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.