Cajones Formation Geometry And Regional Facies Distribution In The Boomerang Hills Area, Bolivia

The Cajones Formation represents one of the most important reservoirs of the Boomerang fields. This reservoir produce gas and condensate mainly from thins sands horizons and the shale's constitutes sometimes a good seal of the Yantata Formation top reservoirs. The Upper Cretaceous (Maastrichtian) Cajones Formation share a gradational contact with the Yantata Formation and it had been affected by the Pre-Tertiary unconformity, which had cut almost all the upper strata section of this sequence in the Boomerang Hills area. The representative sequence for the Boomerang consists of white calcareous sandstones interbeds in a low scale of sandy limestones and shales levels. A striking characteristic of the Cajones Formation is the abundance and diversity of shapes of cemented areas, some of which are well enough defined to be considered concretions. Different cores were cut from some exploration and exploitation wells drilled in the Boomerang Hills area, shown buried stems of woody plants or vertical to subvertical roots and some burrows of large invertebrates. The isopach map of the Cajones Formation thickness indicates thinning from east to west and south to north as a result of Pre-Tertiary unconformity at the top of the sequence. The seismic interval does not present too much thickness and its internal character has a moderate parallelism and continuity. At the northeast of the Boomerang Hills where the depositional systems or facies changes and thinning is not easy to identify the horizons where the seismic lost definition and the Cajones Formation sediments could be interpreted as Tertiary levels.