Carbonate-Siliciclastic Transitions from Cycle to Sequence and Outcrop to Basin — The Quintuco-Vaca Muerta System in the Neuquén Basin, Argentina

Detailed outcrop analysis on two large-scale exposures representing proximal and distal settings and their correlation to the time-equivalent subsurface successions provide new insights into the mixing of carbonate and clastics along the entire depositional profile of the Quintuco-Vaca Muerta System in the Neuquén Basin. This understanding of the relative mixing in both, the shelfal and basinal portions, is paramount for the characterization of conventional as well as unconventional reservoirs.

Both outcrop exposures display a similar stratal architecture; a lower prograding clastic unit is overlain by the aggrading mixed successions, which in turn are capped by an abrupt change from shallow marine to fluvial/deltaic deposits. The carbonate-clastic transitions at cycle scale are represented by gradual changes from siliciclastic to carbonate-rich facies, which occur in both distal (shale - calcareous shale) and proximal (silt/sandstone - ooid grainstone) settings. On the sequence scale pure siliciclastic packages turn up-section into carbonate-dominated intervals within a few tens of meters. This correlation of shallowing and cleaning upward points at an accommodation controlled variation of carbonate content with reduced siliciclastic transport along the shelf during times of low relative sea level, allowing carbonates to be become more dominant. In addition to the vertical succession, a lateral transition is observed along the depositional profile, where over a distance of only 8km pure carbonate facies gradually turns downdip into a silty wackestone facies and calcareous shale.

The subsurface data (cores, well logs, seismic) reveal that the same trends in mixing are present on the large-scale, in other parts of the basin. However, based on seismic correlation, the western areas always remained in shallow marine settings, producing mixed deposits, while the eastern portions of the basin were subject to tectonic uplift that resulted in increased siliciclastic input.

Based on all observations, transitions from siliciclastic to carbonate sedimentation (and vice versa) in the Quintuco-Vaca Muerta System occur in multiple scales in both time (cycles and sequences) and space (across- and along-shelf) and are controlled by the interplay of eustatic sea level, subsidence and regional tectonic pulses.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California