Sequence Architecture, Heterogeneities and Seismic Expressions of a Vaca Muerta Outcrop Analog in the Neuquén Basin, Argentina — Implications for Unconventional Exploration

Zeller, Michael; Reid, Samuel B.; Eberli, Gregor

The Quintuco-Vaca Muerta System in the Neuquén Basin recently received much attention due to its potential as a major unconventional play. However, studies of the Vaca Muerta Formation are generally restricted to 1D reference sections and are not covering the lateral architecture and heterogeneities within the depositional system. Combining fieldwork, satellite imagery and seismic modeling closes this gap and helps to establish rules for predicting facies and heterogeneities from subsurface data sets.

The field site, the Sierra de la Vaca Muerta, offers spectacular insights into the prograding system along a 10km long and 1km thick cross section. Combination of field geology with satellite imagery interpretation documents the sequence architecture of this prograding mixed system and allows the allocation of bed scale heterogeneities within the stratigraphic framework. The studied interval can be subdivided into 8 sequences, representing a very similar scale as the 7 sequences documented in the global sea level chart and pointing therefore at eustatic control on the system.

Within this framework, heterogeneities occur in temporal and lateral scales. While carbonate-rich portions are characterized by abrupt lateral facies transitions and breaks within their steep depositional profile, siliciclastic-rich parts show gently inclined profiles with no breaks and more gradual facies changes.

In the temporal scale, heterogeneities occur both as cycles and events. Meter-scale alternations of pure shale with calcareous shale are the typical cyclic expressions, which occur down-slope of carbonate-rich systems. Mixed turbidites commonly follow just after carbonate sequences, triggered by the steeper depositional gradients. This distribution indicates that carbonate-rich basinal portions would offer both more brittle volumes for fracturing (calc. shales) and at the same time potential relative permeable streaks (turbidites) within the tight shale surrounding.

In order to compare this architecture with subsurface seismic, synthetic seismic models are constructed from the observed facies distribution and rock properties. In the synthetic seismic models the carbonate cycles prove to be easy to detect due to their strong acoustic impedance and geometrical expressions. These findings could offer a way to identify sweet spots for unconventional exploration from seismic, since those are laterally related to carbonate sequence tops, which have a clear seismic signature.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013