The Vaca Muerta formation in the Neuquén Basin in western Argentina is one of the most important shale plays in the world. During the initial assessment of the play as an unconventional resource most of the operators and service companies used parameter estimation methods and reservoir quality definitions extrapolated from North American analogs. As noted by different authors not all the shale plays are similar. In the case of Vaca Muerta, one of the distinctive characteristics is the thickness of the organic rich interval, reaching 350 meters in some parts of the basin, other distinctive feature is related to the depositional settings, Vaca Muerta shales are the result of a marine siliciclastic-carbonatic mixed ramp deposition where total clays are rarely found above 30% in weight. There has been a competition between the continental clastic influx and the authigenous carbonate factory that is reflected in the clear negative correlation between these two matrix components.
The scope of this contribution is the presentation of a workflow built and calibrated specifically for the Vaca Muerta formation to enables the estimation of the volumes of the principal matrix mineral assemblages and of the formation fluids, using commonly available well logs. This work proposes a platform-independent method that is easy to implement. A first objective of the method is the basin-level assessment of vertical and lateral variability of the formation using already existing well-log data. A second objective is the enabling of industrialization workflows for the play development phase when only limited well logs would become available. The method presented here does not pretend to replace a detailed petrophysical evaluation.
The work presented in this contribution is based on the analysis of 20 exploratory wells benefiting from advanced logs and laboratory analysis from cores, covering a wide range of basin regions and thermal maturities. However, to ensure the widest possible applicability, only a reduced set of common open-hole well logs such as Gamma Ray, Resistivity, Sonic, Neutron and Density was considered as input for the method. The estimation of the main petrophysical parameters such as porosity, TOC, matrix volumes and water saturation is achieved through a sequential deterministic workflow.
For the calibration of these parameters we have used laboratory measurements (TOC from coulometry, gas filled porosity from tight rock analysis and matrix mineralogy from XRD) and measurements and models derived from last generation advanced well logs (multipolar sonic, magnetic resonance, dielectric dispersion and elemental spectroscopy).
It will be shown on several examples that the formation properties can be estimated with an acceptable degree of confidence in a wide range of conditions with a minimal set of assumptions. Tentative physical explanations will be provided to decode the reasons that allow the method to be efficient and widely applicable. During the construction of the workflow, some relationships between a priori unrelated formation components surfaced and a limited number of controlling factors was identified. Insights on the origin of these factors and possible dependencies will be briefly proposed.
