Mechanical stratigraphy of the Vaca Muerta Formation, Neuquén Basin, Argentina

Abstract

The Vaca Muerta offers an excellent opportunity to study field development at an early stage. The particularities of how best to characterize this play are developing, and various techniques and processes must be tested. Even in parts of the basin where wells are being drilled in rapid succession, production history is relatively short, and the dynamics of how the production may change with time are still to be anticipated (Rimedio Et al., 2015). With limited core data for calibration, a comparison of several predictive methods was made to estimate rock properties for the Vaca Muerta Formation in a study area consisting of 5 wells. Borehole image logs were interpreted to study pre-existing natural fractures, and a stability assessment was conducted to estimate the reactivation potential of the identified natural fractures. The comparison results in the characterization of an interval with clear vertical trends between an upper, middle, and lower section. In the lower section, organic and siliciclastic contents are highest. In the upper, organic content is lowest, and carbonate content is highest. The middle zone between them is a transitional interval in which the organic matter continuously decreases upwards in depth, in relative proportion with the increase in carbonate content. Borehole image log data were interpreted for the presence of natural fractures, horizontal discontinuity surfaces, concretions, and other heterogeneities. Documented is the presence of a naturally fractured zone in the middle Vaca Muerta and abundant calcareous concretions in the Lower Vaca Muerta. The identified natural fractures are likely candidates for reactivation during hydraulic stimulation based on a stability assessment. The middle of the interval is suitably brittle, organic rich, and has now been documented to contain multiple natural fractures. Although this conclusion was reached independently, it is corroborated by regional literature (Fantin Et al., 2014; Williams Et al., 2014). In addition, geochemical analysis on the quality of the liquid hydrocarbons in the Vaca Muerta indicates that the middle interval has high volumes, with a moderate gas-to-oil ratio, and moderate viscosities (Williams Et al., 2014).