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Acoustic Properties of Unconventional Mineral Combinations From the Vaca Muerta Formation, Neuquén Basin, Argentina


Successful exploitation of unconventional reservoirs requires its reservoir zones to be brittle, porous, and rich in kerogen. Modeling results show that an increasing clay and/or kerogen content, or increasing porosity, will decrease the rocks brittleness. We evaluate the usefulness of forward modeling of rock brittleness using unconventional mineral combinations. Initial model validation is based on samples collected from the Vaca Muerta Formation in Neuquen, Argentina. In addition to porosity and permeability, acoustic properties were measured for all plug samples at a nominal frequency of 1MHz. Carbonate content was determined by crushing part of the sample and dissolving the carbonate portion using 10% hydrochloric acid. Total Organic Carbon (TOC) content was measured on the remaining material using an Elemental Analyzer. The measured acoustic data of mudstone samples from the Vaca Muerta Formation is compared to other mixed siliciclastic samples from the Neuquen Basin, shaley Fontainebleau Sandstone, and clean carbonates from different ages and locations. The Vaca Muerta mudstones show substantially lower compressional and shear velocities than all the other data. To properly capture the influence of clay and kerogen on mechanical rock properties, we calculate mechanical rock properties using a rock physics model and then validate the model using sample measurements. Acoustic measurements of Vaca Muerta outcrop samples support the conclusions derived from forward modeling. Outcrop samples also suggest that if sufficient thickness and lateral homogeneity is present, seismic delineation of brittle and/or clay/kerogen-rich zones is feasible. Gassmann fluid substitution is an important tool in many exploration efforts and is assumed to only work properly in microscopically homogeneous rocks. Clay-rich heterogeneous rocks are thought to be non-compliant with Gassmann's theory. The Vaca Muerta Formation, although considered unconventional, shows generally much less clay content then many other unconventional reservoirs. To evaluate Gassmann applicability, we compare measured brine saturated velocities with velocities calculated using dry measurements and Gassmann fluid substitution. The results suggest that Vaca Muerta outcrop samples are homogenous enough to comply with Gassmann's Theory.