Ultrasonic Vertical Transverse Isotropy in High TOC Mudstones From the Vaca Muerta Formation, Argentina
Unconventional plays consist mostly of a mixture of clastic/carbonate mudstone with varying clay and total organic carbon (TOC) content. Knowledge of factors strongly influencing acoustic properties on log and seismic scale (e.g. porosity, TOC, and clay volume) is essential for successful well placement when brittleness is crucial for effective hydraulic fracturing. It is also imperative to understand the degree of lamination and layering because both are responsible for the degree of acoustic anisotropy and hence required for adequate integration of horizontal acoustic well measurements and correct processing of seismic data. Here we present a study that uses ultrasonic plug measurements to assess vertical transverse isotropy (VTI). Pairs of plug samples are extracted at almost identical locations with both vertical and horizontal orientation to analyze VTI at different stratigraphic positions within a sequence stratigraphic framework. Acoustic properties (compressional and two perpendicular shear waves) are measured at 1 MHz under both wet and dry conditions with varying effective pressure. Porosity and density are measured using a helium pycnometer, and both carbonate content and %TOC are measured on an Elemental Analyzer coupled with a Mass Spectrometer. Compressional velocities vary between 3500 m/s and 6000 m/s while porosity ranges from less than 1% to over 18%. Comparison of plug pairs from almost identical depth reveals an average transverse isotropy constant ε = [Vp90 − Vp0]/Vp0 ranging from near 0 to over .18. Such anisotropy is significant not only in an effort to compare vertical and horizontal well log velocities, but also when well log data is used for acoustic inversion or velocity model building. Strong anisotropy is observed in plugs of identical location and mineral composition. Anisotropy related velocity variation lies well within the range of velocity changes that can be attributed to mineralogy variability. As a result, velocity differences between vertical and deviated wells will remain an unsolvable uncertainty without plug measurements. For adequate log velocity analysis, required for both accurate seismic processing and depth conversions, core plug measurements are imperative to distinguish velocity variations attributable to anisotropy from mineralogy related velocity changes.
AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016