Soft Inorganic Geochemistry applied to Sedimentology and Mineralogy 3D modeling of Black Shale Vaca Muerta formation, Neuquén Basin, Argentina

Gomez, Hugo O.¹; Larriestra, Claudio N.; Nawratil, Alejandro; and Larriestra, Fernando
¹[email protected]

Soft inorganic geochemistry can be defined as the spatial modeling of geochemical data, which puts more emphasis on greater amount of data, its spatial behavior and the relationship with other data types (geology, mineralogy, organic geochemistry and geophysics) rather than chemical analysis precision.

The workflow involves non-destructive chemical analysis of rock samples and the geostatistical processing to integrate with well log and seismic data, producing unconventional resources 3D models. The methodology integrates the non-destructive analysis of the entire sample population available using rapid handheld X Ray Fluorescence analysis (HHXRF), the selection of a representative sample (3 to 5% of data population) for destructive analysis of mineralogy (XRD) and organic geochemistry (TOC analysis). Statistical analysis of relationships between HHXRF, XRD and TOC data are made by regression analysis. The integration of rock and well log data is performed using 1D Gaussian cosimulation (type II Markov model) to produce geochemical and mineralogical logs. Finally, the integration of geochemical and mineralogical logs with seismic attributes is made using a Bayesian approach in sequential indicator cosimulation.

This methodology was applied to build an integrated 3D model for Vaca Muerta formation source rock, in a mature oil field of Neuquén basin, Argentina. Statistical studies showed that the relationship between Molybdenum and TOC is useful for quantitative assessment of source rock volume. Quartz-Zirconium and Calcite-Calcium relationships allowed us to calculate volumes of those minerals and their vertical variation within geological formation. The most important conclusion is that Vaca Muerta is not typical black shale, being at the bottom silicoclastic mudstone with high quartz content (25% average), 52% of calcite and 5% clay. The lower portion of the formation has the highest TOC content (3-13%). The upper Vaca Muerta has 75% of calcite (limestone) and 15% of quartz on average, with values reaching up to 25% in some areas considered as clastic wedges within the limestone. This model allowed us to identify source rock intervals and high brittleness zones due to rock mineralogy, and adding complementary data helped us choose those intervals to complete the well. This analysis permitted the selection of the best stimulation method and the intervals to apply it, having great impact on overall cost and time.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013