Summary
The determination of an accurate lithology and the estimate of an accurate total organic carbon weight fraction, TOC, are essential blocks in any formation evaluation workflow for unconventional shale plays. Different sources of information are commonly used to assess these properties, including measurements on core and on cuttings and measurements from petrophysical logs.
To evaluate the respective accuracy and advantages of these different inputs, Total Austral decided to systematically acquire these measurements in different blocks of Vaca Muerta and to perform comparative analysis.
A total of 82 plugs were extracted from full cores and sent for analysis in Total CSTJF laboratory in Pau, France. These cores were selected based on CT Scanner image in order to include a representative lithology, an important issue when we are dealing with formation presenting a high vertical heterogeneity. Elemental concentrations, mineralogy analysis and TOC measurements were performed and duly quality-controlled. These data represent the reference to which the other measurements will be compared.
Cuttings analysis was performed as part of advanced mud logging service. Elemental concentrations and mineralogical contents were determined. The element-mineralogy quality control of cuttings measurements presents | apparent biases that are discussed and resolved in this paper.– Finally, despite the limitations inherent to measurements on cuttings, a good match with core data is observed.
The mineralogy from logs was obtained from a new generation spectroscopy tool that simultaneously combines capture and inelastic spectra. Quantitative estimation of elements, traditionally difficult to quantify, such as Magnesium, Manganese, Potassium and Sodium is readily achieved with this technology. Advanced interpretation techniques coupled with a priori knowledge of the formation characteristics are used to construct a reliable lithological model from the element weight fractions to accurately estimate the inorganic carbon and derive the TOC. The match between log-derived mineralogy and TOC with core-derived properties is excellent.
The quality of the comparison between core and log relies on several factors, such as core measurements quality control, log quality control, and last but not least definition matching. As devised in this paper, most of the properties measured by core and logs do not follow the same definition, and adequate normalization is necessary prior to comparison.
The validation work presented in this paper allows considering different source of information for petrophysical analysis, depending on operational and development constrains. Each technique presents its own advantages at different stages of development phase.
