Raman Spectroscopy Based Maturity Profiling of the Vaca Muerta Formation, Neuquén Basin, Argentina

摘要

The thermal maturity of organic-rich mudstones is one of the main parameters to evaluate, when appraising a new area in an unconventional shale play project, to decide on the best field development strategy and to define the landing zones. Conventionally, thermal maturity is derived from optical vitrinite reflectance measurements, but this technique has some limitations in marine sediments with lack of terrestrial material. Other techniques, such as Rock-Eval pyrolysis, are destructive and the results can be biased if oil-based mud is used to drill the well. In this contribution, a fast, easy and non-destructive method known as Raman spectroscopy is proposed to estimate the maturity of mudstone samples from the Argentinian Vaca Muerta formation, collected from a wide range of maturities. Raman spectroscopic measurements were executed on a variety of Vaca Muerta shale samples. A complete maturity depth profile was acquired for one well over the entire Vaca Muerta organic shale sequence. Additionally, samples from eight further wells, presenting a wide range in the expected maturity, were examined with the Raman technique. Using a correlation between the Raman spectroscopic signal and vitrinite reflectance, established earlier based on a set of reference samples, containing organic-rich mudstones from a variety of paleo-marine sedimentary basins in North America, thermal maturities were derived for the Argentinian shale samples. For certain samples kerogen was extracted and properties of the isolated kerogen were measured. The Raman results were not only compared to standard maturity indicators such as vitrinite reflectance or Rock-Eval pyrolysis, but also with other non-standard techniques like DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) or results derived from the kerogen properties. This case study in the Vaca Muerta shows a good correlation between the maturity values derived from the Raman measurements and maturities inferred from other methods. The depth profile shows a trend of increasing maturity with depth as expected for such a thick unconventional reservoir.