--> Raman Microspectroscopy of Carbonaceous Material in the South Portuguese Zone: Preliminary Results

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Raman Microspectroscopy of Carbonaceous Material in the South Portuguese Zone: Preliminary Results


The chemical and physical transformations which occur in sedimentary organic matter during diagenesis (maturation/coalification) have been used to study formation temperatures and the thermal histories of sedimentary basins. The thermal maturity for diagenetic conditions can be assessed by methods including vitrinite reflectance, rock-eval pyrolysis and illite crystallinity. However, for late diagenesis to low grade metamorphism, some of these techniques are not reliable or their application has been questionable.

Raman spectroscopy of carbonaceous material has become a widely used quantitative geothermometer for assessing peak metamorphic temperatures attained by rocks in the range of 330 – 650 °C. Recently, this technique has also been used to characterize the thermal maturation of rocks in the diagenesis to low grade metamorphism range (200 – 320 °C).

The Devonian and Carboniferous strata of the South Portuguese Zone have already been comprehensively studied by vitrinite reflectance indicating thermal maturity conditions in the range of late diagenesis to low grade metamorphism (McCormack et al., 2007; Fernandes et al., 2012). The principal aim of this work was to study the Raman spectrum of the carbonaceous material present in rocks of the South Portuguese Zone, and compare these results with vitrinite reflectance, since no studies of this kind are known in this zone.

The samples used in this study are from shales of the Mértola Formation (Mid-Visean) and the Volcano-Sedimentary Complex (early Visean?) penetrated by borehole AC-1 (Fernandes et al., 2012). The vitrinite reflectance measured is consistent (4.65 – 5.55% Roran) throughout the core and does not increase downhole. The palaeotemperatures calculated based on the vitrinite reflectance values are in the range 305 – 325 °C.

Raman microspectroscopy of the carbonaceous material from borehole AC-1 reveal spectra similar to those described by Lahfid et al. (2010) from carbonaceous material in late diagenetic to low-grade metamorphic rocks from the Cenozoic Helvetic Flysh of the Glarus Alps (Switzerland). The Raman spectra obtained show features that indicate palaeotemperatures higher than 250 °C, with some samples indicating temperatures higher than 300 °C (samples from depths 21 m, 421.5 m, 503 m and 728.5 m), due to the presence of a narrow intense D1 band with higher intensity than the G band. These results support the maturity levels assigned to this borehole and for the South Portuguese Zone. They also demonstrate the potential of Raman spectroscopy as a geothermometer for rocks in the mid-late diagenesis to low grade metamorphic range.