Abstract
This work provides detailed information of outcrop gamma-ray logging from the Sinemurian–Pliensbachian organic-rich units (Água de Madeiros and Vale das Fontes formations) of the Lusitanian Basin, which are recognized as one of the most important potential oil source rocks interval of Portugal. The study included total and spectral gamma-ray measurements in outcrop and laboratory, Total Organic Carbon (TOC) content and whole-rock mineralogical characterization by X-ray diffraction, carried out in the two most important outcrop areas of the Western Lusitanian Basin (S. Pedro de Moel and Peniche). The outcrop data was compared with subsurface information. The results show high variability of the gamma radiation (26 to 210 cps) and radioactive elements associated with the lithological, mineralogical and geochemical differences. The TOC data yielded the organic matter enrichment of both formations, where the highest values are observed in the Água de Madeiros Formation (maximum=20.4%). The highest U concentration (11 ppm) is consistent with the highest Total Organic Carbon peak mainly supported by the precipitation of authigenic U (10 ppm). The majority of the analysed samples show an obvious authigenic U enrichment, especially in the Água de Madeiros Formation where it represents the main component of the total U content. A highly significant U/TOC correlation (r up to 0.87) with a low ratio (0.3–0.7 ppm/wt.%TOC) was obtained, confirming that U content can be used as a proxy for organic richness in the studied depositional system. The gamma-ray tool was also useful in the identification of 2nd order Transgressive–Regressive facies cycles (and maximum flooding surfaces) and in the correlation between outcrop and subsurface data.
Highlights
Organic-rich deposits, with dark and laminated marls, are of great importance for petroleum exploration, since they may represent major source rock intervals (e.g., Bodin et al, 2011; Riediger, 2002; Lüning et al, 2000)
It is possible to obtain a linear relationship between the total organic carbon (TOC) and U (Lüning and Kolonic, 2003)
The GR spectrometry performed in the laboratory had a great importance in all correlations established in this study due to their accuracy
Summary
Organic-rich deposits, with dark and laminated marls (commonly named as black shales), are of great importance for petroleum exploration, since they may represent major source rock intervals (e.g., Bodin et al, 2011; Riediger, 2002; Lüning et al, 2000). It may be precipitated to enrich the sediment in authigenic (nondetrital) U, fixed at the sediment-water interface accumulating together with the organic matter (Wignall and Myers, 1988) In this systems, it is possible to obtain a linear relationship between the total organic carbon (TOC) and U (Lüning and Kolonic, 2003). This technique, when associated with the direct analysis of the rocks (biostratigraphy, sedimentology, petrography, geochemistry) can be very helpful, to characterize a potential source rock (e.g., Bodin et al, 2011; Lüning et al, 2005; Lüning and Kolonic, 2003; Myers and Wignall, 1987), and as an important tool for basin analysis, in sequence stratigraphy studies (e.g., Catuneanu, 2006; Collins et al, 2006; Pawellek and Aigner, 2003; Parkinson, 1996), lithostratigraphic characterization (e.g., Raddadi et al, 2005; Martinius et al, 2002; Hadley et al, 2000; Bessa and Hesselbo, 1997) and correlation with subsurface data (e.g., Aigner et al, 1995; Leslie et al, 1993)
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