Outcrop gamma-ray spectrometry: Applications to the Sinemurian–Pliensbachian organic-rich facies of the Lusitanian Basin (Portugal)

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 2 nd order Transgressive-Regressive facies cycles (and maximum flooding surfaces) and in the correlation between outcrop and subsurface data.

New high resolution geochemistry of Lower Jurassic marine sections in western North America: A global positive carbon isotope excursion in the Sinemurian?

“…The marly-rich deepening upward episodes show generally higher values of radioactivity; mainly due to the higher proportion of marls, than the relatively more calcareous shallowing upward intervals. This fact is a quite common feature as indicated by several works concerning the Lower Jurassic of the Cleveland Basin in the UK (van Buchem et al, 1992) and in the Lusitanian Basin of Portugal (Correia et al, 2012). No geochemical analysis to determine if the main source of the radioactivity is U, Th or K have been performed, but in deposits of similar age studied in the Lusitanian Basin, correlation coefficient is slightly higher with K (0.64) than with Th (0.56) and with U (0.47) (Correia et al, 2012).…”

“…van Buchem et al, 1992;Gómez and Goy, 2000;Pawellek and Aigner, 2003;Correia et al, 2012). Obtained gamma-ray log in the Rodiles section ( Fig.…”

“…(see Lüning and Kolonic, 2003). In the Western Europe Lower Jurassic black shales, the quality of U and TOC relationship varies from good, like in the Posidonienschiefer of northern Germany (Mann et al, 1986) and in the Sinemurian-Pliensbachian of the Lusitanian Basin (Correia et al, 2012), to moderate like in the Lower Jurassic of NE England (Wignall and Myers, 1988;van Buchem et al, 1992) and poor, like in the subsurface of the Netherlands, where the bituminous Posidonienschiefer does not correlate with any significant increase in gamma ray activity (Meyer and Nederlof, 1984).…”

Mineralogical investigation of fine clastic rocks from Central Sarawak, Malaysia

“…The relationship between total organic carbon (TOC) and uranium concentration has previously been documented worldwide by different authors (Lüning and Kolonic 2003;Correia 2012). The TOC values range from 0.04 to 1.80%; the results interpreted according to Peters and Cassa (1994) as shown in Table 3 indicate that shale and mudstone samples have poor-to-good hydrocarbon generative potential.…”

The Lower Jurassic at Peniche (Lusitanian Basin): recent advances in Stratigraphy and Sedimentary Geology

“…Afterwards, Duarte et al (2010), based in a detailed biostratigraphic framework (especially complete in the Peniche section), sedimentological data and TOC variation, present a consolidated 2 nd -order transgressive-regressive facies cycles scheme for the Lower Jurassic carbonate series of the LB and compared them with those observed in the Basque-Cantabrian neighbouring basin. Nowadays, these organic-rich levels are intensively studied throughout the basin, using several innovative techniques, including conventional isotope geochemistry in kerogen (Silva et al, 2011b), biogeochemistry (Silva et al, 2012), outcrop gamma-ray spectrometry (Correia et al, 2012), biomarkers and palynofacies (e.g., Ferreira et al, 2010;Silva et al, 2010Silva et al, , 2011a.…”