학술논문
Outcrop gamma-ray spectrometry: applications to the Sinemurian--Pliensbachian organic-rich facies of the Lusitanian Basin (Portugal)/Espectrometria de rayos gamma: aplicaciones a las facies organicas carbonatadas del Sinemuriense-Pliensbachiense de la Cuenca Lusitanica (Portugal)
Document Type
Academic Journal
Author
Source
Journal of Iberian Geology. July 1, 2012, Vol. 38 Issue 2, p373, 16 p.
Subject
Language
Spanish; Castilian
ISSN
1698-6180
Abstract
1. Introduction Organic-rich deposits, particularly 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 [...]
This work provides detailed information of outcrop gamma-ray logging from the Sinemurian-Pliensbachian organic-rich units (Agua 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 Agua 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 Agua 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. Keywords: gamma-ray spectrometry; organic-rich carbonates; Sinemurian, Pliensbachian; Lusitanian Basin Este trabajo proporciona information detallada de espectrometria de rayos gamma de las unidades ricas en materia organica del Sinemuriense-Pliensbachiense de la Cuenca Lusitanica, reconocidas como los depositos con mas potencial para generar petroleo en Portugal. El estudio incluye mediciones de espectrometria de rayos gamma en afloramiento y laboratorio, cuantificacion del carbono organico total (COT) y caracterizacion mineralogica por difraccion de rayos X, llevada a cabo en dos areas de la Cuenca Lusitanica (S. Pedro de Moel y Peniche). Los datos de afloramiento se han comparado con informacion de pozos. Los resultados muestran una alta variabilidad de la radiacion gamma (26 a 210 cps), de los elementos radioactivos asociados a las distintas litologias, y de las caracteristicas mineralogicas y geoquimicas. Los datos de COT evidencian un enriquecimiento en materia organica de toda la secuencia estudiada, en particular de la Formation de Agua de Madeiros (maximo=20.4%). La concentration mas elevada de U (11 ppm) es consistente con el pico principal de COT causado por la precipitation de U autigenico (10 ppm). La mayoria de las muestras analizadas presentan un marcado enriquecimiento en U autigenico, especialmente en la Formacion de Agua de Madeiros que representa el principal componente del contenido total de U. Se ha obtenido una correlacion U/COT muy significativa (que alcanza valores de r=0.87) con una proportion baja (0.3 a 0.7 ppm/%COT), lo que confirma que el valor de la concentracion de U puede ser utilizado para calcular el contenido en materia organica en el sistema deposicional estudiado. La tecnica de rayos gamma tambien ha sido util en la identification de ciclos Transgressivo-Regressivos de 2° orden, de superficies de maxima inundation y en la correlation de datos de superficie con datos de pozos. Palabras clave: espectrometria de rayos gamma; carbonatos ricos en materia organica; Sinemuriense, Pliensbachiense; Cuenca Lusitanica.
This work provides detailed information of outcrop gamma-ray logging from the Sinemurian-Pliensbachian organic-rich units (Agua 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 Agua 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 Agua 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. Keywords: gamma-ray spectrometry; organic-rich carbonates; Sinemurian, Pliensbachian; Lusitanian Basin Este trabajo proporciona information detallada de espectrometria de rayos gamma de las unidades ricas en materia organica del Sinemuriense-Pliensbachiense de la Cuenca Lusitanica, reconocidas como los depositos con mas potencial para generar petroleo en Portugal. El estudio incluye mediciones de espectrometria de rayos gamma en afloramiento y laboratorio, cuantificacion del carbono organico total (COT) y caracterizacion mineralogica por difraccion de rayos X, llevada a cabo en dos areas de la Cuenca Lusitanica (S. Pedro de Moel y Peniche). Los datos de afloramiento se han comparado con informacion de pozos. Los resultados muestran una alta variabilidad de la radiacion gamma (26 a 210 cps), de los elementos radioactivos asociados a las distintas litologias, y de las caracteristicas mineralogicas y geoquimicas. Los datos de COT evidencian un enriquecimiento en materia organica de toda la secuencia estudiada, en particular de la Formation de Agua de Madeiros (maximo=20.4%). La concentration mas elevada de U (11 ppm) es consistente con el pico principal de COT causado por la precipitation de U autigenico (10 ppm). La mayoria de las muestras analizadas presentan un marcado enriquecimiento en U autigenico, especialmente en la Formacion de Agua de Madeiros que representa el principal componente del contenido total de U. Se ha obtenido una correlacion U/COT muy significativa (que alcanza valores de r=0.87) con una proportion baja (0.3 a 0.7 ppm/%COT), lo que confirma que el valor de la concentracion de U puede ser utilizado para calcular el contenido en materia organica en el sistema deposicional estudiado. La tecnica de rayos gamma tambien ha sido util en la identification de ciclos Transgressivo-Regressivos de 2° orden, de superficies de maxima inundation y en la correlation de datos de superficie con datos de pozos. Palabras clave: espectrometria de rayos gamma; carbonatos ricos en materia organica; Sinemuriense, Pliensbachiense; Cuenca Lusitanica.