Geochemistry of Ordovician and Silurian Black Shales, Cantabrian Zone, Asturias and Leon Provinces, Northwest Spain

Maio, Frank ¹; Aramburu, Carlos ²; Underwood, John ³
(1)Basgas Pty Ltd, Dallas, TX. (2) Departamento de Geología, Universidad de Oviedo, Oviedo, Spain. (3) Heyco Energy Group, Dallas, TX.

The Cantabrian Zone is defined as the area to the east of exposed Precambrian rocks of the Narcea Antiform, north of the Cenozoic Duero Basin, and west of the Mesozoic-Cenozoic Basque-Cantabrian Basin. Rocks within the Cantabrian Zone are mostly sedimentary of Paleozoic age, namely Cambrian through Carboniferous.

Organic rich Ordovician and Silurian black shales are important source rocks throughout the world. According to Klemme and Ulmishek, 9% of the worlds’ original petroleum product, including both gas and oil, were generated from rocks of Silurian age. Their presence has been well established on the Iberian Peninsula on outcrops and in boreholes. Time equivalent shales are currently targets for shale gas exploration in other European countries such as Sweden, Poland and Romania. A recent AAPG publication described organic rich Silurian shales in the Lusitanian Basin of northern Portugal; however, a literature search did not uncover any published reports describing the geochemistry of Silurian shales in the Cantabrian Zone. Field samples were therefore required in order to evaluate the source rock potential of lower Paleozoic black shales in northwestern Spain.

Twenty four new samples were gathered from twelve different outcrop locations in the provinces of Asturias and Leon during a May, 2010 field trip. Twenty samples from the Lower Silurian Formigoso Formation and four samples from the middle Ordovician Sueve Formation were analyzed. Tests included X-Ray Diffraction, Pyrolysis, Conodont Alteration Index, Vitrinite-Like Reflectance, Rock/Eval, Dispersed Organic Matter, Kerogen Type and Total Compositional Analysis. The data from these analyses will be shown along with the sample locations.

Successful shale gas plays in the United States display TOC values above 2%. Unfortunately, no Cantabrian Zone samples recorded TOC’s above 2%; in fact, nineteen of the twenty four samples had TOC’s under 1%. In addition, the kerogen type was overwhelmingly inertinite, meaning not only that there is no organic material left to be generated and produced through the process of desorbtion.

Based upon the exceedingly poor results of the geochemical work, the conclusion reached was that the Lower Paleozoic black shales in the Cantabrian Zone have very poor potential for the production of shale gas.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.