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Surface Geochemical Prospecting of Hydrocarbon in Brejeira and Mira Formations, South Portuguese Zone (SPZ), Portugal

Gabriel A. Barberes5, André Spigolon1, Albert Permanyer4, Rui Pena dos Reis5, Paulo Fonseca2, Teresa Barata3 and Rafal Skupio6

1 Geochemistry Division, Petrobras/CENPES, Rio de Janeiro, Rio de Janeiro, Brazil
2 GeoFCUL & IDL, University of Lisbon & Instituto Dom Luis, Lisboa, Lisboa, Portugal
3 CITEUC, University of Coimbra, Coimbra, Coimbra, Portugal
4 Sedimentary Geology Group, University of Barcelona, Barcelona, Spain
5 Department of Earth Science and Center of Geoscience, University of Coimbra, Coimbra, Portugal
6 Oil and Gas Institute, National Research Institute, Krakow, Poland

April 4, 2017 Tuesday 1:30 P.M. AAPG Annual Convention and Exhibition, Houston, Texas

Abstract

The SPZ is located in the southernmost sector of the pre-Mesozoic Iberian Variscan Chain, which is composed by upper Paleozoic shales and greywackes (interbedded), of low grade metamorphism. Previous works based on organic maturation support that SPZ Carboniferous units are mostly over-mature, thus discarding any hydrocarbon generation potential. However, results based on inorganic maturation consider that they are mostly within the gas window. This study aims to evaluate, through the surface geochemical prospecting, hydrocarbon seeps in Mira and Brejeira Formations, from the SPZ, in order to characterize the potential for hydrocarbon accumulation in the subsurface. The sampling areas were determined in accordance with the thorium-normalized methodology, previously defined for the same project. Twenty-one soil samples (2 in Mira Fm., 18 in Brejeira Fm., and 1 in the border between Mira and Brejeira Formations) were used in this study. The collection of samples was performed using a Isojars, with distilled water and a bactericide added to inhibit any bacterial activities. The soil samples were analyzed by gas chromatography with flame ionization detector (GC-FID) to detect the hydrocarbons (C1-C5). The results show the presence of gas, mostly methane (C1), with very low amounts (2 ppm average) in 20 soil samples, which were interpreted as background of the SPZ formations. In the sample from the border between the formations, were found 160 ppm of methane, 51 ppm of ethane, 22 ppm of propane, 8 ppm of butane, and 3 ppm of pentane. The carbon isotopic ratios (δ13C vs. vPDB) obtained for these compounds were -26‰ (C1), -31.2‰ (C2), -32‰ (C3), -32‰ (C4), and -33‰ (C5). The measurements were performed using GC-IRMS system (isotope ratio monitoring-gas chromatography-mass spectrometry). The results presented in this work are preliminary but they reveal the presence of thermogenic gas, emanating from formations with thickness over 4000 meters, characterizing a potential senile unconventional petroleum system. New samples will be collected, including water samples from artesian wells, and submitted to GC-FID and GC-IRMS aiming the characterization of gas seeps and its origin.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017