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Abstract

The South Portuguese Zone (SPZ) is located in the southernmost sector of the pre-Mesozoic Iberian Variscan Chain, which is composed by Carboniferous interbedded shales and greywackes, with commonly low-grade metamorphism. Previous works about organic maturation support that Palaeozoic 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. Natural hydrocarbon seepage has been important drivers of global petroleum exploration as a direct indicator (pathfinder) of gas and/or oil subsurface accumulations (Etiope, 2004). They occur in all petroleum systems (conventional or unconventional) and form the basis for most geochemical, microbiological, and non-seismic geophysical detection methods (Schumacher, 2012), and today it still drives the surface geochemical prospection for oil and gas (e.g., Abrams, 2005). The presence of near-surface migrated hydrocarbons can provide strong evidence of an active petroleum system presence as well as critical information on source, maturity and migration pathways (Abrams, 2013). When applied to hydrocarbon exploration, soils, sediments, air, water and rock samples can be sampled and tested through various analytical methods. For this study the sampling zone was selected based on the geological features such as alignments and faults, thrusts and major fold structures that could work as pathways for the hydrocarbon migration to the surface soil-water system. The collection of all samples was performed using IsoJar®, with distilled water (just for soil/rock samples; water samples do not need to be filled with distilled water) and a bactericide added to inhibit any bacterial activities post sampling. Twenty soil samples were collected for this study, using a drilling machine, a metal tube and a hammer. Thirty-one water samples were collected from artesian wells, boreholes and springs. From the geological prospecting point of view the presence of hydrocarbon gases in SPZ formation is clear and evident. They are present in soil and water in significantly high levels, with light hydrocarbon (C1-C4) values reach more than 1500 ppm in soil samples and more than 70 ppm in water samples (headspace phase). Anomalous presence of some gasoline hydrocarbons (such as toluene) were detected in both soil and water (Barberes et al., 2018). The integration with TOC, vitrinite reflectance, rock-eval pyrolysis and GC-IRMS reveals the presence of a senile unconventional petroleum system, with possible interbedded of levels with different degrees of organic maturation (leading to a mixture of thermogenic gases). Días-del-Río et al. (2003) and Stadnitskaia et al. (2006) reported the seafloor seepage of similar hydrocarbons (similar isotopic composition) in Portuguese and Spanish (Gulf of Cadiz) offshore, above the Palaeozoic basement. References: Abrams, M.A. 2005. Significance of hydrocarbon seepage relative to petroleum generation and entrapment. Marine and Petroleum Geology, 22, pp. 457-477, doi: 10.1016/j.marpetgeo.2004.08.003. Abrams, M.A. 2013. Best practices for the collection, analysis, and interpretation of seabed geochemical samples to evaluate subsurface hydrocarbon generation and entrapment. Offshore Technology Conference, 24219, 21p., doi: 10.4043/24219-MS. Barberes, G.A., Pena dos Reis, R., Spigolon, A.L.D., Fonseca, P.E., Bandeira de Mello, C., Barata, M.T. 2018. Groundwater Natural Contamination by Toluene in Beja and Faro Districts, Portugal. Geosciences, 8, 9, doi: 10.3390/geosciences8010009. Días-del-Río, V., Somoza, L., Martínez-Frias, J., Mata, M.P., Delgado, A., Hernandez-Molina, F.J., Lunar, R., Martín.Rubi, J.A., Maestro, A., Fernández-Puga, M.C., León, R., Llave, E., Medialdea, T., Vázquez, J.T. 2003. Vast fields of hydrocarbon-derived carbonate chimneys related to the accretionary wedge/olistostrome of the Gulf of Cádiz. Marine Geology, 195, pp. 177-200, doi: 10.1016/s0025-3227(02)00687-4. Etiope, G. 2004. New Directions: GEM—Geologic Emissions of Methane, the missing source in the atmospheric methane budget. Atmospheric Environment, 38, pp. 3099-3100, doi: 10.1016/j.atmosenv.2004.04.002. Schumacher, D. 2012. Hydrocarbon Microseepage - A Significant but Underutilized Geologic Principle with Broad Applications for Oil/Gas Exploration and Production. AAPG Search and Discovery, Article #40943. Stadnitskaia, A., Ivanov, M.K., Blinova, V., Kreulen, R., van Weering, T.C.E. 2006. Molecular and carbon isotopic variability of hydrocarbon gases from mud volcanoes in the Gulf of Cadiz, NE Atlantic. Marine and Petroleum Geology, 23, pp. 281-296, doi: 10.1016/j.marpetgeo.2005.11.001.
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5=227>28
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2 5 : 75! 80 @ 2 
2=!!25! !:2 ;
55222<
520
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52!2
5A 2   2 :;  :  
2 5    : 50 )
   5 : 52  +BC! :
22 : 7A  ?; 5D : 5 2
 2   >2 : 22 :8 2  2
222<50):
 5 7,+0 8 : 2   2!  
25!52550):
57,+08:25:!
2!&A,$7(@80
,5<:
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) 2:> <!
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