Hydrocarbon seeps from the unconventional petroleum system of the South Portuguese Zone, Portugal

Abstract

The presence of natural hydrocarbon seepage in the surface (soil–water) system can provide strong evidence (pathfinder) of an active petroleum system, as well as critical information on source, maturity and migration pathways. The South Portuguese Zone (SPZ) is located in the southernmost sector of the pre-Mesozoic Iberian Variscan Chain, which is composed of Carboniferous interbedded shales and greywackes, commonly displaying low-grade metamorphism. For this study, the sampling area was selected based on geological features such as alignments, faults, thrust zones, and major fold structures. Twenty-seven soil samples and 31 water samples from artesian wells, boreholes and springs were collected using specific headspace containers (Isojar®). From the geological prospecting point of view, the presence of thermogenic hydrocarbon gases in SPZ formations is clear and evident. They are present in both soil and water in significantly high levels, especially in the fault/fracture zones, with light hydrocarbon (C1–C5) values reaching more than 1500 mg/L in soil samples. Anomalous presence of some gasoline-range hydrocarbons (such as toluene) was also detected. The integration with total organic carbon, vitrinite reflectance, Rock-Eval pyrolysis and GC-IRMS data from Paleozoic rocks suggests the presence of a senile unconventional petroleum system.