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.
Resumen
La presencia de indicios de hidrocarburos naturales en superficie, tanto en suelos como en aguas, puede ser una evidencia clara de la existencia de un sistema petrolero, así como proporcionar información sobre origen, madurez y vías de migración. La Zona Sur Portuguesa (ZSP) está situada en el sector más meridional de la Cadena Ibérica Varisca pre-Mesozoica, formada por la intercalación de esquistos y grauvacas del Carbonífero de bajo grado metamórfico. El área de muestreo para el presente estudio se seleccionó en base a características geológicas tales como alineamiento, fallas, cabalgamientos y pliegues principales. Veinte y siete muestras de suelos y treinta y una de aguas procedentes de pozos artesanos, sondeos y fuentes naturales se recolectaron usando viales tipo ”headspace” (Isojar®). La presencia de hidrocarburos gaseosos termogénicos en la ZSP tanto en suelos como en aguas es significadamente alta, en especial en las zonas de fallas y fracturas, donde los hidrocarburos ligeros (C1-C5) alcanzan valores mayores a 1.500mg/L en muestras de suelos. Así mismo, también se ha detectado la presencia anómala de algunos hidrocarburos en el rango de las gasolinas, tales como el tolueno. La integración con el conjunto de valores de carbono orgánico total, reflectancia de vitrinita, pirolisis Rock-Eval y GC-IRMS de rocas paleozoicas sugiere la existencia de un sistema petrolero no convencional senil.
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Acknowledgement
This study was financed by CNPq (Brazil), Equinor (Norway–Brazil) and FCT (Portugal). The authors would like to thank Isolab b.v. (The Netherlands) and Isotech (USA) for all analytical support. Special thanks to reviewers for the valuable contributions.
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Appendices
Appendix 1: Geochemical composition of headspace gases recovered from soil samples collected in Isojars® (1 ppm = 1 mg/L)
Sample | mole % in total gas | Gas composition (mole %) (total hydrocarbons = 100%) | Wetness | Carbon isotope ratios (d13C vs. vPDB) | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Number | Type | CO2 | C1 | C2 | C2H4 | C3 | C3H6 | i-C4 | n-C4 | i-C5 | n-C5 | C1/C2 | C1 | C2 | C2H4 | C3 | C3H6 |
SPZ-00 | Soil | 4.2 | 57.8 | – | 29.6 | 4 | 3 | – | 1.5 | – | 4.1 | – | – | – | – | – | – |
SPZ-01 | Soil | 1.1 | 95.9 | – | 4.1 | – | – | – | – | – | – | – | – | – | – | – | – |
SPZ-02 | Soil | 1.2 | 100 | – | – | – | – | – | – | – | – | – | – | – | – | – | – |
SPZ-03 | Soil | 1.3 | 100 | – | – | – | – | – | – | – | – | – | – | – | – | – | – |
SPZ-04 | Soil | 3.1 | 75.3 | – | 19.3 | – | – | – | – | – | 5.4 | – | – | – | – | – | – |
SPZ-05 | Soil | 0.34 | 100 | – | – | – | – | – | – | – | – | – | – | – | – | – | – |
SPZ-06 | Soil | 6.6 | 48.7 | – | 26 | 8.7 | 11.4 | – | – | – | 5.1 | – | – | – | – | – | – |
SPZ-07 | Soil | 1.3 | 100 | – | – | – | – | – | – | – | – | – | – | – | – | – | – |
SPZ-08 | Soil | 4 | 65.2 | – | 28.3 | 2.5 | 1.7 | – | – | – | 2.4 | – | – | – | – | – | – |
SPZ-09 | Soil | 2.6 | 86.7 | – | 8.3 | 1.8 | – | – | – | – | 3.1 | – | – | – | – | – | – |
SPZ-10 | Soil | 5.3 | 61.7 | – | 30.5 | 2.2 | 2.2 | – | – | – | 3.4 | – | – | – | – | – | – |
SPZ-11 | Soil | 0.87 | 100 | – | – | – | – | – | – | – | – | – | – | – | – | – | – |
SPZ-12 | Soil | 3.1 | 82.1 | – | 14 | 3.9 | – | – | – | – | – | – | – | – | – | – | – |
SPZ-13 | Soil | 0.58 | 100 | – | – | – | – | – | – | – | – | – | – | – | – | – | – |
SPZ-14 | Soil | 0.71 | 100 | – | – | – | – | – | – | – | – | – | – | – | – | – | – |
SPZ-15 | Soil | 3.2 | 67.6 | – | 21.3 | 3.4 | 1.7 | – | 1.9 | – | 4 | – | – | – | – | – | – |
SPZ-16 | Soil | 0.87 | 100 | – | – | – | – | – | – | – | – | – | – | – | – | – | – |
SPZ-17 | Soil | 2.1 | 100 | – | – | – | – | – | – | – | – | – | – | – | – | – | – |
SPZ-18 | Soil | 0.46 | 100 | – | – | – | – | – | – | – | – | – | – | – | – | – | – |
SPZ-19 | Soil | 0.89 | 100 | – | – | – | – | – | – | – | – | – | – | – | – | – | – |
F081 | Soil | 4.2 | 49 | 23.8 | 4.5 | 10.2 | 5.8 | 0.58 | 3.9 | 0.66 | 1.6 | 2.06 | − 27.4 | – | – | – | – |
F08 | Soil | 3.1 | 47.6 | 26.9 | 1.2 | 12.8 | 3.7 | 0.63 | 4.7 | 0.78 | 1.7 | 1.77 | − 31 | – | – | – | – |
F09 | Soil | 3.2 | 54 | 17.3 | 9.1 | 7.5 | 7.1 | 0.55 | 2.7 | 0.6 | 1.1 | 3.12 | − 26 | − 31.2 | − 28 | − 32 | − 31 |
F10 | Soil | 0.62 | 53.6 | 17.4 | 10.2 | 7.2 | 7.7 | 0.51 | 2.4 | 0.55 | 0.52 | 3.08 | − 24.8 | – | – | – | – |
F11 | Soil | 0.7 | 80.8 | 6.9 | 6.9 | 2.7 | 2.7 | – | – | – | – | 11.71 | Too small | – | – | – | – |
F12 | Soil | 0.56 | 62.5 | 11 | 11 | 6.5 | 6.5 | – | 2.4 | – | – | 5.68 | Too small | – | – | – | – |
P03 | Soil | 1.1 | 63 | 10.1 | 10.5 | 5.3 | 7 | 0.36 | 1.8 | 1.1 | 0.83 | 6.24 | – | – | – | – | – |
Sample | Carbon isotope ratios (d13C vs. vPDB) | Gas composition (mole ppm) | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Number | Type | i-C4 | n-C4 | i-C5 | n-C5 | C1 | C2 | C2H4 | C3 | C3H6 | i-C4 | n-C4 | i-C5 | n-C5 | Toluene | C6+ |
SPZ-00 | Soil | – | – | – | – | 2 | – | 1 | 0.1 | 0.1 | – | 0.1 | – | 0.1 | < 1 ppm | 2 |
SPZ-01 | Soil | – | – | – | – | 2 | – | 0.1 | – | – | – | – | – | – | < 1 ppm | 2 |
SPZ-02 | Soil | – | – | – | – | 2 | – | – | – | – | – | – | – | – | < 1 ppm | 1 |
SPZ-03 | Soil | – | – | – | – | 2 | – | – | – | – | – | – | – | – | < 1 ppm | 2 |
SPZ-04 | Soil | – | – | – | – | – | – | – | – | – | – | – | – | – | < 1 ppm | < 1 ppm |
SPZ-05 | Soil | – | – | – | – | 2 | – | – | – | – | – | – | – | – | < 1 ppm | 2 |
SPZ-06 | Soil | – | – | – | – | 2 | – | 1 | 0.4 | 0.5 | – | – | – | 0.2 | < 1 ppm | 3 |
SPZ-07 | Soil | – | – | – | – | 2 | – | – | – | – | – | – | – | – | < 1 ppm | 5 |
SPZ-08 | Soil | – | – | – | – | 2 | – | 0.9 | 0.1 | 0.1 | – | – | – | 0.1 | < 1 ppm | 17 |
SPZ-09 | Soil | – | – | – | – | 2 | – | 0.2 | – | – | – | – | – | 0.1 | < 1 ppm | < 1 ppm |
SPZ-10 | Soil | – | – | – | – | 3 | – | 1 | 0.1 | 0.1 | – | – | – | 0.2 | < 1 ppm | 2 |
SPZ-11 | Soil | – | – | – | – | 2 | – | – | – | – | – | – | – | – | < 1 ppm | 6 |
SPZ-12 | Soil | – | – | – | – | 2 | – | 0.3 | 0.1 | – | – | – | – | – | < 1 ppm | < 1 ppm |
SPZ-13 | Soil | – | – | – | – | 2 | – | – | – | – | – | – | – | – | < 1 ppm | 2 |
SPZ-14 | Soil | – | – | – | – | 2 | – | – | – | – | – | – | – | – | < 1 ppm | < 1 ppm |
SPZ-15 | Soil | – | – | – | – | 2 | – | 0.6 | 0.1 | 0.1 | 0.1 | 0.1 | < 1 ppm | 4 | ||
SPZ-16 | Soil | – | – | – | – | 2 | – | – | – | – | – | – | – | – | < 1 ppm | 5 |
SPZ-17 | Soil | – | – | – | – | 1 | – | – | – | – | – | – | – | – | < 1 ppm | 2 |
SPZ-18 | Soil | – | – | – | – | 2 | – | – | – | – | – | – | – | – | < 1 ppm | 1 |
SPZ-19 | Soil | – | – | – | – | 1 | – | – | – | – | – | – | – | – | < 1 ppm | < 1 ppm |
F081 | Soil | – | – | – | – | 96 | 47 | 9 | 20 | 11 | 1 | 8 | 1 | 3 | 2 | 10 |
F08 | Soil | – | – | – | – | 1000 | 565 | 25 | 269 | 78 | 13 | 99 | 16 | 36 | 5 | 117 |
F09 | Soil | − 31 | − 32 | − 31 | − 33 | 160 | 51 | 27 | 22 | 21 | 2 | 8 | 2 | 3 | 5 | 17 |
F10 | Soil | – | – | – | – | 31 | 10 | 6 | 4 | 4 | 0.3 | 1 | 0.3 | 0.3 | < 1 ppm | 2 |
F11 | Soil | – | – | – | – | 3 | 0.3 | 0.3 | 0.1 | 0.1 | – | – | – | – | < 1 ppm | < 1 ppm |
F12 | Soil | – | – | – | – | 3 | 0.5 | 0.5 | 0.3 | 0.3 | – | 0.1 | – | – | < 1 ppm | < 1 ppm |
P03 | Soil | – | – | – | – | 8 | 1 | 1 | 0.7 | 0.9 | – | 0.2 | 0.1 | 0.1 | < 1 ppm | < 1 ppm |
Appendix 2: Geochemical composition of headspace gases recovered from water samples collected in Isojars® (1 ppm = 1 mg/L)
Sample | mol% in total gas | Gas composition (mol%) (total hydrocarbons = 100%) | Wetness | Carbon isotope ratios (d13C vs. vPDB) | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Number | Type | CO2 | C1 | C2 | C2H4 | C3 | C3H6 | i-C4 | n-C4 | i-C5 | n-C5 | C1/C2 | C1 | C2 | C2H4 | C3 | C3H6 |
W02 | Water | 0.85 | 94.5 | – | – | – | – | – | 2.9 | 1.9 | 0.75 | – | – | – | – | – | – |
W03 | Water | 2.1 | 82.7 | – | 7.4 | – | – | – | 1.3 | 5.1 | 3.4 | – | – | – | – | – | – |
W06 | Water | 3.2 | 82.3 | – | – | – | – | – | 3.7 | 8.2 | 5.8 | – | – | – | – | – | – |
W07 | Water | 1.4 | 92.5 | – | – | – | – | – | 1.8 | 3.3 | 2.5 | – | – | – | – | – | – |
W09 | Water | 0.094 | 83.9 | 0.98 | 2.4 | 1.3 | – | – | 3 | 4.4 | 4 | 85.61 | – | – | – | – | – |
W10 | Water | 2.9 | 89.5 | – | – | – | – | – | 1.5 | 5.1 | 4 | – | – | – | – | – | – |
W11 | Water | 0.02 | 95.7 | – | – | – | – | – | 0.67 | 1.8 | 1.8 | – | – | – | – | – | – |
W12 | Water | 2.8 | 87.7 | – | – | – | – | – | 3 | 4.7 | 4.7 | – | – | – | – | – | – |
W13 | Water | 0.59 | 86.4 | – | – | – | – | – | 1.7 | 6 | 6 | – | – | – | – | – | – |
W14 | Water | 1.1 | 89.9 | – | – | – | – | – | 1.3 | 5.2 | 3.5 | – | – | – | – | – | – |
W15 | Water | 0.47 | 79 | – | 5.3 | – | – | – | 2.5 | 5.9 | 7.2 | – | – | – | – | – | – |
W16 | Water | 2.4 | 78.9 | – | – | – | – | – | 3.6 | 10.5 | 7 | – | – | – | – | – | – |
W17 | Water | 3.8 | 82.4 | – | 3.1 | – | – | – | 2.2 | 5.8 | 6.4 | – | – | – | – | – | – |
W18 | Water | 4.1 | 80.3 | – | 7.2 | – | – | – | 2.8 | 5.7 | 4 | – | – | – | – | – | – |
W19 | Water | 3.9 | 86.8 | – | – | – | – | – | 0.92 | 6.5 | 5.8 | – | – | – | – | – | – |
W20 | Water | 3.7 | 86.4 | – | – | – | – | – | 6.9 | 4.7 | 2 | – | – | – | – | – | – |
W21 | Water | 2.7 | 80.3 | – | 11.2 | – | – | – | 4.8 | 2.3 | 1.4 | – | – | – | – | – | – |
W22 | Water | 1.1 | 86.7 | – | – | – | – | – | 6.2 | 4.2 | 3 | – | – | – | – | – | – |
W23 | Water | 0.19 | 85.3 | – | 2.8 | 3.6 | – | – | 4 | 2.1 | 2.1 | – | – | – | – | – | – |
W24 | Water | 0.16 | 83.5 | – | 7.4 | – | – | – | 3.6 | 3.5 | 2.1 | – | – | – | – | – | – |
W25 | Water | 1.8 | 88.1 | – | 4.9 | – | – | – | 2.9 | 2.7 | 1.4 | – | – | – | – | – | – |
W26 | Water | 1.8 | 87.2 | – | – | – | – | – | 4 | 5.6 | 3.1 | – | – | – | – | – | – |
W27 | Water | 2.5 | 93.8 | – | 2.1 | – | – | – | 0.68 | 1.8 | 1.6 | – | – | – | – | – | – |
W28 | Water | 1.3 | 93.7 | – | – | – | – | – | 2.9 | 3.4 | – | – | – | – | – | – | |
W29 | Water | 0.25 | 88.1 | – | – | – | – | – | 3.1 | 5.6 | 3.2 | – | – | – | – | – | – |
W30 | Water | 0.54 | 95.2 | – | – | – | – | – | 4.8 | – | – | – | – | – | – | – | – |
W31 | Water | 2 | 95.3 | – | 1.9 | – | – | – | 2.8 | – | – | – | – | – | – | – | – |
W32 | Water | 0.21 | 100 | – | – | – | – | – | – | – | – | – | – | – | – | – | – |
W33 | Water | 2.7 | 96 | – | 2.7 | – | – | – | 0.56 | 0.43 | 0.29 | – | – | – | – | – | – |
W33-1 | Water | 1.4 | 63.3 | 5.5 | 23 | – | 8.2 | – | – | – | – | 11.51 | Too small | – | – | – | – |
W34 | Water | 3 | 97.1 | 1.3 | – | 0.99 | – | – | 0.42 | – | 0.2 | 74.69 | − 32.7 | – | – | – | – |
Sample | Carbon isotope ratios (d13C vs. vPDB) | Gas composition (mole ppm) | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Number | Type | i-C4 | n-C4 | i-C5 | n-C5 | C1 | C2 | C2H4 | C3 | C3H6 | i-C4 | n-C4 | i-C5 | n-C5 | Toluene | C6+ |
W02 | Water | – | – | – | – | 5 | – | – | – | – | – | 0.2 | 0.1 | – | 3 | 4 |
W03 | Water | – | – | – | – | 4 | – | 0.4 | – | – | – | 0.1 | 0.2 | 0.2 | 3 | 3 |
W06 | Water | – | – | – | – | 3 | – | – | – | – | – | 0.1 | 0.3 | 0.2 | 5 | 6 |
W07 | Water | – | – | – | – | 6 | – | – | – | – | – | 0.1 | 0.2 | 0.2 | 4 | 5 |
W09 | Water | – | – | – | – | 8 | 0.1 | 0.2 | 0.1 | – | – | 0.3 | 0.4 | 0.4 | 3 | 4 |
W10 | Water | – | – | – | – | 5 | – | – | – | – | – | 0.1 | 0.3 | 0.2 | 2 | 3 |
W11 | Water | – | – | – | – | 7 | – | – | – | – | – | – | 0.1 | 0.1 | 4 | 5 |
W12 | Water | – | – | – | – | 3 | – | – | – | – | – | 0.1 | 0.2 | 0.2 | < 1 ppm | < 1 ppm |
W13 | Water | – | – | – | – | 2 | – | – | – | – | – | – | 0.1 | 0.1 | 3 | 3 |
W14 | Water | – | – | – | – | 4 | – | – | – | – | – | 0.1 | 0.2 | 0.2 | 3 | 4 |
W15 | Water | – | – | – | – | 2 | – | 0.1 | – | – | – | 0.1 | 0.1 | 0.2 | 3 | 4 |
W16 | Water | – | – | – | – | 2 | – | – | – | – | – | 0.1 | 0.3 | 0.2 | 2 | 3 |
W17 | Water | – | – | – | – | 3 | – | 0.1 | – | – | – | 0.1 | 0.2 | 0.2 | 5 | 6 |
W18 | Water | – | – | – | – | 3 | – | 0.3 | – | – | – | 0.1 | 0.2 | 0.1 | 4 | 4 |
W19 | Water | – | – | – | – | 2 | – | – | – | – | – | – | 0.1 | 0.1 | 2 | 3 |
W20 | Water | – | – | – | – | 2 | – | – | – | – | – | 0.2 | 0.1 | – | 5 | 5 |
W21 | Water | – | – | – | – | 3 | – | 0.4 | – | – | – | 0.2 | 0.1 | 0.1 | < 1 ppm | < 1 ppm |
W22 | Water | – | – | – | – | 2 | – | – | – | – | – | 0.1 | 0.1 | 0.1 | 5 | 5 |
W23 | Water | – | – | – | – | 3 | – | 0.1 | 0.1 | – | – | 0.1 | 0.1 | 0.1 | 6 | 7 |
W24 | Water | – | – | – | – | 2 | – | 0.2 | – | – | – | 0.1 | 0.1 | 0.1 | 1 | 2 |
W25 | Water | – | – | – | – | 3 | – | 0.2 | – | – | – | 0.1 | 0.1 | – | 5 | 5 |
W26 | Water | – | – | – | – | 2 | – | – | – | – | – | 0.1 | 0.1 | 0.1 | 2 | 2 |
W27 | Water | – | – | – | – | 6 | – | 0.1 | – | – | – | – | 0.1 | 0.1 | 1 | 2 |
W28 | Water | – | – | – | – | 3 | – | – | – | – | – | 0.1 | 0.1 | – | 2 | 2 |
W29 | Water | – | – | – | – | 2 | – | – | – | – | – | 0.1 | 0.1 | 0.1 | 3 | 3 |
W30 | Water | – | – | – | – | 3 | – | – | – | – | – | 0.2 | – | – | 5 | 5 |
W31 | Water | – | – | – | – | 2 | – | – | – | – | – | 0.1 | – | – | 2 | 2 |
W32 | Water | – | – | – | – | 3 | – | – | – | – | – | – | – | – | 2 | 2 |
W33 | Water | – | – | – | – | 12 | – | 0.3 | – | – | – | 0.1 | 0.1 | – | 2 | 2 |
W33-1 | Water | – | – | – | – | 4 | 0.3 | 1 | – | 0.5 | – | – | – | – | 2 | 2 |
W34 | Water | – | – | – | – | 70 | 0.9 | – | 0.7 | – | – | 0.3 | – | 0.1 | 2 | 2 |
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Barberes, G.A., Spigolon, A.L.D., dos Reis, R.P. et al. Hydrocarbon seeps from the unconventional petroleum system of the South Portuguese Zone, Portugal. J Iber Geol 46, 1–19 (2020). https://doi.org/10.1007/s41513-019-00115-x
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DOI: https://doi.org/10.1007/s41513-019-00115-x
Keywords
- Hydrocarbon seeps
- South Portuguese Zone
- Unconventional petroleum system
- Light hydrocarbons
- Surface geochemical prospecting
- Gas