Universidade de Vigo A NATURAL LABORATORY FOR SHALLOW

Universidade de Vigo A NATURAL LABORATORY FOR SHALLOW GAS: THE RÍAS BAIXAS (SPAIN) Contribution 7, by: García-Gil, Soledad *Dpto. Geociencias Marinas, Facultad de Ciencias, Universidad de Vigo, 36200, Vigo, Spain E-mail: sgil@uvigo. es

García-Gil, S. , A Natural Laboratory for Shallow gas. . . THE SETTING G -H M -PH M -H RÍAS BAIXAS (GALICIA) and NORTH PORTUGAL TECTONIC MAP OF IBERIAN PENINSULA. Escale 1: 1. 000 (FROM IGME, 1980) Basement in the rías region is mainly composed of Palaeozoic metamorphic rocks and Hercynian granites with NNW-SSE trendings. LEGEND: G – H: Hercynian Granites M – H: Metamorphic Hercynian Rocks (Palaeozoic). M – PH: Metamorphic Pre-Hercynian Rocks (Cambrian- U. Precambrian). G -H M -H

García-Gil, S. , A Natural Laboratory for Shallow gas. . . RIA DE VIGO Cíes Islands Rande Narrow San Simón Bay

García-Gil, S. , A Natural Laboratory for Shallow gas. . . Satellite image from Galicia. Fault Trends u NW-SE (dextral) v N-S (sinistral) w NE-SW (dextral)

García-Gil, S. , A Natural Laboratory for Shallow gas. . . METHODOLOGY Sparker & Uniboom seismic lines Bottom samples Well SEISMICS STRATIGRAPHY: CONTINENTAL SHELF & RÍAS BAIXAS

García-Gil, S. , A Natural Laboratory for Shallow gas. . . WELL PMB 1 (3538 m ) Galicia Shelf (Data from ENIEPSA, 1984

García-Gil, S. , A Natural Laboratory for Shallow gas. . . multiple SW Ria de Pontevedra multiple NE

García-Gil, S. , A Natural Laboratory for Shallow gas. . . Ría de Pontevedra Q 2 Q 1 T 2 T 1 SW U. Pleistocene-Holocene Last Glacial Maximum Seq. Pleistocene L. Miocene- M. Miocene? U. Oligocene- Miocene? RSP 1 NE Ría de Pontevedra GAS FRONT multiple

García-Gil, S. , A Natural Laboratory for Shallow gas. . . GAS SEAL IS MAINLY COMPOUND BY THE HOLOCENE MUDS. GAS ACCUMULATIONS ALWAYS ARE LOCATED UNDERLYING THESE FINER SEDIMENTS, WHILST ACOUSTIC PLUMES ALSO OCCURS WITHIN AREAS OF COARSER SEDIMENTS (SANDS) RÍA DE AROUSA

García-Gil, S. , A Natural Laboratory for Shallow gas. . . RÍA DE VIGO SEDIMENT DISTRIBUTION ON PRESENT SEA-BED & GAS ESCAPES FEATURES (From García-García, 1999)

García-Gil, S. , A Natural Laboratory for Shallow gas. . . 97 54 39 37 27 96 6 7 SANDY MUD (F(a)) BIOCLASTIC SAND (B) MUDS (F) BIOTURBATED MUDS (Fdb) LAMINATED MUDS (Fl) LAMINATED & BIOTURBATED MUDS (Flb) GRAVITY CORES CORRELATION OF GAS SEAL FACIES

García-Gil, S. , A Natural Laboratory for Shallow gas. . . RÍA DE VIGO SHALLOW GAS EVIDENCES Ø 3. 5 k. Hz mud profiler

García-Gil, S. , A Natural Laboratory for Shallow gas. . . RÍA DE VIGO Ø ECHO SOUNDER PROFILE Ø 3. 5 KHZ SUBBOTTOM PROFILE

García-Gil, S. , A Natural Laboratory for Shallow gas. . . MUROS L 21 MUROS T to L 27 The mounds in the Ría de Muros have been identified on sidescansonar and are typically 5 m in diameter and 1 m high, located in water depths from 10 m to 15 m. (García-Gil et al. 2001)

García-Gil, S. , A Natural Laboratory for Shallow gas. . . Ría de Vigo Vibrocore ZVC-1 June 2001 Bottom 2, 52 m GAS VOIDS Top

García-Gil, S. , A Natural Laboratory for Shallow gas. . . 0 m 0. 38 m 0. 68 m RIA DE VIGO Gas filled pores occurs as white elongated spots with larger deformtion at the edges af the cores 0. 38 m X-RAY GRAVITY CORE CV 1 0. 68 m 0. 98 m

García-Gil, S. , A Natural Laboratory for Shallow gas. . . RIA DE AROUSA Gas front Multiples 3. 5 k. Hz

García-Gil, S. , A Natural Laboratory for Shallow gas. . . MULTIBEAM RIA DE AROUSA POCKMARK

García-Gil, S. , A Natural Laboratory for Shallow gas. . . SOME QUANTITATIVE DATA FROM GAS ESCAPES RIA DE VIGO 176. 4 -40 km 2 TYPE OF GAS ESCAPES POCKMARKS Nº IDENTIFIED 158 GAS PLUMES POCKMARKS MEAN SINGLE DIMENTIONS D: 6. 83 x 4. 62 m Up to 15 m high GAS PLUMES 59 h: 1 m Up to 20 m high D: 5. 88 x 8. 23 m d: 0. 59 x 1. 03 m Up to 10 m high (cell: 1 km 2 ) 19 60 DENSITY IN SEEPING AREA 1. 65 (target/km 2) POCKMARKS 536 1940 H: 1. 3 m D: 60 x 40 m h: 0. 4 m d: 5. 16 m (cell: 5 km 2 ) RIA DE MUROS 125 km 2 GAS PLUMES 165 1168 h: 0. 5 m-1. 5 CELLS OCCURRENCE 19 (5 km 2 ) RIA DE AROUSA 230 km 2 49 20 1. 72 19. 5 8 10. 9 423 to 6 Ad: 97 1 to 15 Ad: 7. 38 5. 08 DENSITY IN RÍA AREA (target/km 2) 0. 89 0. 93 VR: 4027 m 3 2. 3 15. 52 0. 47 (V= 1/3 ∏ R 2 H) FLUX to Water 206. 25 to 5907 t yr-1 670 to 19188 t yr-1 73. 75 to 2112. 2 t yr-1 FLUX to Atmosphere (70%) 144 to 4135 t yr-1 469 to 19188 t yr-1 51. 63 to 1478. 45 t yr-1

García-Gil, S. , A Natural Laboratory for Shallow gas. . . RÍA DE PONTEVEDRA LGM Q 1 LGM Q 2 DOMES LGM: EROSIVE SURFACE FROM THE LAST GLACIAL MAXIMUN

García-Gil, S. , A Natural Laboratory for Shallow gas. . . RÍA DE PONTEVEDRA DOMES (BLACK SPOTS)ARE LOCATED ALONG MAIN FAULT TRENDS

García-Gil, S. , A Natural Laboratory for Shallow gas. . . THERMAL SPRINGS CLOSE TO THE GALICIAN RÍAS THERMAL SPRINGS ARE ALIGNED ALONG NNW-SSE to NE-SW TRENDING FAULTS. THE DOMES ALSO COINCIDE WITH THESE LIGNATIONS AND COULD BE ORIGINED BY RISING THERMAL FLUIDS.

García-Gil, S. , A Natural Laboratory for Shallow gas. . . DIAGENETICAL PROCCESSES. RÍA DE VIGO. SULPHIDES AND SULFATES (García, 2001) AUTHIGENIG PYRITE AND GYPSUM OCCURS WITHIN THE GAS SEAL FACIES. THESE ANOMALIES ARE SHALLOWER AT THE INNER PARTS OF THE RÍA AND DEEPER AT THE EXTERNAL AREAS. THE GAS ALSO ACCUMULATES SHALLOWER AT THE INNER PARTS THAN AT THE OUTER AREAS.

García-Gil, S. , A Natural Laboratory for Shallow gas. . . Carbonate Dissolution by Microboring organisms (García et al. 2002) EDS Microanalysis Dissolution of carbonate grains may be enhance by the occurrence of micro-organisms such as microboring. EDX analysis shows Al-Si coating on microtubular structures, indicating dissolution-recristallization processes.

García-Gil, S. , A Natural Laboratory for Shallow gas. . . KISH BANK SEEP MOUND (SEM) ARAGONITE PYRITE ARAGONITE AUTHIGENIC ARAGONITE AND PYRITE (SIMILAR PARAGENESIS THAN IN THE RÍA DE VIGO) APPEARS IN THE MOUNDS OF THE IRISH SEA WHICH ALSO ARE RELATED WITH

García-Gil, S. , A Natural Laboratory for Shallow gas. . . FAUNAS and GAS (Data from Mijón, O. , 1998) (Data from Ramil et al. , 1997) (Data from Mijón, O. , 1998)

20 cm García-Gil, S. , A Natural Laboratory for Shallow gas. . . Leptogorgia sarmentosa (Esper, 1789). RÍA DE VIGO See slide 26 for location Ca. CO 3 Esclerites

García-Gil, S. , A Natural Laboratory for Shallow gas. . . Sponge on present muds in the inner part of Ría de Vigo 10 cm Suberites ficus (Linnaeus, 1767) Domayo area (See slide 26 for location)

García-Gil, S. , A Natural Laboratory for Shallow gas. . . Alcyionium digitatum Linnaeus, 1758 (Cnidaria, Antozoa, Octochorallia), Ophiura & Echinoderm

García-Gil, S. , A Natural Laboratory for Shallow gas. . . The high productivity within the Rías Baixas has been directly related to the upwelings currents comming into the rías from May to September. These currents are very rich in nutrients. The idea about that active seeps enhance this productivity is proposed in here. The fluid escapes such as seeps and pockmarks would provoke the resuspension from the present sea bed sediments as well as the whole nutrient components contained into sediments. The filter-feeding faunas (eg. mussels) will be favoured by presence of these seeps. Mussel Rafts

García-Gil, S. , A Natural Laboratory for Shallow gas. . . Sampling of gas bubbles 29 th April 2002 shell roots 5 c m peat Low SPRING TIDE.

García-Gil, S. , A Natural Laboratory for Shallow gas. . . Sampling of gas bubbles 29 th April 2002 High SPRING TIDE.

García-Gil, S. , A Natural Laboratory for Shallow gas. . . Abundance 31 5500000 GAS ANALYSIS 29 5000000 Scan Mode 627 (0. 589 min): SOLE 9. D 4500000 4000000 3500000 (Total gases) 3000000 2500000 2000000 1500000 Methane 15 1000000 500000 13 0 m/z--> 17 19 22 24 26 33 36 40 42 44 46 48 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56

García-Gil, S. , A Natural Laboratory for Shallow gas. . . Q 2 - U. Pleistocene-Holocene Seq. YD – Younger Dryas LGM – Last Glacial Maximum dls – downlap surface and maximum flooding surface ts – transgressive surface INNER RÍA OUTER RÍA HST TST ts Q 2 U 4 p. s. b. U 3 U 2 U 1 dls ts L. Miocene – M. Miocene Seq? T 1 U. Oligocene. Miocene? MB GB Hercynian T 2 GB B YD LGM Q 1 Pleistocene Seq. GB ts Metamorphic Basement Palaeozoic STRATIGRAPHIC MODEL FOR THE RÍAS BAIXAS Granite Basement

García-Gil, S. , A Natural Laboratory for Shallow gas. . . GAS GENERATION Organic-rich sediments, in which microbial methane generation occurs, accumulate in the rías environments largely controled by relative sealevel variations (see slide 36). Falling relative sea-level (about 100 m) during the Last Glacial Maximum (18 ka) provoked a prominent unconformity due to subaerial erosion on the shallower continental shelf and whitin the Rías Baixas. Relative sea-level rise from its lowest position allowed the infilling of the existing incised valleys by the Transgressive System Tracts (seismic units U 1 to U 3) and earlier High Stand System Tract Sedimets (U 4 seismic unit, see slide 34). These sediments (TST) are characteriscally organic rich due to the high productivity of such areas. They are inmediately overlain by the finest sediments of the HST that act as seal of the gas. The domes appearing on the Q 2 Sequence could be originated by fluid migration troughtout faults comming from thermal springs.

García-Gil, S. , A Natural Laboratory for Shallow gas. . . 0 ALTITUDE (m) MODEL OF THE RELATIVE SEA-LEVEL CHANGES FOR RÍAS BAIXAS FROM THE LAST GLACIAL MAXIMUM (aprox. 18 ky) +50 -100 Fairbanks (1989) Aloisi et al. (1978) Pinot (1968) García-Gil - et al. (1999) -150 20 15 10 AGE (ky B. P. ) 5 0

García-Gil, S. , A Natural Laboratory for Shallow gas. . . REFERENCES Aloïsi, J. C. ; Monaco, A. ; Planchais, N. ; Thommeret, J. ; and Thommeret, Y. (1978): The Holocene trangression in the Golfe du Lion, southwestern France: paleogeographic and paleobotanical evolution. Géographie Physique et Quaternaire, 32 (2), 145 -162. ENIEPSA (1984): Sondeo Pontevedra Marino B-1. Dirección General de la Energía, Ministerio Industria, Comercio y Turismo, España. Fairbanks, R. G. (1989): A 17, 000 -year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deepocean circulation. Nature, 342, 637 -642. García, T. (2001): Pautas de Organización Geoquímica entre Sedimentos Carbonatados y Siliciclásticos en la Ría de Vigo: Comportamiento Superficial y Diagenético. MSc. Thesis, Dpt. Geociencias Marinas y O. T. , Facultad de Ciencias, Universidad de Vigo, Unpublished, 144 pp. García, T. ; Gago-Duport, L. ; Bastero; S. F. ; Velo, A. ; Santos A. and de la Rosa-Fox, N. (2002). Geochemical self-organization of mixed siliciclasticcarbonate sediments in estuarine-like systems. Geochimica et Cosmochimica Acta, 66: (15 A), A 262 -A 262. García-García, A. (1999): Estudio del Gas de los Sedimentos de la Ría de Vigo Mediante Técnicas Sísmico-Acústicas de Alta Resolución. MSc. Thesis, Dpt. Geociencias Marinas y O. T. , Facultad de Ciencias, Universidad de Vigo, Unpublished, 165 pp. García-Gil; S. ; Croker, P. and Vilas, F. (2001): Seafloor Mounds in the Muros and Arousa Rías of NW Spain. Poster-Abstracts Earth System Processes Global Meeting, Geol. Soc. America and Geol. Soc. London, June 2001, Edinburgh, Scotland, 104. García-Gil, S. ; Vilas, F. ; García-García, A. ; Durán, R. (199): Holocene Storm Delta In Incised_Valley Fill Sediments of Ría de Pontevedra, NW Spain. Poster. AGU Fall Meeting, San Francisco, USA. Eos, Trnsactions, 80 (46), 559. IGME (1980): Mapa Tectónico de la Península Ibérica y Baleares, Escala 1: 1. 000. Ed. by Servicio de Publicaciones-Ministerio de Industria y Energía de España. Mijón, O. , (1998): Antozoos de la Ría de Vigo. MSc. Thesis, Dpt. De Ecología y Biología Animal, Facultad de Ciencias, Universidad de Vigo. Unpublished, 165 pp. Pinot, J. P. (1968): Littoraux wurmiens submergès à l´ouest de Belle-Ile. Bulletin de l´Association Fraçaise pour l´Etude du Quaternaire, 3, 197 -216. Ramil, F. ; Ansín-Agís, J. ; Fernández-Pulpeiro, E. (1997): Soft-bottom hydroids (Cnidaria: Hydrozoa) collected in the Ría de Vigo (NW Spain). Zool. Verh. Leiden, 323, 182 -208.