Presentaci

1
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_at_uvigo.es
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THE SETTING
G -H
M -PH
G -H
M -PH
M -PH
M -H
RÍAS BAIXAS (GALICIA) and NORTH PORTUGAL
TECTONIC MAP OF IBERIAN PENINSULA.Escale
11.000.000 (FROM IGME, 1980)
M -H
G -H
G -H
Basement in the rías region is mainly composed of
Palaeozoic metamorphic rocks and Hercynian
granites with NNW-SSE trendings.
M -H
LEGEND G H Hercynian Granites M H
Metamorphic Hercynian Rocks (Palaeozoic). M PH
Metamorphic Pre-Hercynian Rocks (Cambrian-
U.Precambrian).
3
RIA DE VIGO
Cíes Islands
Rande Narrow
San Simón Bay
4
Satellite image from Galicia.
Fault
Trends
u
NW
-
SE (dextral)
v
N
-
S (sinistral)
w
NE
-
SW (dextral)
5
Sparker Uniboom seismic lines
Bottom samples
Well
SEISMICS STRATIGRAPHY CONTINENTAL SHELF RÍAS
BAIXAS
6
WELL PMB1 (3538 m ) Galicia Shelf
(Data from ENIEPSA, 1984
7
multiple
multiple
8
Ría de Pontevedra
RSP 1
NE
SW
Ría de Pontevedra
GAS FRONT
multiple
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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
10
RÍA DE VIGO
SEDIMENT DISTRIBUTION ON PRESENT SEA-BED GAS
ESCAPES FEATURES
(From García-García, 1999)
11
7
6
96
27
37
39
54
97
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
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RÍA DE VIGO
SHALLOW GAS EVIDENCES
3.5 kHz mud profiler
Ø
13
RÍA DE VIGO
ECHO SOUNDER PROFILE
Ø
3.5 KHZ SUBBOTTOM PROFILE
Ø
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MUROS L21
The mounds in the Ría de Muros have been
identified on sidescansonar sonar 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)
15
Ría de Vigo Vibrocore ZVC-1 June2001
Bottom 2,52 m
Top
16
RIA DE VIGO
Gas filled pores occurs as white elongated spots
with larger deformtion at the edges af the cores
X-RAY GRAVITY CORE CV1
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RIA DE AROUSA
Gas front
Multiples
3.5 kHz
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MULTIBEAM
RIA DE AROUSA
POCKMARK
19
SOME QUANTITATIVE DATA FROM GAS ESCAPES
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RÍA DE PONTEVEDRA
Q1
LGM
Q1
LGM
Q2
Q2
DOMES
LGMEROSIVE SURFACE FROM THE LAST GLACIAL MAXIMUN
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RÍA DE PONTEVEDRA
DOMES (BLACK SPOTS)ARE LOCATED ALONG MAIN FAULT
TRENDS
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THERMAL SPRINGS CLOSE TO THE GALICIAN RÍAS
THE 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.
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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.
24
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.
25
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 GAS ESCAPES (METHANE)
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FAUNAS and GAS
(Data from Mijón, O., 1998)
(Data from Ramil et al. , 1997)
(Data from Mijón, O., 1998)
27
20 cm
Leptogorgia sarmentosa (Esper, 1789). RÍA DE VIGO
See slide 26 for location
CaCO3 Esclerites
28
Sponge on present muds in the inner part of Ría
de Vigo
10 cm
Suberites ficus (Linnaeus, 1767)
Domayo area (See slide26 for location)
29
Alcyionium digitatum Linnaeus, 1758 (Cnidaria,
Antozoa,Octochorallia), Ophiura Echinoderm
30
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 re-suspension 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
31
Sampling of gas bubbles 29th April 2002
5 cm
Low SPRING TIDE.
32
Sampling of gas bubbles 29th April 2002
High SPRING TIDE.
33
Abundance
31
GAS ANALYSIS
29
Scan Mode 627 (0.589 min) SOLE9.D
(Total gases)
Methane
15
13
17
33
19
40
44
26
42
24
46
48
22
36
m/z--gt
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INNER RÍA
OUTER RÍA
dls
p.s. b.
YD
ts
HST
U4
Q2
U3
TST
ts
U2
U1
LGM
ts
Q1
Pleistocene Seq.
GB
GB
Hercynian
T2
L. Miocene M. Miocene Seq?
Granite Basement
GB
T1
B
U. Oligocene- Miocene?
Metamorphic Basement Palaeozoic
MB
STRATIGRAPHIC MODEL FOR THE RÍAS BAIXAS
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GAS GENERATION
Organic-rich sediments, in which microbial
methane generation occurs, accumulate in the rías
environments largely controled by relative
sea-level 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 U1 to U3) and earlier High
Stand System Tract Sedimets (U4 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 Q2 Sequence could be originated by fluid
migration troughtout faults comming from thermal
springs.
36
MODEL OF THE RELATIVE SEA-LEVEL CHANGES FOR RÍAS
BAIXAS FROM THE LAST GLACIAL MAXIMUM (aprox. 18
ky)
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