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Figure 4y
Figura y4 The mapping of the BSR reflector at of samples during the scientific cruise of
Foz do Amazonas indicated an area with the Tucuxi Project, in 2015.
28,000 km², present in seismic sections ob- The gas hydrates recovered through
tained in water depths from 600 to 2800 cores drilled in a water depth of 1900 m,
m, at a depth of about 450 m below the sea inside a channel where the gas escape is
floor (Figure 4y). The BSR occurs discontin- active, showed a composition of CH (85-
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uously in the seismic lines, being commonly 98%), CO (2-15%) and traces of C H . Iso-
2 2 6
associated with the hinge of anticlinal folds topic values of carbon present in methane

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5 o 0’0’’N in the compressive zone of the fan. These indicate a biogenic origin with δ C values
between -81.1 and -77.3‰ (RODRIGUES
reflectors disappear towards the flanks
(KETZER et al., 2022). The reasons for this et al., 2019). The gas hydrates present in
pattern of occurrence are still poorly under- their composition CH (95%), CO (4%)
4 2
stood, but they may be related to the differ- and C H (1%); isotopic analysis of meth-
2 6
ential fluid flow in the structures (PRAEG et ane (δ C between -61.7 and -59.2‰ and
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al., 2018). Sad et al., (1998), using the total δD between -206 and -205‰) and ethane
volume of sediments existing in the GHSZ (δ13C between -31.5 and -30.8‰) indicate
and assuming a saturation of 1.5% of gas a fundamentally biogenic origin with pos-
hydrates present, estimated a potential vol- sible thermogenic mixing (RODRIGUES et
ume of methane in the order of 12 trillion al., 2019). This mixing may have occurred
m³ (or 430 tcf) of CH on the surface. during the vertical migration of thermogenic
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Well 938, drilled during the IOPD 155 gases through the sediments, until reaching
0 o 0’0’’ mission, indicated the presence of dissolved microbial gas deposits at shallower depths.
methane in the sediments. The geochemical This thermogenic influence is possibly re-
analysis of these samples indicates an essen- lated to the connection provided by faults
tially biogenic in situ origin, suggested as the connecting deep gas sources to surface de-
source the formation of methane hydrates posits. According to Rodrigues (2019), the
Foz do Amazonas Fan present in the GHSZ, identified in the seismic recognition of the mixture must be based on

BSR data (ARNING et al., 2013). The presence of analyzes based on the values of 13C-CH4vs.
gas hydrates together in the sediments of the C1/(C2+C3), as can be seen in the analysis
50 0’0’’W 45 0’0’’W
o
o
Amazon Fan was confirmed by the recovery of the continental slope area (Figure 5y).
TWTT (s) 100.000
BSR Seabed –1,5 BIOGÊNICO Figure 5y: Graph showing the relationship
10.000 between the compositions of C1/(C2+C3)
BSR versus 13C of hydrate gases, plumes and
–2 dissolved gases in sediments from the Foz
1.000 do Amazonas Fan. Open circles (HG) and
C1 / C2+C3 Proximal area samples from the Amazon
–2,5 100 solid circles (dissolved gases and plumes).

10 TERMOGÊNICO Fan. Open triangles (HG); solid triangles
(dissolved gases and plumes). Samples
Figure 4y: Map of the Foz do Amazo- lines, coincident with the anticlinal 1 from the continental slope area north-
nas Fan illustrating the BSR occurren- folds, seismic line below (d-d’). -110 -100 -90 -80 -70 -60 -50 -40 -30 -20 east of the Fan.
ce areas mapped on the 2D seismic Source: Authors elaboration, 2022 δ C de CH ( /00 VPDB) Source: Modified from Rodrigues (2019)
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630 ECONOMIA AZUL State of the Art of Gas Hydrate Occurrences 631
630 BLUE ECONOMY