MARINE AND FISHERY SCIENCES 37 (1): 233-240 (2024)

https://doi.org/10.47193/mafis.3712024010102

ABSTRACT. Radar images are commonly applied to recognize and monitor oil seeps on surface

waters over continental shelves. In San Jorge Gulf, Patagonia, between 46° S and 48° S, oil slicks

have been surveyed performing ellipse patterns in response to mesotidal dynamics. These effects

were assigned to recent episodic increments of summer bottom temperatures at depths between 100

and 120 m, which are 2 °C warmer than those recorded during the 20th century. Slicks are assumed

to have their origin from faults already known by the oil industry onshore. The effects here

described should be envisaged in a climate-change scenario leading to temperature increases of the

oceans’ shallow waters, together with other effects such as the human-induced global sea level rise.

Under such warmer conditions seeps from continental shelf floors will become more frequent, and

their contribution to the atmospheric C budget should be globally assessed.

Key words: SAR images, continental shelf, Argentina.

Filtraciones de petróleo desde la plataforma patagónica: su destino termoestérico

RESUMEN. Las imágenes de radar se aplican comúnmente para reconocer y monitorear las fil-

traciones de petróleo en las aguas superficiales sobre las plataformas continentales. En el Golfo San

Jorge, Patagonia, entre los 46° S y 48° S, se han relevado manchas de petróleo que presentan patro-

nes de elipse en respuesta a la dinámica mesotidal. Estos efectos fueron asignados a incrementos

episódicos recientes de las temperaturas del fondo del verano a profundidades entre 100 y 120 m,

que son 2 °C más cálidas que las registradas durante el siglo XX. Se supone que las manchas tienen

su origen en fallas ya conocidas por la industria petrolera en tierra. Los efectos aquí descritos deben

contemplarse en un escenario de cambio climático que provoque un aumento de la temperatura de

las aguas poco profundas de los océanos, junto con otros efectos como el aumento global del nivel

del mar inducido por el hombre. Bajo tales condiciones más cálidas, las filtraciones de los suelos de

la plataforma continental serán más frecuentes y su contribución al balance de C atmosférico debe

evaluarse globalmente.

Palabras clave: Imágenes SAR, plataforma continental, Argentina.

Natural oil seeps have been surveyed at different locations worldwide. The

most accurate figure was about 6 ´105t y-1, although it could vary from 2 ´

105to 2 ´106t y-1 (Kvenvolden and Cooper 2003), while estimates of CH4

contributions to the atmosphere from geological sources indicated 45 Tg y-1

(Kvenvolden and Rogers 2005). Synthetic Aperture Radar (SAR) images are

233

*Correspondence:

fisla@mdp.edu.ar

Received: 7 June 2023

Accepted: 26 July 2023

ISSN 2683-7595 (print)

ISSN 2683-7951 (online)

https://ojs.inidep.edu.ar

Journal of the Instituto Nacional de

Investigación y Desarrollo Pesquero

(INIDEP)

This work is licensed under a Creative

Commons Attribution-

NonCommercial-ShareAlike 4.0

International License

Marine and

Fishery Sciences

MAFIS

NOTE

Oil seeps from the Patagonian shelf: their thermosteric fate

FEDERICO I. ISLA1, 2,* and LUIS C. CORTIZO1, 3

1Instituto de Geología de Costas y del Cuaternario (IGCC), Universidad Nacional de Mar del Plata (UNMDP), Comisión de Investigaciones

Científicas (CIC), Funes 3350, B7602AYL - Mar del Plata, Argentina. 2Instituto de Investigaciones Marinas y Costeras (IIMyC), Universidad

Nacional de Mar del Plata (UNMDP), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Mar del Plata, Argentina.

3Comisión de Investigaciones Científicas (CIC), Buenos Aires, Argentina. ORCID Federico I. Isla https://orcid.org/0000-0002-4930-0907

progressively applied to detect and monitor oil

spills. Wavelengths of C-band are preferentially

applied to L and X-bands for detecting oil slicks

(Marghany 2014; Nunziata and Miglioaccio

2015; Asl et al. 2017; Jafarzadeh et al. 2021).

San Jorge Gulf has been reported as the most

polluted area of the Argentine coast. Highest

hydrocarbon concentrations are associated with

sandy sediments and gravel. This was primarily

attributed to the extraction and transportation of

oil through two loading buoys in Caleta Córdova

and Caleta Olivia (Commendatore et al. 2000)

(Figure 1). During the summer months of the

Southern Hemisphere (January), three oil spills

were detected close to San Jorge Gulf (Argentina)

between 2017 and 2023. They were not related to

known anthropogenic-triggered activities (spills);

instead, they were related to oil seeping from the

bottom of this productive basin. This paper

described and analysed the occurrence and possi-

ble causes of these oil slicks.

San Jorge Gulf is located in the eastern Patago-

nia between 44° S and 47° S. Provinces of Chubut

and Santa Cruz share the gulf. Tidal ranges

increase from 3.5 m at the outer edge to more than

5 m at the interior (Isla et al. 2002). The sedimen-

tary basin below the gulf has been producing oil

and gas since 1910, covering an area of 170,000

km2. San Jorge basin is known for its normal

faults, which are common for the Salamanca For-

mation (Foix et al. 2008). Oil extraction is related

to normal faults deepening 60°-65° prevailing E-

W and ESE-WNW alignments (Sylwan 2001;

Foix et al. 2008). The gulf was originated during

the Holocene sea-level transgression (Desiage et

al. 2023). Today, the sediment is supplied by cliffs

subjected to coastal erosion, and occasionally by

the wind (including ashfalls) and southerly

coastal currents (Desiage et al. 2018). The gulf

circulation is dominated by westerly winds and

tidal currents (Isla et al. 2002). Temporal variabil-

ity of the atmospheric heat flux increases from

spring to summer (Palma et al. 2020).

In order to map these slicks, Sentinel 1-A radar

images (C-band, 5.6 cm wavelength) were

processed from different areas of the outer shelf

of San Jorge Gulf during summer months. The

VV polarization procedure gave better results for

234 MARINE AND FISHERY SCIENCES 37 (1): 233-240 (2024)

Figure 1. Location of seeps and dates recorded by Sentinel radar images.

60°

42° 30'

47° 30'

W65°

those purposes than the HH polarization. The

WV-GRD acquisition mode was preferred for

scenes of 20 ´20 km, spatial resolution of 5 ´

20 m, and vertical angle between 23° and 36.5°.

The Copernicus Open Access Hub (European

Space Agency, ESA) and the Alaska Satellite

Facility of the National Aeronautics and Space

Administration (NASA) were accessed for this

research. Images from the interval 2015-2023

were analysed for different meteorological condi-

tions. Radiometric, geometric and speckle noise

(Lee Sigma and Frost) were performed with con-

version to decibels and geolocalization through a

Shuttle Radar Topography Mission (SRTM)

masking the land territories. The SNAP 8.0 (SeN-

tinel Applications Platform; ESA) program was

applied to get areas, perimeters and axis lengths

of darks stains (Marzialetti 2012). False positives

were assigned to surfactants liquids or biological

detritus. In order to analyse the occurrence of

these seeps during recent summers, the BARDO

statistical database (Baldoni et al. 2008) of the

Instituto Nacional de Investigación y Desarrollo

Pesquero (INIDEP, Argentina) was consulted

(https://www.inidep.edu.ar/datos-oceanograficos).

Oil seeps were registered out of San Jorge Gulf

during summer months of the southern hemi-

sphere (January 2018, January 2019 and January

2023). Seeps of 2018 were reported by CGG

Geoconsulting (2019). As the area is subjected to

tidal ranges between 2 and 4 m, seeps configure

ellipses with longer axis between 3 and 5 km

(Figure 2). A careful examination denoted that

some of these ellipses were oriented into lines.

Episodic temperatures three degrees warmer

were recorded since the year 2000 at 120 m depth

(Figure 3).

As slicks were distributed along lines or in

clusters, they were assigned to faults that are evi-

dent at cliffs (Figure 4) and that have been detect-

ed by reflective seismic (Fígari et al. 1999). Com-

paring beach samplings performed between 1989

and 1995, highest values were recognised at Cale-

ta Córdova and Comodoro Rivadavia, although

no spills were related to these anomalous values

(Commendatore et al. 2000). However, episodic

natural spills at the outer gulf could explain these

higher values.

Although oil seeps have been surveyed at the

Argentine Exclusive Economic Zone (EEZ)

(CGG Geoconsulting 2019), no prospection was

carried out offshore San Jorge Gulf. Biological

impacts of oil pollution in San Jorge Gulf were

already analysed considering them of anthro-

pogenic origin (Klotz et al. 2018).

Significant changes in the deep water tempera-

ture of oceans have been reported worldwide

(Abraham et al. 2013). It was also postulated that

235

ISLA AND CORTIZO: SAN JORGE GULF OIL SEEPS

Figure 2. Partial ellipses from the outer area of San Jorge Gulf captured by radar Sentinel 1A, band C, image taken at 9.10 AM

local time. A) January 29, 2018 (major axis 3.4 km). B) January 11, 2019 (major axis 5 km). C) January 13, 2023.

2018 2019 2023

increments of intermediate water temperatures

could release hydrocarbon gas along the Gulf

Stream current to the uppermost planet cycle

(Phrampus and Horbach 2012). The Argentine

shelf is assumed to be warming in the last years

with a possible migration of the convergence of

Brazil and Malvinas currents due to a southwards

displacement of Brazil current (Risaro et al. 2022;

Chidichimo et al. 2022).

Increase in the tidal ranges could lead to an

increase in the buoyancy of shallow-shelf oil

accumulations (Idier et al. 2017; Pickering et al.

2017). Some models have been proposed to

explain the behaviour of seeps in the Gulf of

Mexico (Asl et al. 2017).

The upward velocity of droplets produced at

the bottom depends on temperature, salinity (both

as function of depths) and the diameter size of

droplets (Najoui et al. 2018). Considering the sea-

level rise trends expected for the next century

(Oppenheimer et al. 2019), it is concluded that

the buoyancy will increase and seeps will there-

fore be more frequent.

In the case of San Jorge Gulf, faults are con-

duits for hydrocarbons to reach the bottom (Figure

5). In Punta Peligro, sediment-filled fissures filled

with friable, ﬁne-grained sandstone strata, suitable

for liquefaction/ﬂuidization processes (Rio Chico

Formation) (Foix et al. 2008) gave idea of the

seepage probability of recurrence. According to

the hook-shaped of the spills in January 29, 2018

and January 11, 2019, it would take at least 3 h the

reversal of tides to produce those shapes. Faults

were also reported to cause natural oil seeps mon-

itored from Lower Congo Basin (Jatiault et al.

2017). According to a collection of SAR images,

these spills lasted 3.25 h in relation to the wind

speed. Considering wind statistics along the year

it was estimated that this site was supplying

4,380 m3y-1 to the ocean surface and therefore

considered the third biggest supply province from

natural leakage (Jatiault et al. 2017). Applying

ENVISAT-ASAR images from the South China

Sea, it is rather difficult to know whether spills

have been generated by gas hydrates or petroleum

seeps (Wang et al. 2013). Although anthropogenic

oil spills have been drastically diminishing world-

wide in the last years (ITOPF 2022), oil tankers

activities (Kluser 2006) along the trip from

Buenos Aires to Tierra del Fuego are suspected of

illegally cleaning of their deposits.

CONCLUSIONS

1) Natural oil seeps were repeatedly detected

close to San Jorge Gulf during the summer

months.

236 MARINE AND FISHERY SCIENCES 37 (1): 233-240 (2024)

Figure 3. Water temperature records at different depths in

San Jorge Gulf. The circle highlights the anomalous

measurements (courtesy of INIDEP).

00 20 40 60 80 100 120 140

Z (m)

1960-1989 1990-1999

2000-2009 2010-2019

Temperature (°C)

237

ISLA AND CORTIZO: SAN JORGE GULF OIL SEEPS

Figure 4. Fault at the coast of San Jorge Gulf (modified after Foix et al. 2008).

Figure 5. Seepage explanation of San Jorge Gulf seepage processes (modified from Najoui et al. 2018).

Sea surface

Oil seep

Wind

Sea surface outbreak (SSO)

Water column

Deﬂection

Seaﬂoor source (SFS)

Seaﬂoor

Fault lineation

Tidal current

2) Anomalous bottom temperature increments in

the gulf have been detected since 2000.

3) C-band and VV polarization radar images were

the most useful to discriminate hydrocarbon

stains related to seeps.

4) The SNAP software permitted to approximate

the direction of these slicks and the wind

velocity at the moment of the image captures.

5) L-band images (Alos Palsar) were compared to

the C images showing less details due to the

longer wavelengths.

ACKNOWLEDGEMENTS

Authors are indebted to researchers from the

Physical Oceanography Lab of the INIDEP (Mar

del Plata, Argentina). CONAE kindly provided

the radar images. Two anonymous reviewers and

the editor made useful comments and sugges-

tions.

Author contributions

Federico I. Isla: conceptualization, investiga-

tion, supervision, validation, writing-original

draft, writing-review and editing. Luis C. Corti-

zo: image processing, formal analysis, investiga-

tion, methodology.

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