Introduction

 

The study area for this work is located in the western South Atlantic on the continental margin of Uruguay. Here we found three sedimentary basins: Punta del Este, Pelotas, and Oriental del Plata (Figure 1). In the Uruguayan territorial sea, seismic evidence for the occurrence of gas hydrate has been identified based on the presence of BSRs in 2D seismic reflection sections.

 

BSRs are frequently used in the indirect diagnosis of gas hydrate accumulations (Sloan, 1998; Pecher and Holbrook, 2000) as geophysical indicators of the base of the GHSZ (gas hydrate stability zone). Because the BSR occurs at a contact between higher acoustic impedance above and lower acoustic impedance below (a negative acoustic impedance contrast), it produces a phase reversal in a reflected acoustic wave compared to the reflection from the seafloor (Max et al., 2006). The base of the GHSZ can have different seismic expressions such as continuous BSRs, segmented BSRs, and high-relief BSRs (Shedd et al., 2009) depending on the geological, fluid and geothermal setting.

 

Offshore Uruguay, initial determinations concerning the presence of BSRs indicated an area of 5,000 km² according to information from seismic surveys performed offshore Uruguay between 1970 and 2002, which were available at that moment in non-digital media (paper and acetate) (de Santa Ana et al., 2004). In several of these seismic lines, associated with post-Miocenic sequences, the presence of several BSRs were identified. In this work we present recent results regarding the identification of BSRs in 2D seismic data acquired in the years 2007 and 2008 as well as the results of the chimney processing analysis performed in some of these seismic sections.

 

Methodology

 

We interpreted more than 10,000 km of regional and semi-detailed 2D reflection seismic sections from surveys shot in 2007 and 2008, using the Kingdom Suite© seismic interpretation software looking for reflectors attributable to the base of the GHSZ based on the known characteristics of the BSRs. The acquisition parameters of these surveys are showed in Figures 2 and 3 (CGG Veritas).

 

From these surveys some lines have been selected for special processing aiming to identify gas chimneys. Gas chimneys are visible in seismic data as columnar disturbances, where the continuity of reflectors is missing, and reflection amplitudes are weaker than in the surroundings areas (Heggland, 2005). The processing for gas chimney identification was performed by Geoinfo-Argentina and dGB-USA, and was accomplished through seismic attribute analysis by training a neural network to recognize chimneys that have been identified in a seed interpretation (Heggland, 2005). The network transforms all attributes into one new attribute, which indicates the probability of chimney at the seismic position. The resulting chimney-probability section can be further enhanced by image processing techniques and iterating the process (Meldahl, 2001). The processing is based on the following workflow:

 

1) Data analysis and attribute selection.

2) Select representative train locations.

3) Calculate seismic attributes.

4) Feed calculated data to neural network and train.

 

In this work we studied some of the results of the gas chimney identification processing in the surroundings of the BSRs in order to find a link between gas hydrate occurrence and gas chimney presence.

 

Discussion

 

The interpretation of the base of the GHSZ on the 2007 and 2008 surveys show a widespread occurrence of the gas hydrate bearing sediments as showed in the Figure 4. As shown in Figure 4, the BSR is present in water depths greater than 500 m and has high continuity in the northern area (Pelotas Basin), but is more discontinuous at Punta del Este Basin and southern part of Oriental del Plata Basin.

 

The presence of BSR at the southern part of Offshore Uruguay represents a new discovery regarding gas hydrates in this area since previous seismic surveys have not reached these areas so there was no information regarding gas hydrate occurrence.  In Figure 5 we present a seismic section from the southern area showing a clear example of BSR and enhanced amplitudes suggesting the presence of free gas below the base of the GHSZ.

 

Regarding the gas chimney identification, Figure 6 shows the results for the processing in the surroundings of an interpreted BSR. Here we can observe the presence of enhanced amplitudes below the BSR and high probability of gas chimneys (shown in green colour) associated with a recent fault system. In Figure 7 we present results of gas chimney identification showing possible gas chimneys ending at the BSR. One explanation of this is that the hydrate bearing sediments are acting as a stratigraphic seal.  The deep source of the gas chimneys present in this section suggests thermogenic origin.

 

Conclusions

 

The BSRs extend over an area of approximately 22,000 km² and show that the potential for this non-conventional resource offshore Uruguay is higher than thought. We also found seismic evidence suggesting the existence of free gas below the GHSZ (sub-hydrates prospects) through the presence of increased amplitudes below the BSR. Additionally, the results of the processing for identifying chimneys are consistent with the BSR interpretation suggesting in some cases thermogenic origin of the gases that reach the hydrate-bearing layers.

 

 

References

 

Heggland, R., 2005, Using gas chimneys in seal integrity analysis: A discussion based on case histories, in P. Boult and J. Kaldi (eds.) Evaluating fault and cap rock seals: AAPG Hedberg Series, no. 2, p. 237-245.

 

Max, M.D., A.H. Johnson, and W.P. Dillon, 2006, Oceanic gas hydrate character, distribution, and potential for concentration in M.D. Max, A.H. Johnson, and W.P. Dillon, (eds.) Economic geology of natural gas hydrate, v. 9, p. 105-130.

 

Meldahl P., R. Heggland, B. Bril, and P. De Groot, 2001, Identifying Faults and Gas Chimneys Using Multiattributes and Neural Networks: The Leading Edge, v. 20/5, p. 474-482

 

Pecher, C.K. and W.S. Holbrook, 2000, Seismic methods for detecting and quantifying gas hydrates in M.D. Max (ed.), Natural gas hydrate in oceanic and permafrost environments: Kluwer Dordrecht, p. 257-294.

 

de Santa Ana, H., N. Ucha, L. Gutiérrez, and G. Veroslavsky, 2004, Gas hydrates: Estimation of the gas potential from reflection seismic data in the continental shelf of Uruguay: Revista de la Sociedad Uruguaya de Geología, no. 11, p. 46-52.

 

Shedd, B., P. Godfriaux, M. Frye, R. Boswell, and D. Hutchinson, 2009, Occurrence and Variety in Seismic Expression of the Base of Gas Hydrate Stability in the Gulf of Mexico, USA: Fire in the Ice, Winter, p. 11-14

 

Sloan, E.D., 1998, Clathrate hydrates of natural gas, 2^nd edition: Marcel Dekker, New York, 705 p.

 

Website

 

CGG Veritas website, Marine Data Library Offshore Uruguay, http://www.cggveritas.com/default.aspx?cid=1744&lang=1

 

 

 

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