Print this pageAbstract: Marine Gas Hydrates Along the Brazilian Margin
Sad, Alberto Roque E.; Silveira, Desiderio P.; Silva, Sérgio Rogério P.; Maciel, Rosangela R.; Machado, Marco Antonio P. - Petrobras/E&P
Gas hydrates occur in many places around the world and have brought great interest to the international scientific community. They contain extremely large amounts of methane, considering that 1 m3 of hydrate can be converted to 164 m3 of methane at the surface, and constitute a new exploratory frontier as a promising energy source for the coming century. However, little is known about their natural environment in subsurface deposits, due to the high instability of these compounds under STP conditions. Speculative global estimates on total amount of gas hydrates in these deposits converge at around 700,000 TCF of methane. Such estimates do not take into consideration the presence of free gas trapped by the hydrate layer. This work aims at discussing potential occurrences of gas hydrates offshore Brazil, particularly in the Pelotas and Foz do Amazonas basins, located at the extreme portions of the Brazilian continental margin.
The gas hydrate occurrence situated in the Pelotas Basin, at the southernmost portion of the Brazilian margin, extends for an area of 45,000 km2 (Fig. 1), based on the distribution of the Bottom Simulating Reflector (BSR). This surface, parallel to the sea bottom, corresponds to a high amplitude reflection on seismic lines. The BSR crosses reflectors associated with sediments of a large prograding marine system, Miocene to Recent in age, named Cone do Rio Grande. The area of occurrence ranges in bathymetry from 500 to 3,500 m, and the thickness of the sedimentary package that contains the gas hydrates, the Hydrate Stability Zone (HSZ), is around 600 m. The P-wave velocity varies from 2,500 to 2,800 m/s and decreases abruptly under the BSR, suggesting the presence of free gas. This area hasn?t been drilled yet and the only available geological information is based on the interpretation of seismic lines, spaced 15 to 20 km apart. In the Foz do Amazonas Basin, at the northernmost portion of Brazilian margin, the evidence for gas hydrates in seismic data occur in an area of 28,000 km2 (Fig. 2) and the thickness of the HSZ is about 450 m. This interval comprises an upper part of a thick prograding prism of siliciclastic sediments, composed by Upper Miocene and younger sediments. The BSR occurs in an area where the bathymetry ranges from 600 to 2,800 m, and the seismic reflection data are spaced 5 to 20 km apart. Evidence for hydrates in core samples (honeycomb structures) was observed on site 935 (Leg 155) from the Ocean Drilling Program (ODP) in this basin.
The limited amount of data available in these regions does not allow genetic interpretation or quantification of recoverable reserves. However, the evidence for gas hydrates observed on seismic profiles may constitute an important asset for the future, particularly if technological advances in extracting gas from hydrates result in economically viable projects. Based on parameters obtained from ODP - Leg 164 Scientific Prospectus, that drilled and sampled gas hydrates in the Blake Ridge region, a thickness of 200 m and 1.5% of concentration were used for preliminary volume estimations. The Pelotas Basin deposit would have about 780 TCF of methane on surface, while Foz do Amazonas deposit can reach 430 TCF, both comparable to the largest ones worldwide.
AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil