Shallow gas occurrence in a Brazilian ría (Saco do Mamanguá, Rio de Janeiro) inferred from high-resolution seismic data

Semi-enclosed areas, such as rías, offer low-energy sedimentary conditions, which allow the necessary environment for the deposition of clays and organic-rich deposits that generate gas through methanogenic bacteria. In Saco do Mamanguá (Rio de Janeiro, Brazil), acoustic data from low frequency echo sounder and high-resolution seismic-reflection data from chirp sonar and boomer from three surveys - February 2009 and January and June 2014 - enabled for the first time the identification of acoustic disturbances related to gas occurrences. Saco do Mamanguá hosts shallow gas accumulation throughout its entire area, expressed as acoustic turbidity, enhanced reflections or acoustic blanking, intra-sedimentary plumes and acoustic plumes. However, gas occurrence evidence decreases towards the open ocean. A lithological control occurs on the sealing layer in the inner area, whereas the smaller gas occurrence in the outer area can be due to (1) absence of a sealing layer and/or (2) the re-oxidizing of methane as more oxygenated waters are available. We propose a sequence stratigraphic model in which the gas formation is Holocene or either pre-Holocene age.