Shallow gas features in incised-valley fills (Ria de Vigo, NW Spain): a case study

摘要

Areas with gas accumulations and gas seeps, where gas escapes from the seabed to the water column have been mapped in the fill of a submarine incised valley, the Ria de Vigo. The various gas features have been classified into four types according to their specific seismic signatures: (1) acoustic blanket, (2) acoustic curtains, (3) acoustic columns, and (4) acoustic turbidity. At the same time, three types of gas escapes features have been distinguished: (1) acoustic plumes, (2) cloudy turbidity, and (3) pockmarks. Calculations indicate mean densities of 1.7 acoustic plumes km(-2) and 1.6 pockmarks km(-2). Estimations of the gas fluxes towards the atmosphere range from 144.37 to 4134.9 t yr(-1). It is concluded that sedimentary facies is the main factor determining whether gas accumulates or seeps, and also determines the specific type of accumulation or seep at each location. (C) 2002 Elsevier Science Ltd. All rights reserved. [References: 33]