Estruturas rasas de gás em sedimentos no estuário Potengi (nordeste do Brasil)

Abstract

ABSTRACT. Areas with gas accumulations and gas seeps, where gas escapes from the river-bottom to the water column, have been mapped in the fill of a fluvial valley, the Potengi estuary. 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, two types of gas escapes features have been distinguished: (1) acoustic plumes and (2) pockmarks. The neotectonic activity in the semigraben of the Potengi river may be one of the responsible factors for the migration of gas coming from the seismofacies located below the present river level where it is possible to recognize the connection with the level of the gas source through the acoustic column type seismic signature.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.Keywords: acoustic records, shallow gas accumulations, gas seeps, Potengi estuary. RESUMO.Áreas com acumulações e escape de gás foram identificadas no estuário Potengi. As várias ocorrências de gás foram classificadas em quatro tipos, deacordo com suas assinaturas sísmicas específicas: (1) blankets ou coberturas acústicas, (2) cortinas acústicas, (3) colunas acústicas e (4) turbidez acústica. Ao mesmo tempo, dois tipos diferentes de escape de gás foram identificados: (1) plumas acústicas e (2) pockmarks . A atividade neotectônica na porção fluvial do sistema Potengi pode ser um dos responsáveis pela migração de gases provenientes das sismofácies localizadas abaixo do atual nível do rio, ondeé possível reconhecer a conexão com o nível da fonte do gás, através da assinatura sísmica do tipo coluna acústica. Conclui-se que a fácies sedimentaré provavelmente o fator determinante da acumulação e escape de gás, como também do tipo específico de acumulação ou escape em cada local.Palavras-chave: registros acústicos, acumulações de gás raso, escape de gás, estuário Potengi.Universidade Federal do Rio Grande do Norte -UFRN, Cx.

Shallow gas seismic structures: forms and distribution on Santa Catarina Island, Southern Brazil

“…An example is the feature type known as Acoustic Cover, which HOVLAND andJUDD (1988) andMAZUMDAR et al (2009) classified as Acoustic Blanking or Acoustic Masking and JENSEN and BENNIKE (2009), MISSIAEN et al (2002), DUARTE et al (2007, GARCIA-GARCÍA et al (2007), EMEIS et al (2004), LEE et al (2005), ORANGE et al (2005 called Acoustic Blanking; TAYLOR (1992) and FRAZÃO and VITAL (2007) described them as Blankets, GARCIA-GIL et al (2002) as the Acoustic Blanket. The references show that there is an overlapping of names referring to the classification of the different features, with different names being used for similar accumulations.…”

“…The shallow gas structures occur in various geomorphological features in the form of an Acoustic Blanket (Acoustic Blanking), Acoustic Curtains, Acoustic Columns, Acoustic Turbidity, Turbidity Pinnacles and Intra-sedimentary Plumes, among others types of seismic reflectors (GARCIA-GIL et al, 2002;FRAZÃO;VITAL, 2007;WESCHENFELDER et al, 2006;WESCHENFELDER et al, 2016). The seepage may also occur in the form of Acoustic Plumes and Pockmarks, among others (GARCIA-GIL et al, 2002;FRAZÃO;VITAL, 2007;WESCHENFELDER et al, 2006;WESCHENFELDER et al, 2016). The spatial distribution of the different types of gas accumulation depends on the grain size, porosity and type of sediment and rock in which the gases are trapped (FRAZÃO;VITAL, 2007).…”

“…The seepage may also occur in the form of Acoustic Plumes and Pockmarks, among others (GARCIA-GIL et al, 2002;FRAZÃO;VITAL, 2007;WESCHENFELDER et al, 2006;WESCHENFELDER et al, 2016). The spatial distribution of the different types of gas accumulation depends on the grain size, porosity and type of sediment and rock in which the gases are trapped (FRAZÃO;VITAL, 2007). The porosity of the sedimentary facies where the gas accumulates and the sealant facies are the main factors that determine the type of gas accumulation (FRAZÃO;VITAL, 2007).…”

“…The spatial distribution of the different types of gas accumulation depends on the grain size, porosity and type of sediment and rock in which the gases are trapped (FRAZÃO;VITAL, 2007). The porosity of the sedimentary facies where the gas accumulates and the sealant facies are the main factors that determine the type of gas accumulation (FRAZÃO;VITAL, 2007). According to LEE et al (2005), the gas accumulations may have two origins: biogenic (by the action of bacteria that degrade organic matter), usually consisting of methane; and thermogenic (organic structures broken down under the appropriate conditions of temperature and pressure) that produce methane plus other hydrocarbons.…”

Marine substrate response from the analysis of seismic attributes in CHIRP sub-bottom profiles

“…This is because shallow gas is associated with fine-grained sediments (FRAZÃO;VITAL, 2007), and when the gas is close to the water/seabed interface it forms a strong impedance contrast, because the speed of the acoustic wave decreases considerably when it comes into contact with the gas (WILKENS; RICHARDSON, 1998), thus constituting a boundary that will reflect a great amount of energy.…”