The control of palaeo-topography in the preservation of shallow gas accumulation: Examples from Brazil, Argentina and South Africa

Incised valley paleoenvironments interpreted by seismic stratigraphic approach in Patos Lagoon, Southern Brazil

“…The data was acquired by a GeoPulse Sub Bottom Profiler, from GeoAcoustics (Great Yarmouth, United Kingdom), and was processed in SonarWiz software (Chesapeake Technology, California, United States). These seismic data were analyzed in previous works with distinct approaches, but still provide a unique source of information and are necessary to support our interpretations (Weschenfelder et al 2005(Weschenfelder et al , 2006(Weschenfelder et al , 2008a(Weschenfelder et al , 2008b(Weschenfelder et al , 2010a(Weschenfelder et al , 2010b(Weschenfelder et al , 2014(Weschenfelder et al , 2016. Average velocities applied were 1,500 m/s for the water and 1,650 m/s for sediments (Jones 1999).…”

“…Sedimentologic samples were tied to seismic data. Besides the classic parameters, the presence of gas also can be used to suggest the presence of some paleoenvironments (Weschenfelder et al 2016).…”

“…The layers are horizontal in the central portion of the valleys and tend S1: Sequence 1; S2: Sequence 2; S3: Sequence 3; SU: subaerial unconformity; SU1: subaerial unconformity 1; TS: transgressive surface; MFS: maximum flooding surface; FSST: falling-stage system tract; U1: Unit 1; LST: lowsand sytem tract; U2: Unit 2; TST: transgressive sytem tract; U3: Unit 3; HST: highstand system tract; U4: Unit 4. to be inclined when onlaping the valley's axes, where they form a wedge shape. Some reflectors of Facies C show high amplitude and trap the gas accumulated in older deposits (Weschenfelder et al 2006(Weschenfelder et al , 2016. Such trapping layers are interpreted as stiff muddy sediments and are mappable for long distances.…”

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

“…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).…”

“…In Brazil, the presence of these features has been described by various authors such as VITAL and STATTEGGER (1997) in the lowest stretch of the Amazon River; FIGUEIREDO and NITTROUER (1995) and FIGUEIREDO et al (1996) in the Amazon submarine delta and COSTA and FIGUEIREDO (1998) on the Amazon Continental Shelf; BAPTISTA NETO et al (2011) in Rodrigo de Freitas Lagoon -Rio de Janeiro; BENITES et al (2015) in Saco do Mamangua, Rio de Janeiro; BAPTISTA NETO et al (1996), QUARESMA et al (2000) and CATANZARO et al (2004) in Guanabara Bay, Rio de Janeiro; WESCHENFELDER et al (2006) in Patos Lagoon -Rio Grande do Sul associated with low-lying paleotopographical features such as paleo river channels and/or valleys; SCHWARZER et al (2006) in the Rio Açu Canyon and FRAZÃO and VITAL (2007) in the Potengy estuary, both in Rio Grande do Norte; FÉLIX (2012) in the Bertioga Channel, São Paulo;andSOUZA et al (2011), DEMARCO et al (2012), GUESSER et al (2012) andWESCHENFELDER et al (2016) presented preliminary results for the North Bay and the Conceição Lagoon -on the Santa Catarina Coast (Figure 1). In summary, gas-charged sediment and exhaust gases escaping into the water column occur in Brazil on the continental shelf and in rivers, bays and lagoon systems, from north to south, normally associated with sediments rich in organic matter.…”