Anomaly3

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Anomaly 3, Browse Basin

Location: Browse Basin
Intensity: High, localised gas chimneys
Amplitude Anomalies: Strong
Area: Small
Data Degradation: High

Several seismically prominent gas chimneys are present near the Londonderry–1 well location (Fig. 11), on the Yampi Shelf, Browse Basin. Detailed water column sniffer surveying over these chimneys (O’Brien et al, 1998b; 2000) showed that these chimneys produce only minor geochemical anomalies within the overlying water column (typically 2– 5 times background). Consequently, whilst these chimneys have a strong seismic response, they have probably been produced by a relatively small amount of leaking gas. These chimneys are invariably located where the regional seal thins significantly onto topographically prominent tilt blocks. Here, the seal capacity is reduced sufficiently to allow migrating gas to leak through the seal, forming gas chimneys (O’Brien et al, 1998b; 2000). These chimneys effectively represent areally restricted (point) sources of seepage, and thus only small fluxes of hydrocarbons pass through them.

Figure 11. Gas chimneys developed over landward-dipping tilt blocks near Londonderry–1 well, Yampi Shelf, Browse Basin.