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Shallow gas system - interpreting anomalies

Seismic anomalies indicating leakage are common in most hydrocarbon basins. The proper interpretation of such features is important both for geohazard assessment and as an exploration tool. By interpreting the data in an integrated manner, a more reliable model for gas migration in the shallow section and to the seabed can be made. TNO has demonstrated the added value of such an integrated approach in a recent North Sea study.

The approach uses a 3D (or 2D) seismic survey made available by the client. The seismic data are carefully examined for any expressions of seismic anomalies related to the presence of gas. These may include smaller or larger bright spots and flat spots indicating the trapping of gas and seismic anomalies indicative of leakage. The latter may include gas chimneys, leaking fault systems, shallow enhanced reflectors and shallow disturbed zones. Where a 3D survey Is available, dTEct - a licensed product of dGB Earth Sciences - could be used to invert seismic chimneys from the seismic volume through a neural network based multi-attribute approach. At the end of this first phase, the resulting model shows the occurrence and distribution of seismic anomalies likely to be related to gas.

Acquiring additional data

The second phase of the study comprises the acquisition of additional data using a marine vessel. For example, for an average North Sea licence block about one week of boat-time is normally needed to collect seafloor sediment samples (using the vibrocore method) and acquire both multi-beam and high-frequency acoustic surveys over selected identified zones of interest. Multi-beam data reveal the presence of seafloor bathymetry anomalies associated with gas venting (such as pockmarks and carbonate mounds), whereas high-frequency, sub-bottom profiles reveal disturbances, due to the presence of gas in the uppermost 20 metres of the subsurface, as well as gas-plumes in the water column. From the vibrocores retrieved from the shallowest 4-5 metres of sediment, sub-samples are analysed in terms of their gas content. The concentrations of the lighter hydrocarbon fractions can be measured and stable isotopes determined.

Value of integration

The interpretation of geochemical anomalies is integrated with the interpretation of the various seismic and acoustic methods, yielding a comprehensive model for the migration of gas in the shallow section and to the seafloor. This model can then be integrated with existing models for the generation and migration of hydrocarbon in the deep subsurface. Such a model contributes to a better understanding of the entire hydrocarbon system of the area.

Contact

Dr. Johan ten Veen

  • geology
  • seismic interpretation
  • structural geology
  • geomodels
  • petroleum geology
Email

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