Conventional 3D surveys, although designed and processed with clear exploration objectives in mind, often contain much valuable information in the shallow section This information can add significantly to shallow gas studies based on conventional high resolution seismic techniques. Examples from the North Sea are used to illustrate how an integrated approach to interpretation can lead to a better understanding of the shallow geology and to a more accurate and reliable assessment of drilling hazards, particularly shallow gas. A new approach site investigations has been tried and tested and has been found to be both time and cost effective.


In recent years there has been a rapid increase in the use of 3D seismic surveys for exploration and development purposes. Oil and gas producing provinces all over the world are gradually becoming covered by 3D seismic, and the use of 3D in new exploration areas is fast becoming routine. The opportunity exists to make use of these existing datasets in drilling hazard studies of the shallow section. The challenge faced by the seismic interpreters working in this field is how to use this data in the most efficient way.

This paper shows how high resolution 2D seismic. acquired as part of a site survey programme, and existing 3D seismic can be interpreted together to overcome some of difficulties experienced when using either data se[ in isolation. Although the exploration 3D cannot provide the same resolution as high resolution 2D seismic, existing 3D data sets can provide essential information on the shallow geology which can lead ultimately [o a more refined and more reliable assessment of potential hazards, The approach can becaused to assess new with little or no existing high resolution coverage and can also help design site survey programmed. The approach is both time and cost effective.

The paper concentrates on shallow gas, but reference is also made to other tophole drilling hazard.

Conventional Shallow Gas Studies

Shallow gas is one of the most serious hazards offshore engineers face when drilling exploration and development well.

Although not restricted 10the shallow section (top 1000 metres below seabed), it is in this interval where shallow gas, if encountered unexpectedly and without the necessary precautions in place, poses the greatest threat to the drilling operation and the safety of the rig. Incidents vary from costly delays and lost wells to catastrophic blowouts, sometimes involving loss of life.

During the seventies and eighties these incidents occurred with alarming regularity which led to the development of site survey specifically designed to identify potential gas hazards prior to drilling. Site surveys are now carried out routinely to investigate the shallow section and provide essential information on water depths, seabed obstructions, jack-up foundation conditions or anchoring conditions, as well as drilling hazards.

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