The Impact of Broadband Seismic for Imaging and Reservoir Geophysics
Pharez, Stephen J.
Petroleum Geo-Services, Weybridge, United Kingdom.
Seismic data has contributed significantly to reducing the risk in hydrocarbon exploration and production. There have been many step change developments; cdp stacking, migration, 3D, even time lapse, but for more than sixty years the principles of marine acquisition with an acoustic source and hydrophone receivers have remained the same. Unfortunately the data quality is affected by the ghost - the result of an almost perfect reflection of the acoustic wavefield from the sea surface. Up-going waves are reflected back as down-going waves with a reversed polarity, and interfere constructively for certain frequencies and destructively for other frequencies. This phenomenon occurs both on the source side and on the receiver side and the affected frequencies depend solely on source and receiver tow depths. Recent developments of a dual-sensor streamer and a ghost free source look set to revolutionize the quality of marine seismic data by the ability to separate the up-going and down-going wavefields. Ghost free acquisition provides a broader bandwidth with improved noise characteristics ideal for accurate imaging and reservoir property analysis.
This paper reviews the enabling technologies, but will concentrate on the value of the resulting data for exploration and production. Being able to capture the full bandwidth reflected by the subsurface, leads to improved imaging and interpretation in complex areas such as the Moere Marginal High in the Norwegian Sea. The broad bandwidth (2.5 - 175Hz) reveals deep Cretaceous sediments under Eocene basalt/volcanics and slump structures. Stratigraphic layers can be more readily interpreted and a pre-stack inversion result will be presented.
Pre-stack simultaneous relative inversion with additional low frequency seismic velocity information from the ghost free data has achieved significant benefits for the use of pre-stack inversion in a prospectivity workflow. The broadening of the low frequencies also represents a key improvement for lithology and fluid prediction and absolute elastic properties estimation. Further work in the Carnarvon Basin of the NW Australian shelf has found a very good agreement between the elastic property variation in the well and the pre-stack inversion results, to the extent that the distribution itself follows the well elastic behavior. Increased reliability of the data delivers increased confidence in the resulting estimated properties and leads to reduced risk.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012