AAPG Annual Convention and Exhibition

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Ocean Bottom Seismic Inspires Major Structural Update at South Arne Field, Danish North Sea

Abstract

Keywords: OBS, Brown Field development, Structural Interpretation, Imaging, Uncertainty The South Arne field produces oil from chalk reservoirs of Danien and Maastrichtian age, at ca 2650-3000 m depth in the Danish North Sea. The seismic image of the field is generally of high quality, and two 4D monitor surveys have been used to successfully constrain the production of the reservoirs, including discrimination of the units being produced. The main recovery mechanism is water injection with additional support from compaction. The overall development plan consists of alternating producers and injectors. While the seismic image quality in general is very good, a gas cloud in the overburden greatly deteriorates the image underneath, to the point where the conventional streamer seismic image cannot be used for structural and stratigraphic interpretation. Figure 1 Seismic image at reservoir level. Panel a shows streamer data, panel b shows PP OBS data and panel c shows PS OBS data through the gas cloud obscured area. A major normal fault is revealed under the gas cloud. To overcome this challenge, Hess acquired an Ocean Bottom Seismic (OBS) survey over the field. The OBS project had several objectives, including: - Improving the image in the gas cloud obscured area for structural and stratigraphic interpretation using both PP and PS data - Serve as a base line for future 4D OBS - Optimize further field development The updated structural interpretation under the gas cloud has revealed reservoir thickness significantly greater than previous models had indicated, directly impacting the in-place volumetric assessment and that the existing wells are sub optimally placed to effectively recover resources. The fault pattern and geometries are on trend with the area outside of the gas cloud, and with the better data we have a greatly improved handle on fault throw, directly impacting juxtaposition and flow behavior between different reservoir units. The presentation will highlight how the OBS data was interpreted and integrated into the structural framework, and how the new information is used to support reservoir modeling and reduce risk and uncertainty in further field development.