Cores from the Atlantic Field, Kopervik Fairway, Outer Moray Firth, UK

At first sight the 'tank of sand' provided by the Kopervik/Captain Sandstone Member reservoir of the Atlantic Field is a development geologist's and reservoir engineer's dream. However, the challenges posed are several: (1) The top of the reservoir is not generally visible on seismic data, hence geologically derived isopachs are important in creating top structure maps. The isopachs are best controlled by mapping individual sand units; however distinguishing between these units is difficult even in core, let alone on log data. Petrography, textural characteristics, palaeomagnetically derived flow directions, heavy minerals, biostratigraphy, and geological models developed from other discoveries along the Kopervik fairway indicate that the sands may be deposited by a mixture of the 'sausage' and 'sock' mechanisms of Law et al (2000). (2) The 'tank' may be subdivided by permeability barriers or baffles in the form of thin shale units or cemented zones. Atlantic Field will be developed with only two wells so these barriers may prove significant and it is important, although difficult, to predict their locations and lateral extents. (3) The high-permeability (multi-darcy) reservoir, together with the thin hydrocarbon column and active aquifer, mean that development wells must be designed to prevent early catastrophic water breakthrough. Accurately drilling such wells will not be easy when the reservoir cannot easily be resolved on seismic data, mapped or depth converted.