PETROLEUM GEOLOGY/Exploration 297 Figure North Sea play types: (A) a truncated tilted fault block, typical of the northern North Sea; (B) a drape or pinchout trap, typical of the central graben; (C) a salt induced structure typical of the central graben; and (D) a horst block capped by salt, typical of symbol, source rock; pink, salt; blue, chalk reservoir; red, gas; the southern North Sea Yellow, sandstone reservoirs; brown, with green, oil Reproduced with permission of Nautilus Limited particularly true in geologically complex areas, where the cost of a three-dimensional survey can be considerably less than the cost of a mislocated well A traditional two-dimensional seismic survey is shot along single lines to provide vertical slices through the Earth’s crust; from a series of parallel and intersecting lines, the three-dimensional structure of the subsurface can be interpreted (Figure 8) However, the assumption is that the reflections seen on each line originate within the plane of the vertical slice, whereas in reality many of the reflections originate from inclined rock layers that may be hundreds of metres outside the plane of the seismic line; also, the gaps between the lines may hide faults and other geological complexities These limitations have been overcome by the use of three-dimensional seismic surveys, which employ multiple sound sources and simultaneous recording by lines of detectors laid out in a dense grid to