Oil has been discovered on the sea floor of the North Sea.

Literal

North-sea [of-の] sea-floor [at-で] oil [subject-が] found-is.

北海 ('North Sea') is distinct from 北海道 ('Hokkaido'). 海底 ('sea floor, seabed') is a compact compound: 海 + 底 ('bottom'). 発見されている is the passive ~ている of 発見する ('to discover') — '[has been] discovered [and is known].' A news-style scientific observation.