The Irish Sea has a long and varied geological history from the earliest Carboniferous (330 million years’ ago (mya)) up to the recent ice age (1 mya). With respect to the formation and accumulation of natural gas in the East Irish Sea the main geological units are the Carboniferous (320 mya) where the initial organic material needed for creating natural gas is first deposited, Triassic sandstone reservoir (250 mya) where the natural gas is held, and Triassic Salt (220 mya) which seals the top of the reservoir to ensure the natural gas accumulation is trapped in place.

However, the 112/25 Isle of Man gas accumulation is slightly different as the reservoir rock is Permian sandstone (260 mya) and the top-seal is Permian salt (250 mya).