Three hundred million years ago,
the present-day North Sea was unrecognizable. What is now
deep water was a shallow sea, shown in light blue. Wherever
you see light blue, oil was forming under organic-rich waters.
Where you see dark blue -- deep water -- shales were laid
down, which helped trap oil.
The coastlines shifted dramatically as sea level rose and
fell. What is now under water might have been dry land, and
Volcanoes ejected blankets of ash and lava that trapped oil.
Great sand dunes swept northward, and formed sandstone that
would hold oil.
Today, more than 150 oil fields dot the North Sea, from The
Netherlands to the Shetland Islands. No two fields are exactly
the same, but all owe their birth to the rise and fall of
sea level over millions of years.