Nice explanations of oil exploration, what is oil and refining and the exploration history of the area.
Lots of pictures and explanations, partly aimed at primary school level but good basic information and excellent links to other sites.
This PDF contains a good summary of petroleum formation and the exploration industry with some fun illustrations.
Platts report on gas storage projects in the UK
Simplified version of the UK geology, very helpful for starters looking at geological maps as the detail can be overwhelming at first.
Large seismic exploration company. Has a good slide show showing the main steps in seismic exploration.