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Some people lost their jobs. 
Seriously, the usually have a pretty good idea of what's down there. VERY SIMPLIFIED: First seismic analysis (and this is where some pretty hefty computing power comes in useful), then if a "potential" reserve is identified, you might well use a mobile (semi-submersible or jack-up) rig to drill some exploratory wells. In fact these might be tied off at the top and used for later production if found good. Only then, when they are really sure, is a full rig used.
And these days (a) the N. Sea is pretty damn well mapped under the surface (b) we have subsurface technology which means you can have a wellhead on the sea floor tied back into the pipeline infrastructure with no permanent rig above it and (c) horizontal/directional drilling means that very you can get to oil about 7 miles sideways from a rig!
The result? Very few new rigs in the N Sea. Deepwater (eg gulf of mexico), and off Nigeria is where it's at now, and here there are some quite big single reserves to attack. So you should hit something black and sticky
The N Sea has the rigs from when there were bigger reserves there, and now has the technology to get at the little reserves that were uneconomic before using the same rigs.
But in the end, it's an expensive and fairly risky business. We write a lot of exploration expense off, as does everyone. A single hole in the ground can cost USD 15 million.
Seriously, the usually have a pretty good idea of what's down there. VERY SIMPLIFIED: First seismic analysis (and this is where some pretty hefty computing power comes in useful), then if a "potential" reserve is identified, you might well use a mobile (semi-submersible or jack-up) rig to drill some exploratory wells. In fact these might be tied off at the top and used for later production if found good. Only then, when they are really sure, is a full rig used.
And these days (a) the N. Sea is pretty damn well mapped under the surface (b) we have subsurface technology which means you can have a wellhead on the sea floor tied back into the pipeline infrastructure with no permanent rig above it and (c) horizontal/directional drilling means that very you can get to oil about 7 miles sideways from a rig!
The result? Very few new rigs in the N Sea. Deepwater (eg gulf of mexico), and off Nigeria is where it's at now, and here there are some quite big single reserves to attack. So you should hit something black and sticky
The N Sea has the rigs from when there were bigger reserves there, and now has the technology to get at the little reserves that were uneconomic before using the same rigs.
But in the end, it's an expensive and fairly risky business. We write a lot of exploration expense off, as does everyone. A single hole in the ground can cost USD 15 million.
Computers taking over the controll.
We had a hand pump on a well here from back when my Father grew up here (He's 'bout 9 years older than you or -old- in other words 
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