Reuters: Improving the odds: why shale really is revolutionary: Kemp

Exploring for oil and gas is like gambling: it's all about playing probabilities, and using technology and skill to improve the odds. Until a well is bored into the ground - possibly thousands of feet below the surface, at a cost of millions or even tens of millions of dollars - it is impossible to know for sure whether it will find hydrocarbons in commercially producible quantities.

Some exploratory wells prove to be "dry holes" or "dusters". Others find some useful oil or gas but often in only modest amounts. Only a few turn out to be "elephants". Success in the exploration business depends on maximising the number of profitable elephants while minimising the number of costly dusters.

Exploration companies spend a fortune on seismic, gravity and magnetic surveys, as well as acquiring old well records and employing massive computers to crunch all the resulting data to improve their odds of finding large accumulations of oil or gas, a process known in the industry as "de-risking".
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