An excellent example of the unevenness of information on the web: http://askville.amazon.com/geological-effect-removing-oil-underground/AnswerViewer.do?requestId=7186071
Here’s the question I googled: what does extracting petroleum do to the earth? The link above was my second hit. Wow. No input that oil extraction might damage the earth in any way. This discussion is on askville by Amazon.
“How Oil Drilling Works” on the How Stuff Works website offers solid introductory information. Doesn’t address the effects of oil drilling on the Earth but helps immensely to understand the process.