Well, Obstruction is usually physical in nature. You physically get in the way of an officer and his car, for example.
Simply refusing to answer a cop's questions, can never be Obstruction. It's a 5th Amendment right.
You really don't know what questions can or possibly could, implicate you. Hell, an officer may be trying to see if you'll slur your words in some way to charge you with public intoxication... There, it wouldn't even matter what you say, but the manner in which you say it.
But, besides that, there are a bunch of other problems with speaking with police, even if you think that the questions they ask are not going to implicate you.
Here is a good video on the subject:
Simply shutting up, unfortunately, is not enough to express your rights. They will continue to question you. Saying that you want a lawyer is good, but I've seen that be not enough, as well (depending on your dialect).
I would say something like, "I invoke and refuse to waive my 4th, 5th and 6th Amendment rights. Please do not question me further or request consent to search me"