What would happen to the world oil and gas market if Green cars were mass produced and what effect would it have on companies in that industry?

Well, the theoretical effect would be that the price of
oil would lower as supply increased and demand decreased (which is what happened with
the onset of the recent recession), but the overall and real impact of making the
majority of the world's cars either more efficient or gasoline-free is hard to
say.

Remember that only about 50% of the oil that is pumped
out of the ground is used for transportation fuel, with the rest going for a wide
variety of products from plastics to cosmetics. (follow link for more complete
list)

The companies you're referring to are huge and very
profitable, so it stands to reason that they would resist a move towards fuel efficiency
or alternative fuels.  Their other option, which they are fully capable of, is to
embrace green technology and become the major innovator and supplier of it, which would
make them more, not less profitable.