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At the first sight it looks like they're still in the Tyrannosaur paddock. But why would there be Bachiosaurs grazing?

The movie script describes the scene as:

GRANT, LEX, and TIM make their way through Jurassic Park. Far in the distance, there's another ROAR.

The group was on their way through the park, they must have left the T. rex Paddock and reached the Brachiosaur Pen.

The fact that the T. rex roar is 'far in the distance' indicates that the group has left the T. rex paddock.