I could only see this happening in the Far Lands due to the extremely buggy code.
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It's the "in the shape of a portal" part that's improbable. To form obsidian, the lava has to be source blocks, not flowing, so that's unlikely itself. And then to reach the bottom parts, the water couldn't just be flowing from the top, it would have to be coming in at multiple points on the sides. And even then, the middle of the portal would be full of something (flowing water or lava, if nothing else).
Like, if it's in one of those areas where water+lava are just pouring out everywhere, could it happen?
Of course it would require somebody to use some Flint+Steel on it, but could the frame be generated by the terrain?
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is it probable, not really
you would need obsidiean to form on the surface in the necessary portal shape and have lightning strike it
Would be ****ing awesome if it did though.