"immense territory" is correct and usable in written English.
You can use this phrase to describe a large expanse of land. For example, "The Roman Empire once encompassed an immense territory that extended throughout much of Europe."
This is an immense territory, roughly equivalent to the combined surface of India, Pakistan, and Afghanistan.
In their decades in power, the Communists set up communities like Baikalsk all over their immense territory.
Traditionally, the Chickasaw were a seminomadic people who patrolled the immense territory that they claimed for themselves and raided tribes far to the north; like many conquering peoples, they integrated the remnants of these tribes into their culture.
He traveled an immense territory selling commercial paper; he was on a train 12 nights out of 14. (He was later one of the first investment bankers to fly).
… So many musical ideas are seething within me … Now that I have broken the chains of routine, I see an immense territory stretching before me, which academic rules forbade me to enter.
In huge wordless and immersive spreads Grill evokes the world of the wolves, showing the immense territory in which they roam and the vast starlight skies under which they travel.
The supremely cunning Leopold II, King of the Belgians, secured exclusive private title to the immense territory--roughly the size of the United States east of the Mississippi--surrounding the Congo River Basin..
The second of the human factors, the nomads, inhabited the immense territories beyond the northern frontiers.
It won and (in the case of the costly American war of independence of 1775-83) lost immense territories.
He also claimed Louisiana, comprehending the immense territories watered by the Mississippi, and the rivers which empty into it, by the title of discovery.
But the numbers are only the start of a journey of immense proportions and contrasts, from imposing Niagara Falls to high-rise Chicago, with the vast territory in between full of small-town charisma and timeless charm.
