In the future, the world is in a parlous1 state. Earth people of the future (let us call them Futurians as in the answer by DWKraus) are desperate but they still have a number of time portals in working order. They decide to raid the past. They will travel back in time and steal resources that have lasting value, including oil, metals, and machinery.
The portals are open at one end in the future and simultaneously open at the other end in the target time in the past. The portals have zero length apart from the frame that contains the mechanism and acts as the entrance/exit. The time travellers2 can move back and forth easily between the two time periods and even run roads or pipelines through the portals for mass transport.
What they fail to realise is that they are robbing their own past and thereby impoverishing their ancestors. Thus they themselves created the barren world that led to their own downfall.
Let us focus on oil. The ancestors lose their oil because it is stolen by the Futurians. The Futurians are short of oil because it was stolen from the ancestors who therefore were unable to pass it on to their descendants.
This leads to a paradox — or does it? If we handwave the existence of time travel, is the scenario possible from a resources point of view? How can I make this work? How can I balance the books?
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1 parlous is a real word