Subsurface reservoir with properties staying constant across the reservoir volume: 

Single-phase flow or multi-phase flow with uniform distribution of phases over the flow volume and no phase slippage:

There are no homogenous reservoir in real life but the concept plays important academic role and also has a good number of practical applications to characterize the distributed field properties with some average value. 

See also

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Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petroleum Geology / Petroleum reservoir