Saturated Black Oil Reservoir

A saturated black oil reservoir refers to a type of hydrocarbon reservoir where the oil is in equilibrium with the gas phase at the reservoir conditions (temperature and pressure). 

Here’s a Breakdown of the Concept

Black Oil  -  A term used in petroleum engineering to describe crude oil with certain characteristics.  It typically has a low gas-to-oil ratio (GOR), a higher viscosity, and is usually dark in color, hence the name "black oil."  Black oil can range in composition, but it generally contains a higher percentage of heavier hydrocarbons.
Saturated Reservoir  -  The reservoir pressure is at or below the bubble point pressure.  The bubble point is the pressure at which gas begins to come out of solution from the oil.  In other words, the oil is "saturated" with gas; any reduction in pressure will cause the gas to separate from the oil.
Saturated Black Oil Reservoir  -  The oil contains the maximum amount of dissolved gas it.