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It allows compensating the pressure difference between various reservoir locations related to the differences in their elevation. 

If reservoir structure is flat then initial Reservoir Pressure at formation top is going to be constant across the field and so will be the Datum Pressure but with a constant shift accounting for the height between formation top and Datum.

If reservoir structure is non-flat then initial Reservoir Pressure at formation top will be varying across the field while the  Datum Pressure will be constant across the field.

This particularly helpful in Reservoir Pressure analysis during production so that areal Reservoir Pressure distribution recalculated to Datum shows only those pressure variations across the field which are related to production and not to the field structure.If the reservoir structure is flat than 

See Also

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Petroleum Industry / Upstream / Petroleum Engineering / Subsurface E&P Disciplines / Reservoir Engineering / Reservoir pressure

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