Abstract

The presence of a complicated, variable-depth Oil Water Contact (OWC) in the Early Miocene L-III carbonate reservoir of the Mumbai High field has been well established. The OWC dips towards the SW along a curved profile, but the Gas Oil Contact (GOC) is flat. Very little is known about the possible mechanisms that could have produced this complex fluid contact. In the absence of horizontal pressure gradient, gravity should produce a flat OWC. In many fields around the world, where non-flat fluid contacts are observed, the contacts could be described as segmented, tilted or curved OWCs. Commonly believed mechanisms which produce such types of contacts are: fault compartmentalization, hydrodynamic flow, ongoing charge and reservoir property variation. All these mechanisms fail to explain the tilted OWC of Mumbai High. This paper proposes that another mechanism: structural adjustments after the migration of hydrocarbons into the paleo-trap, might have resulted in tilting or curving the originally flat OWC of Mumbai High. Such a phenomenon is likely to be observed in oil-wet low-permeability carbonate reservoirs. Imbibition related hysteresis combined with diagenesis induced property degradation in the water-leg are the possible mechanisms that can prevent the OWC from equilibrating even after cessation of structural evolution.

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