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Geology/migration of oil


Hi Sir. My question is that how migration of oil is more efficient through faults? the faults can become active or it may destroyed the nearby reserviors at the time of its (fault) formation or its activeness.

Dear Wasim,

Thanks for sending me your question.

A fault is a zone of general weakness in rock surface where some kind of observable movement or displacement along two sides has taken place.Thrust or reverse faults tend to converge over the strata due to compressional forces and, therefore there is a possibility that neighborhood area distorted the reservoir. Tensional or gravity fault forms  a gap between two strata when faulting takes place, therefore, there is a possibility that reservoir has improved due to fracturing.

However, after faulting if sedimentation continues the fault gap either filled by new sediments or adjacent overlying rocks slide down and filled the fault gap. During faulting substantial grinding and rubbing of rock grains, due to the tectonic forces, takes place which alter the grains and geometry of the rock and form polished a crystalline surface   along the fault path which may be very impermeable resulting diminishing of the  reservoir  if not totally destroyed.

So, the faults may act both ways, may destroy the reservoir or may also enhance the quality, so we have to be careful in the fault analysis.

Therefore, we have to see few aspects of any fault; determine type of faulting, fault gap. gap filling material, lithology juxtaposed across the fault, the distance of the fault throw and the age of the fault.

Old fault tends to be healed up by later sediments filling processes while the new faults likely to be either open or partially filled.


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Syed Tariq Hasany


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