For a low permeability, water-wet reservoir under water flooding development quite a large amount of oil remains in the rocks as residual oil while the oil recovered is only 20-30%. The amount of residual oil is far larger than that in the high permeability reservoirs. Research in this article has identified two major causes of this situation. One is due to the lithology of the reservoir. Such influence on residual oil comes from 1) the content of medium to fine sand and silt 2) the granular sizes; 3) the reservoir pore throat sizes; and 4) the effective fluwable pore volume. The four parameters are interdependent, each reflecting one aspect of their relationship to the residual oil. The other is the capillary "entrapping force" in the low permeability reservoir on the residual oil. It is indicated through theoretical calculation that the residual oil saturation rises up as the permeability goes down.

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