In the Gulf Coast area salinity of formation water increases with decrease of shale porosity. The higher salinity probably results from ion filtration or deionization by shales (water passes through the shales but most salt ions are trapped there), as proposed by Overton and Timko.

If the shale beds are interbedded with permeable sands, the maximum reduction in shale porosity must occur immediately above and below the sands, because the maximum water appears to be expelled from these parts of the shales. The porosity of the central zones of the shale beds seems to stay at relatively high levels, at least during the earlier stages of compaction and water expulsion. The fluid pressure in the margins thus will become lower than that in the central zones, and as a result the water will move from centers to margins toward the sands.

Corresponding salinity distribution in shales will be reciprocal to the porosity, because of the ion-filtration effect of the shales. In other words, the salinity in the shales will tend to increase toward sand beds. The osmotic-flow direction in this case is from the less concentrated point to the concentrated, or from the centers to the margins of the shales toward the sands. This osmotic fluid flow is an addition to the compaction fluid flow mentioned above, and could facilitate fluid migration from shales to sands.

Water expelled by compaction is interpreted to be about one third as saline as that of the original sea water in the Gulf Coast, and that moved by osmosis is fresh. In these fresher waters, if all other conditions are the same, hydrocarbon solubility is higher than in more saline water. This factor, therefore, favors hydrocarbon migration from shales to sands.