Four possible causes of primary hydrocarbon migration are: (a) forces resulting from sediment compaction (movement of compaction water); (b) aquathermal effect (water expansion owing to increasing temperature); (c) osmotic effect because of variation of formation-water salinity caused by ion filtration by shales; and (d) montmorillonite-illite conversion (montmorillonite dehydration). The relative importance of these causes may be related to another problem--the form of oil and gas at primary migration stage, that is, whether it is in solution in water or in its own phase.
A larger proportion of migrating oil must be in its own phase because of the relatively low solubility; most of the migrating gas must be in solution in water.
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In the business of petroleum exploration, the topic of migration has enjoyed a great bounty of ideas and opinions while, at the same time, suffering a famine of facts. This publication tontains papers on geologic constraints on migration mechanisms, oil migration limitations, the importance of water-mineral-organic matter interactions, and the nature of shales.