In argillaceous and carbonate-evaporite source beds, much of the oil-generating organic matter is concentrated along bedding surfaces (varves and laminae). During the principal phase of oil generation, when adequate thermal energy is available, 25 to 30 wt. % of the original organic matter commonly is converted to liquids, mainly bitumen, with a relatively small percentage of water. Part of the bitumen is then thermally cracked to crude oil before oil expulsion occurs. Substantial amounts of gas, principally hydrocarbons with some CO₂ and N₂, are also generated. Much of the water and CO₂ is generated before oil is formed.

The release of fluids from the organic matter causes a reduction in the volume of the residual solid organic matter; however, this volume decrease is offset by the considerably greater volume of the generated fluids. Thus, the volumes of generated products plus residual organic matter represent a substantial net volume increase relative to the volume of the organic matter at the start of significant oil generation. Consequently, very high pressures result locally along the bedding surfaces if the laminae are adequately sealed. Eventually, these localized, transitory, high fluid pressures will develop along the bedding in most parts of the source-rock sequence if the entire source-rock system is sealed and confined. This generated fluid pressure supplements aquathermal pressure caused y thermal expansion of water but is more focused in time and space and is, therefore, a more important factor in internal migration and expulsion than aquathermal pressure.

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