Most oil accumulations in clastic sediments originated from organic matter in fine-grained source-bed facies. As the organic matter in the source beds matured to form intermediate oil-precursor products, clay-mineral transformations supplied water as a migration medium. Meanwhile, compaction from overburden greatly increased fluid pressures in the source beds to establish a pronounced pressure gradient from source beds to associated carrier or reservoir-rock systems.
Initially, migration of the hydrocarbon-bearing waters was imperceptible because of the extremely fine pore network. However, eventually the high fluid pressures facilitated the formation of very small zones of weakness which upon further consolidation of the rock became very small microfractures. Concomitantly, during transformation of montmorillonite to illite, grain size and pore size increased. Thus micropathways developed to permit a reasonably effective extraction and "drainage" of mobile organic matter. The adsorption capacity of kerogen for this mobile component was weakened by the moderately high temperatures prevalent at depths of several thousand feet or more. Accordingly accessibility of this oil-precursor material to the water-migration medium increased.
Where dips were moderately high or a positive structure was nearby, lateral updip or up-structure migration along source-rock bedding surfaces was favored. Cross-bed migration pathways were much less permeable, but high-pressure gradients between the source bed and on overlying or underlying sandstone overcame this cross-bed barrier. However, extensive upward migration across a series of interbedded sandstones and mudstones would be unlikely for two reasons: (1) mudstone units are relatively impermeable; and (2) each mud-stone unit constitutes a pressure barrier to fluids in an underlying sandstone.
The overall movement of fluids in basins with compacting sediments is both vertical and lateral. The only vertically upward migration for extensive distances would be in thick, flat-lying monotonous sequences of unchanging lithology.