A logical approach to the riddle of migration is to take a good look at the destination--where oil and gas are found and where we should have the most positive information. If we can understand what is going on in the traps, it should enable us to look back along the migration trail with special insight as to what has been happening there. That insight could extend all e way back to the "source."
Traps are the most logical places for hydrocarbon mixtures to be put together as distinct oil and gas fluids; traps are not just passive receivers or containers of hydrocarbon mixtures put together elsewhere. Effective oil and gas traps of different well-known styles have in common a very important feature. Structurally and stratigraphically they are designed to discharge waters from depth. Thus they function as active focal mechanisms to gather and process feedstock waters carrying hydrocarbons and other organic matter. It is a forced-draft system. The concept adds an exciting new dimension to the anticlinal theory. It honors all factual observations around oil and gas deposits.
Very simple, the most important function of a trap is to leak water while retaining hydrocarbons. The water can leak because the enclosing membranes and cover are water soaked, like a wick. The hydrocarbons and other organic materials are separated from the waters as they pass through the trap. The separation is caused by abrupt changes--in pressure, temperature, and possibly salinity--which are related to the basic change in direction of feedstock (water) movement from lateral to upward. Coalescence of hydrocarbons makes bubbles or globules which cannot move so easily as water. The ultimate composition of a trapped hydrocarbon mixture depends on the residence times of the various components, which in turn depend on (1) what the water carries, (2) what the trap retains, and (3) the pore-volume exchange rate.