Expulsion of hydrocarbons from a shale source rock can be divided in four stages. In the first stage, only a small amount of hydrocarbons can be expelled in water solution and by diffusion. Compaction and hydrocarbon concentration gradient are the major driving forces, whereas their corresponding hydrocarbon expulsion amounts make up 30% and 70% to the total, respectively. In the second stage, in addition to transport by water solution and by diffusion, source rocks expel a large quantity of gas in free phase. In the third stage, the most important feature is that source rocks expel oil as a separate phase and gas in oil solution. Hydrocarbon expulsion by diffusion through the source rock organic network, dehydration of clay minerals, and thermal expansion of fluids and rocks are the three major driving forces in the second and the third stages, whereas the corresponding hydrocarbon expulsion accounts for 40–60%, 10–20%, and 5–10%, respectively, of the total amount expelled. In the fourth stage, source rocks mainly expel dry gas as a free phase. Volume expansion of kerogen products and capillary force are the two major driving forces for hydrocarbon expulsion. The expulsion accounts for 60% and 30% to the total gas expulsion of this stage, respectively, for each driving force. Hydrocarbon expulsion, including the hydrocarbon expulsion threshold (HET), the relative phases and the dynamics, are controlled by two factors: the hydrocarbon generation amount, and the ability of source rocks to retain hydrocarbons. Source rocks cross the HET and begin to expel a large quantity of hydrocarbons when the generated hydrocarbons have met all of the needs for hydrocarbon retention. HET is divides the processes of hydrocarbon expulsion into the various four stages.
Experimental Simulation of Hydrocarbon Expulsion in Semi-open Systems from Variable Organic Richness Source Rocks
