Hydrocarbon accumulation threshold quantitatively expounds the regularity of hydrocarbon distribution. Hydrocarbon accumulation threshold refers to the least amount of oil and gas dissipated during the process of pool formation in the accumulation system. Dissipated hydrocarbon includes those left within the source rock

detained in the reservoir bed

discharged before cap-rock being formed

adsorbed by adjacent rock

and dissolved in water

as well as oil and gas diffusion. The mechanisms of hydrocarbons dissipation within the pool-forming system mainly include being adsorbed by rocks

blocked by capillary and dissolved by subsurface water and oil. The manners for hydrocarbons dissipated out of the pool-forming system mainly involves oil and gas diffused upward

oil and gas dissolved in formation water-flow and destroyed by tectonic movement after the formation of pools. Statistics

physical modeling experiment

multiphase percolation theory and geological phenomena analyses prove that hydrocarbon accumulation threshold

like hydrocarbon generation threshold and hydrocarbon expelling threshold

exist objectively.The essential research of hydrocarbon accumulation threshold is modeling and calculating the amount of hydrocarbon disspation. On the basis of studying the regional geological condition

dividing different accumulation systems

and geological analyses of source

cap and reservoir rocks

the research is

firstly

determining the temperature and pressure conditions of pool formation and the history of oil/gas reservoir evolution

as well as measuring the amount of hydrocarbon dissipation through adsorption

dissolution and diffusion for the sake of finding out the main factors controlling the dissipation and confirming the quantitative relationship;secondly

modeling the total amount of hydrocarbon generation in the source rocks and the total amount of hydrocarbon dissipation; and lastly

discriminating the accumulation threshold and recovering the critical geological condition.In the geological history

hydrocarbon begins to accumulate when the amount of hydrocarbon generated in source rock reaches or exceeds the least amount of oil and gas dissipated. Accumulation period

amount of hydrocarbon migration and accumulation and the largest reservoir size are all constrained by hydrocarbon accumulation threshold. The research of hydrocarbon accumulation threshold of 28 accumulation systems in Jiyang depression shows that the least amount of oil and gas dissipation is from 0.036 billion tons to 1.84 billion tons

which increases with the increase intensity of hydrocarbon expulsion

range of migration and accumulation and dip angle of objective formation and decreases with the increase of sand-stratum ratio

tectonic movements and denuded thickness. Finally

on the basis of calculation and comparison the amount of hydrocarbon generation

the resources

largest oil/gas field and favorable targets can then be predicted.