Abstract:

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.

Key words: accumulation threshold, accumulation system, resource evaluation, Jiyang depression