As compared to the device developed by our predecessors, the newly developed adsorbed gas degasification-collection device has a series of advantages such as wide sample application range, large sample inlet, high vacuum, short-time sample smashing, low crushing temperature and water-free gas collection, which ensure the geochemical characteristics of acquired adsorbed gas samples to be the same as those of the adsorbed gases on corresponding hydrocarbon-source rocks. The results showed that the acquired adsorbed gases are composed mainly of hydrocarbon gas and carbon dioxide gas, with the hydrocarbon gas accounting for more than 80%, and can be measured reliably for their δ13C1-δ13C3 data, even δ13C4-δ13C5 data. The results of carbon isotope test and analysis satisfy the needs for the geochemical study and application of adsorbed gases. The above new techniques of experimental geochemistry are helpful for establishing the new direct natural gas-source correlation method, proving that the previous usual method of indirect natural gas-source correlation is scientific and authentic, thus providing the experimental basis for the study and application of adsorbed gases on hydrocarbon-source rocks.
SHI BaoguangWANG XiaofengXU YongchangZHEN Jianjing
Many years of experimentation have led to the development and improvement of equipment and methods used to make gas source rock correlations.By crushing samples using a ball mill and directly collecting adsorbed gases in the absence of aqueous media under high vacuum conditions,most possible interference factors,such as atmospheric pollution,crushing-induced pyrolysis,and gas collection by aqueous media are eliminated.This greatly enhances the volume percentage of hydrocarbon gas in the acquired adsorbed gases,with maxima up to more than 80%.The actual measurement of carbon isotopic series can be carried out to such an extent as to be δ13C1-δ13C5.A preliminary study using newly established equipment and methods has indicated the following.(1) The carbon isotopic composition of ethane in adsorbed gases on hydrocarbon source rocks can be used to distinguish the types of source rocks.This is consistent with results obtained by using organic geochemical parameters for source rocks,and illustrates that it is highly feasible to use the carbon isotopic composition of ethane in natural gases as a parameter for distinguishing the types of source rocks.(2) The thermal evolution degree(Ro) of hydrocarbon-source rocks calculated in terms of the carbon isotopic composition of methane in adsorbed gases on hydrocarbon source rocks agrees well with the vitrinite reflectance actually measured in the source rocks.This confirms the reliability of the relationship between the carbon isotopic composition of methane in natural gases and the thermal evolution degree determined using statistics.(3) Finally,a direct gas source correlation method for natural gases has been established,and the expression of log Ro vs.δ13C1 established in terms of actually measured δ13C1 values of methane in absorbed gases.Ro values of hydrocarbon source rocks have been established as well,thus creating favorable conditions for precise oil-and-gas source correlations in exploration areas.
