STUDY ON THE SEGREGATIVE MIGRATION OF NATURE GAS IN EASTERN JUNGGAR BASIN, NW CHINA

Gas geochemistry has recently been shown to enhance information regarding the geological history of hydrocarbons. In this paper, graphical representations of physico-chemical processes affecting the chemical and isotopic signatures of natural gases are exemplified. These diagrams are based on experimental studies and the use of basic statistics to extract significant and synthetic parameters from the geochemical data. From the chemical and isotopic ratios, a statistical analysis yields two very important parameters. The first parameter, using mainly the C₂₊ fraction of the gas, relates to maturity and the second parameter, involving the proportions and δ¹³C values of methane, indicates mainly segregative migration. Positive values of the second parameter indicate that gases accumulated far from their source, whereas negative values correspond to residual gas pools after leakage of a part of the fluids. A tentative reconstruction of the gas history has been performed two gas pools in Junggar basin: Carbon isotopic composition of natural gases in Carboniferous gas reservoir in the Klameli and Wucaiwan gas pools can reflect the dominance of Carboniferous coal type gas despite its high mature degree. Results show that in the Klameli gas pool, the major fault and the unconformity between the Carboniferous and Permian systems act as a drain for hydrocarbon migration. The hydrocarbon kitchen is located its two south-north sides of Dishuiquan and Dongdaohaizi sags, associating with a clear correlation between the second parameter and the distance of its two sides of sags. In the Wucaiwan gas pool, the hydrocarbon kitchen is located its west sides of Dongdaohaizi sags, associating with a clear correlation between the second parameter and the distance of Dongdaohaizi sags. Its natural gas migration is from west to east. Using diagrams based on experimental work and statistical data analyses, we have shown for two gas pools that it is possible to reconstruct the hydrocarbon pathways, directions of migrations, and degree of maturity of the source rocks associated with the gas. The use of all the available geochemical data affecting the chemical and isotopic signatures of hydrocarbon gases improves the usefulness of these gases in hydrocarbon exploration.