Genesis Of Permo-Carboniferous Coal-formed Gas And Its Difference From Jurassic

Coal,carbonaceous mudstone and gray shale in Permo-Carboniferous coal measures are common,but from which hydrocarbons are is uncertain .The influence of igneous rock to the composition of hydrocarbons and carbon isotope of coal-formed gas is worth to explore. There is not better methods to identify the coal-formed gas from Carboniferous - Permian and Jurassic. Aiming at these questions ,using low maturity coal, carbonaceous shale, gray shale and igneous rock to do thermal simulation experiment, and draw the difference of composition of hydrocarbon gas from different lithology ,and ascertain the lithology which produce more hydrocarbon gas in Linqing Depression and Baxian depre- ssion . Explore the impact of the composition of light hydrocarbon gas and carbon isotope for the existence of different igneous rock in coal measures.Raised the identification meth- ods between Permo-Carboniferous and Jurassic coal-formed gas.By the analyses of hydrocarbon gas generated from the thermal simulation experiments of samples of coal,coaly mudstone and grey mudstone in Linqing Depression,the result shows that the light hydrocarbon compositions between the coal and grey mudstone have an obvious difference,but the composition characteristics between the coal and coaly mudstone is similar.The light hydrocarbon compositon of natural gas from well GG4 is greatly similar with the coal's,and partly with the coaly mudstone's,in view of the above,the author concludes that coal is the main gas-source rock of the Permo-Carboniferous coal-bearing series in Linqing Depression,and the subdominant is coaly mudstone ;the light gas composition of the lacustrine mudstone in Baxian Depression differs greatly from the coaly mudstone, and is partly similar with the coal's. Compared with the natural gas,the author concludes that the dominant gas-source rocks of the Permo-Carboniferous coal-bearing series in Baxian Depression are coal and lacustrine mudstone.The experiments of coal with basalt,diorite-porphyrite, lamprophyre and diabase shows that igneous rocks have effect on the composition of coal-formed gas,making the paraffin and cycloalkane parameters have an obvious change, for instance CC5/MCC5,nC5/(2-MC5+3MC5),heptane values, moldovite index and so on; basalt,diorite-porphyrite, lamprophyre all definitely make the carbon isotope of coal-formed gas change heavy,especially lamprophyre,which changes the carbon isotope of ethane and propane heavier respectively by 1.1‰and 2.1‰, the effect of diorite-porphyrite is secondary and basalt have little effect on the carbon isotope of coal-formed gas.The thermal simulations of coal from C-P and J suggest that the constitutes of lighter hydrocarbon of the two kinds of coal gas are obviously different.For example, Before 550℃, nC7/MCC6 > 3 in Fangzi group,Whereas in Taiyuan group,it is converse.In high temperature (>500℃),Ben/nC7 is greater than 40 in Taiyuan group, but is less than 40 in Fangzi group etc.Lighter hydrocarbon parameters plate and C7 trigonometry Plate can differentiate these two kinds of coal gases.In all, conclusion is that the natural gas from Gubei gu1 well is from C-P coal gas,and there are some gas from J interfused.