GC-MS analysis of solvent extracts from marine shale of Longmaxi Formation in South China

The marine shale of Longmaxi formation of Silurian in South China is the main exploration target for shale gas in China.In order to study the chemical nature of soluble organic matter in marine shale of Longmaxi formation in South China,Soxhlet extractions of shale samples from Longmaxi formation of Silurian in Fuling of Chongqing,Zhaotong of Yunnan Province and Yichang of Hubei Province were carried out with four polar solvents,including n-hexane,chloroform,acetone and methanol.It was found that the extraction rate of marine shale of Longmaxi Formation in South China is generally low,ranging from 0.02% to 0.73%.In comparison,the shale in the lower part of Longmaxi Formation in Zhaotong area of Yunnan Province exhibits the highest extraction rate,followed by the lower shale of Longmaxi Formation in Fuling of Chongqing and Yichang of Hubei Province,and the shale in the middle part of Longmaxi Formation in Fuling area of Chongqing shows the lowest extraction rate.By comparing the efficiency of different extraction solvents,the extraction rate of chloroform is the highest,followed by methanol and acetone,and n-hexane extraction rate is the lowest.The soluble organic small molecular compounds of Longmaxi shale in South China are composed of trace aromatic hydrocarbons,a small amount of aliphatic hydrocarbons and a large number of heteroatom compounds.The aliphatic hydrocarbons are mainly alkanes and alicyclic hydrocarbons with more esters of hetero-atomic compounds,followed by halogenated hydrocarbons and fewer alcohols and ketones,among which phthalates and 1-iodo-2-methylundecanone play an important role.There is a slight difference in the composition of soluble organic matter in shale in different areas,which is manifested as that there are relatively few aliphacic hydrocarbons in the middle shale of the Fuling Longmaxi formation in Chongqing,more aliphatics and halogenated hydrocarbons in the lower shale of the Fuling Longmaxi formation in Chongqing,and less alcohols and slightly more ketones in the lower shale of the Longmaxi Formation in Zhaotong of Yunnan Province.The solvents of n-hexane,methanol and acetone have good dissolution ability for the small oxygen-containing compounds in the shale,while the solvents of chloroform have good dissolution ability for the fatty hydrocarbon,halogenated hydrocarbon and small oxygen-containing compounds.The main components of n-hexane and methanol extracts are esters;chloroform extracts are esters,halogenated hydrocarbons and aliphatic hydrocarbons;acetone extracts are esters,amides,ketones and alcohols.The study shows the solvent extraction yield and the content of aliphatic hydrocarbons and halogenated hydrocarbons in soluble organic matter of the marine shale of the Longmaxi formation are associated with sedimentary environment and sediment composition.Therefore,in a sense,it can be concluded that the solvent extraction yield and the content of aliphatic hydrocarbons and halogenated hydrocarbons can be regarded as the parameter of indicating sedimentary facies.