Abnormal Concentration And Origin Of Heavy Hydrocarbon In Anthracite Based On Refined Desorption Process

Current classical theory and hypotheses could not explain the reason of abnormallyhigh concentration of heavy hydrocarbon of anthracite in Zhina coalfield, western Guizhou.Around this scientific problem, the paper had in-depth discussion with thegeological-geochemical origin of the heavy hydrocarbon in anthracite by means of refineddesorption experiments, gas isotopic geochemistry and related ancillary test and analysis,which used four series of coalbed gas samples as objects. Firstly, it is found that naturaldesorption rules of primary coal and mylonitic coal is significantly different, the formerpresents two-stage change but the latter reflects three-stage characteristic, and themolecular sieve effect derived from ink pores in mylonitic coal restricts the flow ofcoalbed gas is the root cause of this difference. Secondly, it is concluded that heavyhydrocarbon in primary coal mainly occurs in closed pore, while that in mylonitic coalmainly in open pore, and its geochemical characteristics vary significantly at differentdesorption stage. The more crushed of coal samples, the higher concentration of heavyhydrocarbon, heavy hydrocarbon concentration in closed pore is extremely high. Thirdly,by analyzing the pore structure of coal crushed powder with different particle size, thededuction of―protection effect of heavy hydrocarbon in closed pore of anthracite‖, whichwas proposed earlier by our research group, is explained and proved. In microenvironmentof closed pore, the increasing pore pressure suppressed and postponed the cracking ofheavy hydrocarbon. Heavy hydrocarbon remains high initial concentration in closed poresince closed system blocks material exchange of heavy hydrocarbon with the outside, so itstill be preserved up to now. Heavy hydrocarbon generated before the end of―oil window‖in closed pore. Fourthly, there is a significant negative linear correlation between heavyhydrocarbon concentration and coal particle size, which indicates that higher heavyhydrocarbon concentration of mylonitic coal maybe derived from its pulverizing coalstructure. Fifthly, this paper first found that the values of δ13CCH4of anthracite can be up to-4.6‰in closed pore. Through comprehensive analysis, it is believed that the4anthracitesamples have inorganic mantle-derived methane and noble gas in closed pore. Thepresence of inorganic gas in closed pore of anthracite in Zhina coalfield is closely relatedto the activities of Emei mantle plume, four deep regional faults from the interior andboundary of Zhina coalfield provide good channels and ways for the upward migration ofmantle-derived gas. Inorganic gas entering Upper Permian coal seams was mainly occur atdiagenesis stage and Yanshan thermal metamorphic stage.