华北上古生界煤层甲烷稳定碳同位素组成与煤层气解吸——扩散效应

华北上古生界浅层煤层甲烷稳定碳同位素具有颁布范围广、组成偏轻的总体分布特点，在进入高煤级煤阶段之后开始接近或落入腐殖型常规天然气甲烷稳定同位素分布范畴。该区浅层煤层甲烷稳定碳同痊素偏轻的现象主要起源于热力馏背景之上叠加的解吸－扩散效应，由此导致了煤层气原生带→过渡带→解吸带的垂向分带特征。

STABLE CARBON ISOTOPE COMPOSITION AND DESORPTION-DIFFUSION EFFECT OF THE UPPER PALEOZOIC COALBED METHANE IN NORTH CHINA

The stable carbon isotope of the shallow Upper Paleozoic coal bed methane (CBM) in North China is generally more various in value and lighter in composition than those of the normal humic-type natural gas, and approaches to or falls in the range of the norm al humie-type gas in the stage of the high-rank coals. This might be primarily originated from the CBM desorption-diffussion effect overlapped upon the thermodynamical isotope fraction mechanism, which resulted in the vertical zoning such as the primary, transitional and desorbed zones. The depth limits among different zones are various with districts, which is associated with the depth of CBM weathered zone and the rank of coal reservoirs, i.e. the deeper the weathered zone, the deeper the desorbed zone, and the higher the coal rank, the shallower the desorbed zone. As for the gas coal reservoirs, the primary zone is deeper than 1500m, the transitional zone is located between 1500m and lO00m, and the desorbed zone is shallower than lO00m. In the high-rank (meagre coal and anthracite) Coal reservoirs deeper than the weathered zone, there is hardly the stable carbon isotope fraction be cause the absorbability of the coal organic matters to CBM increases greatly. Furthermore the coalbed methane in the middle-rank coal (flame to lean coa1) reservoirs that have been explored was almost migrated and had the desorption-diffusion origin. The study on the desorption-diffusion effect above-mentioned is helpful to the evaluation of the CBM-accumulated conditions in North China. If some geological controls such as the tectonic evolution, combed-buried history and covering strata were favorable to the CBM preservation, the desorption-diffusion effect might be favorable to the CBM accumulation. Conversely, the coalbed methane might escape substantially.