POOL-FORMING PERIODS AND GAS SOURCES OF WEIYUAN GASFIELD
POOL-FORMING PERIODS AND GAS SOURCES OF WEIYUAN GASFIELD
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发表时间：
2003
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通讯作者：
Dai Jin-xing
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作者：
Dai Jin-xing
威远气田不仅是中国最古老的储层（震旦系灯影组）和最古老的烃源岩（寒武系九老洞组）的大气田，也是世界地质年代最古老的气藏之一。尽管该气田储集层和烃源岩年龄很古老，但天然气成藏很晚，为喜马拉雅期。关于该气田的天然气成因，存在多种说法。一类是无机成因，认为气藏中He、Ar含量较高，平均40Ar/36Ar值高达7 000，认为包括烃类气体的天然气可能来自地球深处。另一类是有机成因，分为三种观点：a）灯影组气藏是其自身藻白云岩裂解气； b)气源主要来自灯影气藏盖层,即九老洞组; c)灯影组气藏为灯影白云岩和九老洞泥岩的混合气。根据灯影组气藏δ13C1、δ13C2和δ13C1-31‰，判断烃类气体为有机成因。灯影组平均TOC值仅为0. 12%，难以成为有效烃源岩；九老洞组TOC平均值为0. 97% ,烃类气体应来自九老洞组。气藏中 R/Ra 值为 0. 021 和 0. 022 表明氦为地壳源无机成因。同时，由于震旦系白云岩中铀、钍矿物含量较低，九老洞组铀、钍矿物含量较高，气藏中的氦也主要来自于九老洞组铀、钍的衰变。因此，灯影组天然气主要为九老洞组有机成因气。
Weiyuan gasfield is not only the great gasfield with the oldest reservoirs (the Sinian Dengying Formation) and the oldest source rocks (the Cambrian Jiulaodong Formation) in China but also one of the gas pools with the oldest geologic ages in the world. Though the ages of reservoirs and source rocks in this gasfield were very old, the natural gas in it accumulated and pool -formed very late, in the Himalayan period. There are various opinions concerning the gas origin of the field. One is inorganic origin, which believes the natural gas including hydrocarbon gases maybe come from the deep of the Earth because of the high He and Ar contents in gas pools and the high mean 40Ar/36Ar value of up to 7 000. The other is organic origin, which is divided into three viewpoints: a) the gas pools in the Dengying Formation are cracking gases from its own algal dolomite; b)the gas sources mainly come from the caprocks of the Dengying gas pools, that is , the Jiulaodong Formation; c)the gas pools of the Dengying Formation are mixed gases from the Dengying dolomite and the Jiulaodong argillutite. According to δ13C1 δ13C2 and δ13C1-31‰ in the gas pools of the Dengying Formation, it is concluded that the hydrocarbon gases are organic origin. The Dengying Formation has the average TOC value of only 0. 12%, so is difficult to become effective source rocks; and the Jiulaodong Formation has the average TOC value of 0. 97% , so the hydrocarbon gases should come from it. The R/Ra values of 0. 021 and 0. 022 in gas pools suggest the crust-source inorganic origin of helium. It the meantime, as the uranium and thorium mineral contents are low in the Sinian dolomite and higher in the Jiulaodong Formation, the helium in gas pools also mainly comes from the uranium and thorium decay of the Jiulaodong Formation. As a result, the natural gas in the Dengying Formation is mainly organic origin gases from the Jiulaodong Formation.