威远气田成藏期及气源

POOL-FORMING PERIODS AND GAS SOURCES OF WEIYUAN GASFIELD

戴金星

DAI Jin-xing

中国石油, 勘探开发研究院, 北京, 100083

Research Institute of Petroleum Exploration and Development, CNPC, Beijing 100083, China

威远气田不仅是中国储集层最老(震旦系灯影组)和烃源岩最老(寒武系九老洞组)的大气田,而且也是世界上地质时代最古老的气藏之一。尽管该气田的储集层和烃源岩时代很早,但它的天然气聚集成藏很晚,是在喜马拉雅期。关于气田的气源研究有多种认识:一种认为是无机成因,因为气藏中的He和Ar含量高,40Ar/36Ar平均高达7 000,故认为包括烃气在内的天然气可能是来自地球深部;另一种认为是有机成因气。关于后者又有3种观点:a)灯影组气藏是该组含藻白云岩自生自储的裂解气;b)气源来自或主要来自灯影组气藏的盖层九老洞组;c)灯影组气藏是来自灯影组白云岩和九老洞泥页岩的混合气。作者根据灯影组气藏中δ13C1<δ13C2,δ13C1<-31‰,确定烷烃气是有机成因的;灯影组有机碳平均值仅为0.12%,难成为有效烃源岩,而九老洞组有机碳平均值为0.97%,故烷烃气应来自后者。由于气藏中R/Ra为0.021及0.022,说明氦是壳源无机成因;同时震旦系白云岩含铀、钍矿物低,而九老洞组则较高,故气藏中的He也主要由九老洞组铀、钍衰变来。由此得出灯影组天然气主要来自九老洞组的有机成因气。

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.

https://doi.org/10.11781/sysydz200305473