POOL-FORMING PERIODS AND GAS SOURCES OF WEIYUAN GASFIELD

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.