塔中地区石炭系及部分奥陶系储层天然气的成因

GENESIS OF NATURAL GAS IN THE CARBONIFEROUS AND PARTIAL ORDOVICIAN RESERVOIRS OF TAZHONG REGION

塔里木盆地塔中地区石炭系及部分奥陶系储层内赋存着形式不同的凝析气和油田伴生气 ,其甲烷碳同位素值很接近。根据腐泥型有机质生烃演化模式和天然气碳同位素的分馏原理 ,分析认为 ,凝析油气是在生油高峰之后形成的 ,因而凝析气的甲烷碳同位素值应比油田伴生气的甲烷碳同位素值重一些。而实际情况则是凝析气和油田伴生气甲烷碳同位素值几乎一致 ,且甲、乙烷碳同位素值的差值小 ,显然不符合正常的碳同位素分馏原理。用单一成因的观点难以解释这种现象。对此 ,根据天然气组分及碳同位素特征 ,结合天然气组成ln(C2 /C3 )与(δ13 C2 -δ13 C3 )关系图判识其成因 。

The condensate gas and associated gas with various occurrences were found in the Carboniferous and partial Ordovician reservoirs of Tazhong region in Talimu Basin and their methane carbon isotopic values are close to each other.In light of the hydrocarbon generating pattern of sapropel type organic matter and the fractional principle of carbon isotope in natural gas,through analyzing it is considered that the condensate gas was generated after the most active oil generation time.So the methane carbon isotopic value of the condensate gas should be heavier than that of the associated gas.Practically,both of them are almost consistent with each other and the difference between the methane carbon isotopic value and the ethane carbon isotopic value is small,which is not accordant with the normal fractional principle of carbon isotope obviously.Such a phenomenon can be hardly explained by the view of a single genesis.Therefore,on the basis of the natural gas component and carbon isotope properties,in combination with the genetic judgement by ln(C 2/C 3)～( δ 13 C 2- δ 13 C 3)relation curve it is pointed out that this kind of natural gas is mainly the crude oil cracking gas,i.e.the gas was formed from cracking the crude oil in an ancient oil reservoir,then entered into the Carboniferous and Ordovician reservoir beds.