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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 ,指出这种天然气主要是原油裂解气———深部地层古油藏的原油裂解后以气相运移方式进入石炭系及奥陶系储层
Carboniferous and some Ordovician reservoirs in the Tazhong area of Tarim Basin have different forms of condensate gas and oilfield associated gas, and the methane carbon isotope values are very close. Based on the hydrocarbon generation evolution pattern of sapropelic organic matter and the fractionation principle of carbon isotope of natural gas, it is considered that the condensate hydrocarbon is formed after the peak of oil production, and therefore the methane carbon isotope value of condensate gas should be higher than the methane carbon isotope Some value. However, the actual situation is that the condensate gas and the methane in the oilfield have almost the same carbon isotope values, and the difference between the carbon isotope values of A and E is small, obviously not in accordance with the normal carbon isotope fractionation principle. This single phenomenon is difficult to explain. In this regard, according to the composition of natural gas and carbon isotope characteristics, combined with natural gas composition ln (C2 / C3) and (δ13 C2-δ13 C3) relationship to identify its origin, pointed out that this natural gas is mainly crude oil pyrolysis gas --- deep formation Crude oil from the ancient reservoirs is pyrocarbon-typed into the Carboniferous and Ordovician reservoirs by gas migration