塔中地区奥陶系天然气地球化学及成因与分布特征

塔里木盆地塔中地区奥陶系天然气烃类气体以甲烷为主,含量主要分布在75%~95%之间,重烃(C2+)含量低,为3%~7%,干燥系数介于0.73~0.99之间;非烃气体以N2和CO2为主,具高N2、低CO2特征,含量分别为4%~8%和2%~6%。天然气δ13C1值为-63.1‰~-37.7‰,δ13C2值为-40.6‰~-24.8‰。天然气在平面上存在较明显差异,Ⅰ号带东段天然气干燥系数高,N2含量高、CO2含量低,δ13C1值较低,以热裂解成因为主;Ⅰ号带西段干燥系数偏低,N2含量低、CO2含量较高,δ13C1值较高,以热解成因的伴生气与混合成因气为主;Ⅰ号带中段及主垒带卡1区天然气δ13C1值介于两者之间,但前者天然气组分变化比较大,以原油伴生气和腐泥型有机质热解生成的干气为主,而后者天然气干燥系数变化较大,以热解生成的原油伴生气为主。天然气性质上的区带性差别可能与断裂及裂缝的发育程度有关,天然气成藏受喜马拉雅期构造运动的影响,以晚期成藏为主。

Geochemistry of Ordovicial Natural Gases and Its Origin and Distribution in Central Tarim Basin,China

Ordovician natural gas in the central Tarim basin mainly consists of methane with 75%-95%of CH4,3%-7%of C2+ and 0.73-0.99 of C1/(C2+).Non-hydrocarbon mainly is N2 and CO2,with 4%8%and 2%-6%,respectively.The carbon isotope composition of methane and ethane is from-63.1‰ to-37.7‰ and-40.6‰ to-24.8‰,respectively.The natural gas in the eastern part of No.1 structural belt has high dryness coefficient,high N2 content,low CO2 contents,and isotopically light methane(13C1),indicating origin of thermal cracking gas.In contrast,natural gas in the western part is characterized as low dryness coefficient,low N2 content,high CO2 contents and isotopically heavy methane,suggesting mixing of associated cracking gas and mixed gas.Geochemical characteristic of natural gas in the central part of No.1 structural belt and the Ka.1 area was fallen into that of natural gas between eastern and western parts above;the former is mixed of associated gas and primary gas with high dryness coefficient that was sourced from sapropelic,and the later is of thermal cracking gas of crude oil.The regional difference of gas geochemistry would be related to fault and fissure.The time of gas reservoir formation was happened during Himalayan era.