柯克亚油气田混合来源天然气的地球化学特征

GEOCHEMICAL CHARACTERIZATION OF MIXING NATURAL GAS IN KEKEYA FIELD, TARIM BASIN, CHINA

柯克亚油气田是我国开发比较早的一个油气田,但由于受混源和其他因素的影响,对气源的认识众说纷纭。通过对天然气的地球化学特征的分析,并参照国内外关于甲烷、乙烷、丙烷碳同位素关系方面的资料,提出了如下看法:柯克亚天然气碳同位素相对较重,与塔里木盆地下古生界海相天然气的碳同位素组成区别较大;根据天然气40Ar/36Ar组成,柯克亚油气田的气源不是单一来源;柯克亚油气田绝大多数天然气的甲烷和乙烷碳同位素值关系符合Faber推断的 型有机质方程直线关系,说明有机质属于较好的有机质类型,或者是由原油裂解形成的天然气;除了K2井和K18井天然气成熟度较低以外,绝大多数天然气的成熟度大约在1.8%～2.2%之间,属于高过成熟气,但也混有一些成熟度在0.9%～1.2%的源岩形成的天然气;推测天然气主要来源于石炭系—二叠系源岩,而混有成熟度低的源岩形成的天然气则有可能来自侏罗系源岩。

This paper is mainly concentrated on the geochemical characteristics and origin of gas of Kekeya field in the Tarim basin, NW China. Many dispute viewpoints about origin of gas and condensate existed although the field has been found in 1977. This study shows that Permian mudstone is main source rock of oil and gas. The carbon isotopic value of natural gases in kekeya region are more heavy than that of other natural gases in Tarim basin which are mainly derived from lower Palaeozoic marine source rocks. In addition, the 40Ar/36Ar and 3He/4He ratios are also indirect evidence for gases mixing. The relationship between carbon isotopes of methane and ethane is coincidence with Faber equation of gas derived from Organic matter Ⅰ, Ⅱ. This will also approve that gases are mainly derived from source rocks which have relatively good organic matter type. Most maturity of gas is estimated as 1.8%~2.2% besides K2 and K18 well based on the formula. They belong to overmature gases which are generated by Carboniferous\|Permian source rocks. In addition, the gas derived from 0.9%~1.2% Ro source rocks (Jurassic coal\|bearing strata) may also be mixing.