鄂尔多斯盆地大气田的烷烃气碳同位素组成特征及其气源对比

鄂尔多斯盆地是中国第二大沉积盆地,至2002年底已发现4个储量为10¹¹m³以上的大气田.上古生界与煤系有关的烃源岩形成了苏里格、榆林、乌审旗3个位于碎屑岩中的煤成气大气田,在下古生界奥陶系马家沟组碳酸盐岩中发现了靖边气田(其气源仍存在争议).对4个大气田125个气样烷烃气碳同位素分析成果的研究表明,上古生界大气田烷烃气碳同位素组成较重,数值分布域小,表现了煤成气特征,其δ¹³C_iC4>δ¹³C_nC4,各个气田仅发现单项性碳同位素倒转.下古生界靖边气田δ¹³C₁频率主峰值和δ¹³C_B,δ¹³C_T与上古生界具有相似性,也表现出煤成气为主的特征.但靖边气田具有多项性碳同位素倒转,δ¹³C₁,δ¹³C₂,δ¹³C₃数值分布域大,δ¹³C₂较轻,表现出了以煤成气为主油型气为辅的混合气特征.煤成气和油型气的气源均来自上古生界的石炭-二叠系煤系和太原组含煤地层中有机碳丰度高的石灰岩,否定了有机碳含量约为0.20%的马家沟组碳酸盐岩是油型气烃源岩的结论.

Alkane carbon isotopic composition and gas source in giant gas fields of Ordos Basin

The Ordos Basin is secondary big sedimentary basin of China. Four giant gas fields had been proven in the basin，including Sulige，Yulin，Wushenqi and Jingbian gas fields with proven reserves over 100 billion cubic meters for each gas field to the end of 2002.The hydrocarbon source rocks of the Sulige，Yulin and Wushenqi gas fields are coal measures of the Upper Paleozoic.The Jingbian gas field was found in the carbonates of Majiagou Formation in the Ordovician of the Lower Paleozoic. There are still existing arguments about the gas sources of Jingbian gas field.The alkane carbon isotopes analyses of 125 gas samples collected from the above-mentioned four gas fields show that the carbon isotopes of giant gas fields in the Upper Paleozoic are heavy，and the range of values is narrow. The gases are characterized by coal-formed gas. The δ¹³C_iC4 value is bigger than the δ¹³C_nC4 value. Reversal of carbon isotope values in single type appears in the Upper Paleozoic gas fields. In the Lower Paleozoic Jingbian gas field，the δ¹³C₁ dominant peak values，the δ¹³C_B values and the δ¹³C_T values are similar to those in the Upper Paleozoic.However，there are still some different characteristics in Jingbian gas field from other three gas fields.The reversals of carbon isotope values are of multiple types. The values of δ¹³C₁，δ¹³C₂ and δ¹³C₃ are ranged widely，and the δ¹³C₂ value is smaller.These characteristics show that the gases are mixed from mainly humic and some sapropelic sources.It is suggested that the humic and sapropelic sources are all from C-P coal measures and the Taiyuan Formation carbonates with a high abundance of organic matter in the Upper Paleozoic rather than the conventional idea suggested that sapropelic gases are mainly from the Majiagou Formation carbonates with the total organic content of 0.20%.