关键词: 碳同位素, 烷烃气, 大气田, 气源对比, 鄂尔多斯盆地

Abstract: The Ordos Basin is the secondary biggest 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 %.

Key words: carbon isotopes, alkane hydrocarbon gas, giant gas field, gas source correlation, Ordos Basin