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 δ 13C iC4 value is bigger than the δ 13C 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 δ 13C 1 dominant peak values, the δ 13C B values and the δ 13C 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 δ 13C 1,δ 13C 2 and δ 13C 3 are ranged widely,and the δ 13C 2 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%.