GEOCHEMICAL CHARACTERISTICS OF NATURAL GAS AT GIANT ACCUMULATIONS IN CHINA

Abstract

A total of six giant gasfields each with proven reserves of more than 100 billion cubic metres are present in the Ordos and Tarim Basins, China. The gasfields are Sulige, Yulin, Wushenqi, Da'niudi and Jingbian in the Ordos Basin, and Kela 2 in the Tarim Basin. In this paper, we report on the geochemical characteristics of the natural gas at these fields based on studies of gas composition (150 samples), together with stable isotope data (143 samples) and helium isotope data (21 samples). Results show that the gases have high contents of C 1‐4 alkanes (generally over 90%) and minor contents of CO 2 (below 3%). The generally high δ 13 C values of C 1 ‐C 4 hydrocarbons indicates a significant contribution from humic source rocks such as Permian‐Carboniferous and Mid‐Lower Jurassic coal measures. δ 13 C 1 , δ 13 C 2 , δ 13 C 3 and δ 13 C 4 values for gases in Upper Palaeozoic reservoirs were ‐35 to ‐32‰, ‐28 to ‐24‰, ‐27 to ‐24‰, and ‐23.5 to ‐22‰, respectively. The δ 13 C iC4 value is higher than that of the δ 13 C nC4• He 3 /He 4 ratios vary from 10 ‐7 to 10 ‐8 , indicating that the helium is of crustal (as opposed to mantle) origin. The CH 4 / He 3 ratio is 10 10 ‐ 10 11 , indicating that the CH 4 is of biogenic origin. None of the data is consistent with an abiogenic origin for gas in the Ordos Basin.