GEOCHEMICAL CHARACTERISTICS OF NATURAL GAS FROM GIANT ACCUMULATIONS IN CHINA
J. Dai1,2,3*, J. Li1, W. Ding3, G. Hu1, X. Luo1, L. Hou3, S. Tao1, W. Zhang4, G. Zhu1
1 Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China
2 Department of Earth Science, Zhejiang University, Hangzhou 310027, China
3 Faculty of Natural Resourses and Information Technology, University of Petroleum Beijing, 102200, China
4 Institute of Exploration and Development, Changqing Oilfield company, PetroChina, Xi’an 710021, China
*Corresponding author. Email: djx@petrochina.com.cn
ABSTRACT
Six giant gasfields with proven reserves of more than 100 billion cubic metres (bcm) or 3.53 trilllion cubic feet (tcf) are present in the Ordos and Tarim Basins, China. These gasfields are Sulige, Yulin, Wushenqi and Jingbian in the Ordos Basin, and Kela 2 in the Tarim Basin. In this paper, we report on the geochemical characteristics of gases 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 C1-4 alkanes (generally over 90%) and minor CO2 (below 3%); The generally high δ13C values of C1-C4 hydrocarbons indicates a significant contribution from humic source rocks such as Permian-Carboniferous and Mid-Lower Jurassic coal measures. δ13 C1, δ13C2, δ13C3 and δ13C3 values for gases in Upper Palaeozoic reservoirs were -35 to -32‰, -28 to -24‰, -27 to -24‰, and –23.5 to -22‰, respectively. The δ13CiC4 value is higher than that of the δ13CnC4.
He3/He4 ratios vary from 10-7 to 10-8, indicating that the helium is of crustal (as opposed to mantle) origin. The CH4/He3 ratio is 1010 to 1011, indicating that the CH4 is of organic origin. None of the data is consistent with an abiogenic origin for gas in the Ordos Basin.