Evidence for late charge of H2S-poor dry gas in the Ordovician in Tazhong area
He Hong, Li Hongxia, Zhang Ke, Tao Xiaowan, Cai Chunfang
1. Institute of Petroleum Exploration and Development, SINOPEC, Beijing 100083;
2. Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029;
3. Research Institute of Petroleum Exploration and Development, Tarim Oilfield Company, PetroChina, Korla, Xinjiang 841000;
4. Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083
Natural gases in the Ordovician of Tazhong area show very different geochemical characteristics. The gas in the middle and east part of the No.1 fault-slope zone is characterized by high gas dryness coefficient and δ13C isotope value, and is similar to the oil-cracking gas in the Cambrian in well TS1. The gas from the Cambrian was H2S-poor and charged into the petroleum pools of the middle and east part during the Himalayan orogeny, which is supported by the following lines of evidence: 1)the gas has methane δ13C value of about 3‰ higher than anticipated from the natural gas plot by Chung et al.(1988); 2)a linear relationship between gas dryness coefficient and its δ13C value; and 3)a inverse linear relationship between gas dryness and the concentration of H2S. These features suggest that there exists a mixing between H2S-poor, 13C-rich methane-dominated dry gas and H2S-rich, 13 C-poor wet gas. H2S in the Ordovician has δ34S values from 14‰～20‰, being significantly lighter than the H2S in the ZS1(33‰), supporting the different origin. The charge of the Cambrian dry gas may have resulted in increase in gas/oil ratios and wax content, gas dryness coefficient and positive shift in methane δ13C value.