摘要
Sixty-five natural gas samples were collected from 19 oil-gasfields in theQaidam basin, China. The chemical composition and carbon isotope values of the samples weremeasured, and the geochemical characteristics and origin of the natural gases were studied. Thegases can be divided into biogenic gases, sapropelic oil-type gases, mixed type oil-type gases,coal-type gases and mixed gas. The delta^(13)C_1 values of the biogenic gases are very small and theC_2^+ contents of them are very low, ranging from -68.2 per thousand to -61 .8 per thousand and0.06 percent to 0.20 percent respectively. They have heavy delta D and delta^(13)C_(CO_2), showing aCO_2 reduction pathway. They ,are distributed in the East depression region and derived from theQuaternary source rocks. The sapropelic oil-type gases have small delta^(13)C_2 values and highC_2^+ ranging from -36.6 per thousand to -28.6 per thousand and from 33.01 percent to 47.15 percentrespectively. The mixed type oil-type gases have delta^(13)C_2 values and C_2^+ contents varyingfrom -28.6 per thousand to -24.8 per thousand and from 4.81 percent to 26.06 percent respectively.Both sapropelic oil-type gases and mixed type oil-type gases are associated with oil-type oils,distributed in the West depression region and derived from the Tertiary saltwater lacustrinesapropelic source rocks and humic source rocks respectively. The delta^(13)C_2 values of thecoal-type gases are extremely high and the C_2^+ contents are very low, changing from -23.3 perthousand to -12.5 per thousand and from 0.06 percent to 18.07 percent respectively. The coal-typegases in the Nanbaxian gasfield and the Lenghu oil-gasfields in the North fault block belt arederived from the Middle Jurassic coal-measures source rocks, whereas those in the West depressionregion are derived from the Tertiary saltwater lacustrine humic source rocks. Compared with someother basins in China, the natural gases there have obviously heavier delta^(13)C due to the heavierdelta^(13)C of different types of kerogens of the Tertiary saltwater lacustrine source rocks in theWest depression region of the basin. The mixing of natural gases is common in the West depressionregion, but the mixed gases are formed by sapropelic oil-type gases, mixed type oil-type gases orcoal-type gases, respectively, of different levels of maturity. Most of the sapropelic oil-typegases and mixed type oil-type gases in the west part are thermally immature and low-mature, but thecoal-type gases in the West depression region and the North fault block belt are mature and high- toover-mature.
Sixty-five natural gas samples were collected from 19 oil-gasfields in theQaidam basin, China. The chemical composition and carbon isotope values of the samples weremeasured, and the geochemical characteristics and origin of the natural gases were studied. Thegases can be divided into biogenic gases, sapropelic oil-type gases, mixed type oil-type gases,coal-type gases and mixed gas. The delta^(13)C_1 values of the biogenic gases are very small and theC_2^+ contents of them are very low, ranging from -68.2 per thousand to -61 .8 per thousand and0.06 percent to 0.20 percent respectively. They have heavy delta D and delta^(13)C_(CO_2), showing aCO_2 reduction pathway. They ,are distributed in the East depression region and derived from theQuaternary source rocks. The sapropelic oil-type gases have small delta^(13)C_2 values and highC_2^+ ranging from -36.6 per thousand to -28.6 per thousand and from 33.01 percent to 47.15 percentrespectively. The mixed type oil-type gases have delta^(13)C_2 values and C_2^+ contents varyingfrom -28.6 per thousand to -24.8 per thousand and from 4.81 percent to 26.06 percent respectively.Both sapropelic oil-type gases and mixed type oil-type gases are associated with oil-type oils,distributed in the West depression region and derived from the Tertiary saltwater lacustrinesapropelic source rocks and humic source rocks respectively. The delta^(13)C_2 values of thecoal-type gases are extremely high and the C_2^+ contents are very low, changing from -23.3 perthousand to -12.5 per thousand and from 0.06 percent to 18.07 percent respectively. The coal-typegases in the Nanbaxian gasfield and the Lenghu oil-gasfields in the North fault block belt arederived from the Middle Jurassic coal-measures source rocks, whereas those in the West depressionregion are derived from the Tertiary saltwater lacustrine humic source rocks. Compared with someother basins in China, the natural gases there have obviously heavier delta^(13)C due to the heavierdelta^(13)C of different types of kerogens of the Tertiary saltwater lacustrine source rocks in theWest depression region of the basin. The mixing of natural gases is common in the West depressionregion, but the mixed gases are formed by sapropelic oil-type gases, mixed type oil-type gases orcoal-type gases, respectively, of different levels of maturity. Most of the sapropelic oil-typegases and mixed type oil-type gases in the west part are thermally immature and low-mature, but thecoal-type gases in the West depression region and the North fault block belt are mature and high- toover-mature.
作者
ZHANG Xiaobao, HU Yong, DUAN Yi, MA Liyuan, MENG Zifang, HE Peng,ZHOU Shixin and PENG DehuaState Key Laboratory of Gas Geochemistry, Lanzhou Institute of Geology, Chinese Academy ofSciences, Lanzhou, Gansu 730000
Research Institute of Petroleum Exploration and Development, Qinghai Petroleum Company,CNPC, Dunhuang, Gansu 736200
基金
supported by the Major Program(Grant No.KZCX2-111)of Knowledge Innovation in the Field of Resources and Environment funded by the Chinese Academy of Sciences
Chinese National Natural Science Foundation(Grant nos.49972051 and 40172053).
