The Tertiary in the West Depression Region of the Qaidam Basin has the typical inland salty lacustrine deposits in China. 34 natural gas samples were collected from 13 oil fields in the West Depression Region in the basin, the chemical compositions and carbon isotopes of methane, ethane, propane, and butane were measured. According to the carbon isotope characteristics of natural gases in combination with geochemical characteristics of the source rocks and crude oils, the natural gases can be divided into sapropelic associated gas, mixed organic matter-derived associated gas, coal-derived gas and mixed gas. The carbon isotope characteristics, origin and distribution of the natural gases are considered to be related with the types and distribution of organic inputs in lake environments with different salinities. The lake salinity can regionally forecast the distribution of the different genetic types of natural gas. Compared with the different genetic types of natural gas from other Chinese basins, the carbon isotopes of the heavy hydrocarbons of the natural gas from the Tertiary salt lacustrine facies in the Qaidam Basin are extremely heavier. Therefore, this should be considered when natural gases are genetically classified in the basin.
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 Tertia
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
