Geochemical characteristics and origin of natural gases in the Qaidam Basin, China

Sixty-five natural gas samples were collected from 19 oil-gasfields in the Qaidam basin, China. The chemical composition and carbon isotope values of the samples were measured, and the geochemical characteristics and origin of the natural gases were studied. The gases 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 the C-2(+) contents of them are very low, ranging from -68.2parts per thousand to -61.8parts per thousand and 0.06% to 0.20% respectively. They have heavy deltaD and delta(13)C(CO2), showing a CO2 reduction pathway. They are distributed in the East depression region and derived from the Quaternary source rocks. The sapropelic oil-type gases have small delta(13)C(2) values and high C-2(+) ranging from -36.6parts per thousand to -28.6parts per thousand and from 33.01% to 47.15% respectively. The mixed type oil-type gases have delta(13)C(2) values and C-2(+) contents varying from -28.6parts per thousand to -24.8parts per thousand and from 4.81% to 26.06% 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 lacustrine sapropelic source rocks and humic source rocks respectively. The delta(13)C(2) values of the coal-type gases are extremely high and the C-2(+) contents are very low, changing from -23.3parts per thousand to -12.5parts per thousand and from 0.06% to 18.07% respectively. The coal-type gases in the Nanbaxian gasfield and the Lenghu oil-gasfields in the North fault block belt are derived from the Middle Jurassic coal-measures source rocks, whereas those in the West depression region are derived from the Tertiary saltwater lacustrine humic source rocks. Compared with some other basins in China, the natural gases there have obviously heavier delta(13)C due to the heavier delta(13)C of different types of kerogens of the Tertiary saltwater lacustrine source rocks in the West depression region of the basin. The mixing of natural gases is common in the West depression region, but the mixed gases are formed by sapropelic oil-type gases, mixed type oil-type gases or coal-type gases, respectively, of different levels of maturity. Most of the sapropelic oil-type gases and mixed type oil-type gases in the west part are thermally immature and low-mature, but the coal-type gases in the West depression region and the North fault block belt are mature and high- to over-mature.