Geochemical Characteristics of Natural Gas and Hydrocarbon Charge History in the Western Qaidam Basin, Northwest China

Abstract

31 natural gases in the western Qaidam Basin of China were collected and analyzed for gas composition including light hydrocarbons (C5-C7) and carbon isotopic characteristics. Based on genetic type obtained from C1-C3 and C7 fractions, four types of gases are identified: oil-type gas, coal-type gas, biodegraded gas, and mixed gas. The oil-type gas is the predominant-type gas in the western Qaidam Basin; coal-type gas is mainly distributed in the Zhahaquan and Nanyishan fields; mixed gas is mainly in the Zhahaquan, Wunan, and Nanyishan fields; and biodegraded gas is mainly distributed in the Huatugou and Yuejinerhao fields. According to the empirical relationship between δ13C1 and the equivalent vitrinite reflectance (Ro, %) of source rock, the Ro values of gas range from 0.6% to 1.5%, with an average value of 0.9%. The generation temperatures of major reservoired hydrocarbons (GTMRH) calculated from the C7 components range from 115.6°C to 141.7°C, with an average value of 126.5°C. These two maturity indicators have relatively positive correlation and reveal that the maturity of gas increases from west to east in the southwestern Qaidam Basin. Moreover, combining GTMRH with the homogenous temperature of petroleum inclusions, it is inferred that major petroleum charge in the western Qaidam Basin mainly occurred during the late period of the Himalayan movement. Deep hydrocarbon fluid sources were found in the Shizigou, Yingdong, Zhahaquan, and Nanyishan fields; thus, the deep reservoirs of paleouplifts adjacent to the hydrocarbon-generating depressions are estimated as a favorable area for further exploration in the western Qaidam Basin.

Stable carbon isotopic compositions of individual light hydrocarbons in the C5–C7 range in natural gas from the Qaidam Basin, China

“…Genetic type of natural gas C 1 -C 3 and C 5 -C 7 fractions in natural gas may be dominated by different sources (Chen et al, 2020;Dai, 1993). Thus it is necessary to investigate the genetic type of gas based on these two fractions before studying the genesis of light hydrocarbons.…”

“…The procedure to obtain the molecular and stable carbon isotopic compositions of gas follows Chen et al (2020). The C 1 –C 4 hydrocarbons were examined by using a gas chromatograph (GC).…”

Microbial communities and their roles in the Cenozoic sulfurous oil reservoirs in the Southwestern Qaidam Basin, Western China

“…The carbon and hydrogen isotope tests of natural gas were conducted at the China Petroleum Exploration and Development Research Institute. Experiment details can be referred from previous studies 76 .…”

Light Hydrocarbon (C5–C7) Geochemistry in the Northern Margin of the Qaidam Basin, Northeastern Tibetan Plateau: Gas Mixing and Hydrocarbon Charge History

“…11 Dieckmann et al (2002) 55 proposed that the expulsion temperature calculated from C 7 light hydrocarbons can reflect the average temperature of hydrocarbon expulsion. Chen et al (2020) 58 proposed that the expulsion temperature could be regarded as the generation temperature of the major reservoired hydro-carbons (GTMRH). The GTMRH has a close relationship with homogeneous temperatures of petroleum inclusions and can reflect the major hydrocarbon charge in the western Qaidam Basin.…”