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.
Highlights
The Qaidam Basin is the largest petroliferous sedimentary basin inside the Tibetan Plateau, northwest China, and significant amounts of oil have been produced in the western Qaidam Basin [1]
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 distributed in the Huatugou and Yuejinerhao fields
The gas composition of light hydrocarbons (C5-C7) and carbon isotopes was analyzed from 31 gases in the western Qaidam Basin
Summary
The Qaidam Basin is the largest petroliferous sedimentary basin inside the Tibetan Plateau, northwest China, and significant amounts of oil have been produced in the western Qaidam Basin [1]. Oil and gas exploration has made considerable progress in the Yingdong, Zhahaquan, and Yingxi fields in the western Qaidam Basin. Some newly produced oil has lower viscosity and density with a higher gas-oil ratio, suggesting that they may be derived from deeper strata. Little research has been conducted to investigate the genetic type and maturity of these newly produced gases. A systematic analysis of natural gas is necessary to deepen the recognition of gas origin and charge history in the western Qaidam Basin. The hydrocarbon components of natural gas mainly consist of C1-C5 compounds with trace C5+ light hydrocarbon compounds. The C5-C7 light hydrocarbons are the focus in current researches and this paper. Partial light hydrocarbons would dissolve in the gas phase because natural gas can be taken as the solvent and the volatility of light hydrocarbon
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