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

ABSTRACT: Twenty natural gases from the northern margin of the Qaidam Basin were collected and examined for the composition and stable carbon isotopic characteristics of gas and light hydrocarbons (C5-C7). The results reveal that the carbon isotopes of iso-alkanes and cyclo-alkanes in light hydrocarbons are mainly controlled by the bioprecursors, whereas the carbon isotopes of n-alkanes and aromatics in light hydrocarbons are primarily influenced by the bioprecursors and maturity. Based on the genetic types obtained from C1-C3 and C5-C7 fractions, three types of gases are identified: coal-type gas, oil-type gas, and mixed gas. Coal-type gas dominates the northern margin of the Qaidam Basin, oil-type gas is mostly distributed in the Lenghu no. 3 field, and mixed gas is mainly developed in the Dongping and Mabei fields. According to the maturity obtained from δ13C1 and heptane and isoheptane ratios, the petroleum charge period is studied in combination with burial history and hydrocarbon generation history, and the result is roughly well matched with the research of homogeneous temperatures of petroleum inclusions. Furthermore, the generation temperature of the major reservoired hydrocarbons calculated from C7 light hydrocarbon compositions ranges from 125.0 to 135.9 °C, suggesting that a major petroleum charge event may occur primarily during the Shangganchaigou period (N1) in most fields. The deep paleo uplifts adjacent to the hydrocarbon-generating depressions at the margin of the basin and uplift zone in the inner basin are estimated as favorable areas for further exploration in the northern margin of the Qaidam Basin.

SUBMITTER: Chen K

PROVIDER: S-EPMC8655948 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The northeastern margin of the Tibetan Plateau (NE Tibetan Plateau) exhibits active geological structures and has experienced multiple strong earthquakes, with M ≥ 7, throughout history. Particularly noteworthy is the 1920 M81/2 earthquake in the Haiyuan region that occurred a century ago and is documented as one of the deadliest earthquakes. Consequently, analyzing seismic risks in the northeastern margin of the Tibetan Plateau holds significant importance. The b value, a crucial parameter for seismic activity, plays a pivotal role in seismic hazard analyses. This study calculates the spatial b values in this region based on earthquake catalogs since 1970. The study area encompasses several major active faults, and due to variations in b values across different fault types, traditional grid-search methods may introduce significant errors in calculating the spatial b value within complex fault systems. To address this, we employed the hierarchical space-time point-process (HIST-PPM) method proposed by Ogata. This method avoids partitioning earthquake samples, optimizes parameters using Akaike's Bayesian Information Criterion (ABIC) with entropy maximization, and theoretically allows for a higher spatial resolution and more accurate b value calculations. The results indicate a high spatial heterogeneity in b values within the study area. The northwestern and southeastern regions exhibit higher b values. Along the Haiyuan fault zone, the central rupture zone of the Haiyuan earthquake has relatively higher b values than other regions of this fault zone, which is possibly related to the sufficient release of stress during the main rupture of the Haiyuan earthquake. The b values vary from high in the west to low in the east along the Zhongwei fault. On the West Qinling fault zone, the epicenter of the recent Minxian-Zhangxian earthquake is associated with a low b value. In general, regions with low b values correspond well to areas with moderate-strong seismic events in the past 50 years. The spatial differences in b values may reflect variances in seismic hazards among fault zones and regions within the same fault zone.

Dataset Information

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

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