Oil & Gas Geology ›› 2025, Vol. 46 ›› Issue (2): 530-549.doi: 10.11743/ogg20250213

• Petroleum Geology • Previous Articles     Next Articles

Impacts of volcanic ash alteration on element anomalies in fine-grained mixed deposits and paleoenvironmental reconstruction: A case study of the 2nd member of the Permian Lucaogou Formation, Tiaohu-Malang Sag, Santanghu Basin

Yongshuai PAN1,2(), Bo LIU1,2(), Yizhuo YANG3, Tong QU4, Zhilong HUANG1,2,3, Xiongfei XU5   

  1. 1.State Key Laboratory of Continental Shale Oil,Northeast Petroleum University,Daqing,Heilongjiang 163712,China
    2.Key Laboratory of the Ministry of Education for Terrestrial Shale Oil and Gas Reservoir Formation and Efficient Development,Northeast Petroleum University,Daqing,Heilongjiang 163712,China
    3.State Key Laboratory of Petroleum Resources and Engineering,China University of Petroleum (Beijing),Beijing 102249,China
    4.Research Institute of Exploration and Development,Changqing Oilfield Company,PetroChina,Xi’an,Shaanxi 710018,China
    5.Research Institute of Exploration and Development,Tuha Oilfield Company,PetroChina,Hami,Xinjiang 839009,China
  • Received:2024-12-05 Revised:2025-02-26 Online:2025-04-30 Published:2025-04-27
  • Contact: Bo LIU E-mail:Panyongshuai@nepu.edu.cn;liubo@nepu.edu.cn

Abstract:

Volcanic ash input holds significant impacts on the reconstruction of sedimentary environments in saline lacustrine basins. Specifically, when abundant trace elements from volcanic ash enter lakes and are preserved in tandem with sediment, the element contents in lacustrine strata cannot accurately reflect the original sedimentary setting. Focusing on the 2nd member of the Permian Lucaogou Formation (P2l2) in the Tiaohu-Malang Sag within the Santanghu Basin, we investigate the mechanisms by which volcanic ash alteration affects element anomalies in fine-grained mixed deposits using major, trace, and rare earth element analyses, gas chromatography-mass spectrometry (GC-MS) of saturated hydrocarbons, whole-rock X-ray diffraction (XRD), total organic carbon (TOC) content determination, and thin section observation. Accordingly, the saline lacustrine sedimentary environment under the action of volcanic ash is reconstructed. The results indicate that the P2l2 consists of fine-grained tuffaceous materials and carbonates, with the total content of felsic and carbonate minerals reaching up to 95 % on average, suggesting gradational mixed sedimentation at facies margins in a broad sense. Under the influence of alteration such as early dissolution in water and mid-late devitrification and organic acid corrosion, substantial volcanic ash thermodynamically unstable released various high-abundance nutrient ions (including metal ions) into pore fluids, which significantly interfered with the elements including Ni, Co, S, P, and Ga in the fine-grained mixed rocks of the P2l2. As a result, these elements cannot accurately reflect the original sedimentary environment. Biomarker compounds, together with the analysis of major and trace element data after screening and correction, reveal a hot and arid climate during the deposition of the P2l2, featuring limited water supply to the lacustrine basin and extremely high salinity and low oxygen concentration of the lake. Volcanic activity played a significant role in regulating the sedimentary environment of the P2l2. The transition from intense to intermittent volcanic eruptions corresponded to an increasingly hot and arid climate, along with elevated water evaporation, water salinity, and oxygen concentration, leading to the deposition of various types of rock assemblages.

Key words: volcanic ash, saline lacustrine facies, alteration, element anomaly, sedimentary environment, Lucaogou Formation, Permian, Tiaohu-Malang Sag, Santanghu Basin

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