136-9 Study on Hydrocarbon Generation Characteristics of Organic-Rich Tuff and Tuffaceous Shale in the Chang 7 Member of the Yanchang Formation, Ordos Basin

Session: Micro-Nano Scale Pore-Fracture Architecture and Fluid Dynamics in Shale and Coal Reservoirs

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Abstract:

In the Yanchang Formation's Chang 7 Member of the Ordos Basin, ancient volcanic eruptions were frequent, driven by the continuous compressive collision of the Qinling Orogenic Belt to the south. These eruptions produced large amounts of volcanic ash, which entered the Chang 7 Member lake basin either through aerial deposition or water transport, forming extensive tuff or tuffaceous laminae. Previous studies have conducted preliminary research on the thick tuff layers in the Chang 7 Member from the perspectives of petrology, genesis, and sedimentation, suggesting that tuff could serve as a target for future unconventional oil and gas exploration. The  R_o of the Chang 7 Member primarily ranges from 0.5% to 1.3%, with most developed tuff exhibiting massive or laminar structures. The OM content in the tuff typically ranges from 0.2% to 5%. However, the hydrocarbon generation characteristics of organic-rich tuff and laminated tuffaceous shale during thermal evolution remain unclear.

This study focuses on the Chang 7 Member of the Yanchang Formation in the Ordos Basin. Using samples of massive organic-rich tuff and laminated tuffaceous shale from Well Z40, we conducted geochemical, thin-section, and hydrocarbon generation pyrolysis experiments. A systematic analysis was performed on the oil and gas generation components, followed by a comparison of hydrocarbon yield variations, differences in hydrocarbon generation intensity, and hydrocarbon expulsion efficiency.

The study demonstrates that massive tuff exhibits significantly higher expulsion efficiency than laminated tuffaceous shale, with earlier expulsion onset and hydrocarbon contributions from both kerogen and heavy oil components. Thermal evolution analysis shows tuff achieves higher organic matter maturity under identical conditions, while in laminated shale, expulsion efficiency correlates positively with TOC content. Massive organic-rich tuff exhibits significantly higher hydrocarbon expulsion efficiency than laminated tuffaceous shale due to its high volcanic glass content, low clay mineral content, and dispersed organic matter distribution, which create highly efficient migration pathways. In laminated tuffaceous shale, organic matter is stratified and tightly bound to clay minerals, requiring higher thermal maturity (R_o>0.6%) and elevated TOC content to form a continuous organic network that enhances expulsion efficiency. The study reveals that the key factors controlling the differences in hydrocarbon expulsion efficiency between the Chang 7 Member's organic-rich tuff and shale are pore structure, organic matter distribution, and thermal maturity, providing critical insights for unconventional shale oil exploration and development.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8652

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