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AAPG Bulletin, Preliminary version published online Ahead of Print 18 December 2023.

Copyright © 2023. The American Association of Petroleum Geologists. All rights reserved.


Multi-phase pools caused by gas invasion in deep Ordovician carbonates from the Tazhong area, Tarim Basin, China

Zhiyao Zhang11 , Yijie Zhang2 , Guangyou Zhu2 , Jianfa Han3 , and Linxian Chi2

1 School of Earth Resources, China University of Geosciences, Wuhan 430074, China
2 Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China
3 Research Institute of Petroleum Exploration and Development, Tarim Oilfield Company, PetroChina, Korla 841000, China

Ahead of Print Abstract

Unraveling the charge histories of pools with complex petroleum fluid phases is crucial for effective exploration and fluid prediction. Oil and gas samples from multi-phase pools in the Tazhong area of the Tarim Basin, China, were analyzed using complementary geochemical (e.g., gas chromatography (GC), two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC×GC-TOFMS), compound-specific carbon isotope analysis (CSCIA), and pyrolysis simulations) and geological data to better understand their origins and spatial distribution. The integration of these data suggests that the petroleum in these multi-phase pools was significantly impacted by various secondary geochemical processes, including oil cracking, thermochemical sulfate reduction (TSR), and gas invasion. Oil and gas in deep Cambrian pools were altered by oil cracking and TSR due to high temperatures of over 170 °C (320 °F) at depths of over 8,500 m (27,900 ft), leading to the generation of secondary products, including diamondoids, organosulfur compounds (OSCs), and TSR-altered H2S-rich cracking gases. This deep Cambrian gas, with diamondoids and OSCs dissolved in the vapor phase, migrated upward through strike-slip faults and invaded previously charged oil pools in Ordovician carbonates, changing the reservoir fluid characteristics and fluid phases. Thus, condensates were formed owing to the introduction of excessive deep gas into the primary oil. The amount of gas invasion decreased with increasing distance from the strike-slip faults, thus forming multi-phase pools with a spatial distribution pattern. Oil pools near strike-slip faults are more affected by gas invasion than weakly altered volatile oil pools and unaltered oil pools with greater distances away. The oil pools near the faults form condensate pools that show enrichment of H2S and carbon isotopic fractionation in C2-C4 gas components. This study provides new insights into the causal mechanism and distribution of multi-phase pools in superdeep strata and has great potential for petroleum exploration in deeply buried Ordovician carbonates in the Tarim Basin.

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Please cite this AAPG Bulletin Ahead of Print article as:

Zhiyao Zhang , Yijie Zhang , Guangyou Zhu , Jianfa Han , Linxian Chi: Multi-phase pools caused by gas invasion in deep Ordovician carbonates from the Tazhong area, Tarim Basin, China, (in press; preliminary version published online Ahead of Print 18 December 2023: AAPG Bulletin, DOI:10.1306/12212318282.