Earth Science Frontiers ›› 2025, Vol. 32 ›› Issue (5): 131-149.DOI: 10.13745/j.esf.sf.2025.2.3

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Sedimentary units composition and evolution characteristics of the slope fan system of the Upper Ordovician in the Shunnan area, Tarim Basin

WU Pengfei1,2(), WU Jun1,2,*(), FAN Tailiang1,2, LIU Qian1,2, ZHANG Weiguo1,2, YANG Suju3, XIA Yongtao3, LAN Mingjie3   

  1. 1. School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China
    2. Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Enrichment Mechanism, Ministry of Education, China University of Geosciences (Beijing), Beijing 100083, China
    3. Exploration and Production Research Institute, Sinopec Northwest Oil Field Company, Ürümqi 830011, China
  • Received:2024-12-20 Revised:2025-02-18 Online:2025-09-25 Published:2025-10-14
  • Contact: WU Jun

Abstract:

Slope fan system developed in the Upper Ordovician in the Shunnan area of the Tarim Basin. Multiple wells have detected gas anomalies in these strata, indicating its potential value for oil and gas exploration. However, the characteristics of the slope fan system are poorly understood, hindering exploration progress. This study utilized high-precision 3D seismic and logging data from the Shunnan area to clarify the composition of the sedimentary units within the slope fan system and to reveal its evolutionary filling process and controlling factors. The results are as follows: (1) The slope fan system in the Upper Ordovician in the Shunnan area comprises three sedimentary units: channels, sandy lobes, and muddy lobes. These units exhibit significant differences in their seismic and log responses. The channels correspond to “U” and “V” shaped down-cut filling seismic reflections with medium-strong amplitude, medium-low frequency, and good continuity. They are dominated by siltstone, and the GR and AC curves are box-shaped. The sandy lobes correspond to laminar or mound-shaped seismic reflections with similar seismic attributes. They are dominated by siltstone and silty mudstone, with the GR curve showing medium-high values and the RD curve showing medium-low values, both fluctuating significantly. Muddy lobes also correspond to laminar or mound-shaped reflections. They are dominated by mudstone and silty mudstone, with the GR curve showing high values, the RD curve showing low values, and both fluctuating slightly. (2) We identified a total of 8 sandy lobes, 2 muddy lobes, and numerous channels in the slope fan system in the study area. Based on cross-cutting relationships, the sedimentary filling process is divided into three stages: No. 1-3 Sandy lobes and channels were developed in the early stage; No. I and II Muddy lobes, No. 4 sandy lobe, and channels were developed in the middle stage; and No. 5-8 Sandy lobes and channels were developed in the late stage. (3) The evolutionary filling process of the slope fan system was controlled by tectonic activity, sea-level change, and sediment supply. Tectonic activity provided the topographic relief and source of sediments for the system. Sea-level change controlled sediment supply and the location of sedimentary units. Source rock composition and sediment supply determined the types and scales of the sedimentary units. (4) The Upper Ordovician in the Shunnan area has a good source-reservoir-seal assemblage. Furthermore, the strike-slip faults enhance reservoir space and facilitate hydrocarbon migration between source and reservoir rocks, indicating that favorable zones for hydrocarbon accumulation are located near the strike-slip fault zones.

Key words: sedimentary units, controlling factors of sedimentary evolution, slope fan system, the Upper Ordovician, Shunnan area in the Tarim Basin

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