Conference Paper

The significance of fine-grained gravity-flow deposits in the Late Triassic Yanchang Formation (Ordos Basin) for unconventional petroleum exploration

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Abstract

Ever more sedimentological research focuses on the transport and depositional processes of fine-grained sediments, and more specifically of fine-grained gravity-flow deposits. The main reason is the ongoing exploration for shale oil and gas. Whereas the exploration for hydrocarbons was directed for a century towards sediments with a high porosity and permeability (particularly limestones and sandstones), it has become clear in the past decade that fine-grained sediments can also contain significant and economically exploitable volumes of hydrocarbons. The relatively recent focus on fine-grained gravity flows and their deposits implies that still much less is known about their characteristics than about those of, for instance, sandstones and limestones. The complexity and fluctuations of the transport and depositional conditions of fine-grained gravity flows are so large that it is commonly difficult to interpret the genesis of sediments precisely and reliably. This implies that the relatively new research field of fine-grained gravity flows and their deposits still needs new field data, experimental results and theoretical modelling. An interesting case study 1 concerns the Triassic Yanchang Formation in the Ordos Basin, which is the most important oil producing unit in China. The formation, which consists of 1000-1300 m of fluvial, deltaic and lacustrine sediments, contains abundant fine-grained gravity-flow deposits. The most important types of gravity-induced processes that developed on the steep margins of the lacustrine basin were slumps, debris flows, and turbidity currents. Hyperpycnal flows, resulting from sediment-laden fluvial floods also contributed significantly. These different types of gravity flow could transform into one another, and also induce other gravity flows. The Yanchang Formation houses huge shale-gas resources. Even if only 50% of the geologically probable reserves of the gas can be recovered from the shale section of the YC7 oil member, this would amount to 1.1506 × 10 12 m 3. This indicates the great potential for unconventional oil and gas. More detailed studies on unconventional hydrocarbons and on fine-grained sediments will show the vital significance of more research into fine-grained gravity-flow deposits.

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  • A Fan
  • R Yang
  • A J Van Loon
  • Y Wei
  • Z Han
  • C Zavala
A. Fan, R. Yang, A.J. van Loon, Y. Wei, Z. Han, C. Zavala, Journal of Asian Earth Sciences, 2018, in press.