Cyclic Sedimentation of the Amiran Formation for Subsurface Characterization in the Zagros fold–thrust belt (Application of Markov Chain Method in depositional sequences interpretation)

Abstract

This paper proposes an extension of Markov chain model to characterize of geological formations and to make conditioning on any number of well data possible. In this study, lithofacies cycles of the Amiran Formation at type and Pirshamsedin sections are examined by Markov method and entropy analysis as well as field observations, and depositional environment has interpreted by proposed ideal sequence. To do this, deposits of this formation have divided into six lithofacies associations including olive green shale, channel conglomerate, alternation of shale and sandstone with ripple and parallel lamination, alternation shale and sandstone, fine to coarse grained sandstone and conglomerate with chert pebbles. The chain statistics reveals a fining-upward cyclical order of superposition throughout the Raniganj sequence. Markov chain analysis shows that Amiran Formation's sediments are formed from coarsening upward cycles. A complete cycle from base to top consists of shale with ripple and laminated sandstone interbeds, shale, conglomerate with chert pebbles and channel conglomerate. The entropy analysis also shows coarsening upward cycles are asymmetrical. Ideal sequence obtained from the Markov's method is similar to Bouma model that reveals sedimentation is done by deep sea turbidity currents in lower to upper part of sub marine fans. The deposits of debris flows and high density turbidity currents are also interpreted as Channel and inter-channel deposits facies.

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