Integrating Markov Chain Analysis, Cyclostratigraphy and Sequence Stratigraphy to Decipher the Oligo‐Miocene Asmari Formation's Evolution in the Zagros Basin

ABSTRACT

The findings of this research are intended to help with reconstruction of paleoenvironmental and paleogeographic conditions of the Asmari Formation in the Oligo‐Miocene (Chattian‐Burdigalian), a siliciclastic‐carbonate reservoir in the Zagros Basin. The study utilises a multidisciplinary approach, incorporating statistical methods and cyclostratigraphy analysis. Quantitative results from quasi‐independence Markov chain analysis, combined with the Entropy model, suggest that the lithofacies types of the Asmari Formation exhibit a Markovian nature and represent symmetrical shallowing upward cyclicity. These cycles occurred on an inner, middle and outer ramp, forming a homoclinal carbonate ramp. Through cyclostratigraphy analysis of petrophysical logs, the study identified interactions among Earth's orbital parameters (eccentricity, obliquity, precession) within the Asmari Formation. Using a floating astronomical time scale, the calibrated ages indicate that the preserved stratigraphic record of the Asmari Formation in the studied wells spans approximately 4.05 to 5.26 Myr. Compared with the standard Chattian‐Burdigalian timescale, this shortened duration indicates that the preserved Asmari succession represents an incomplete sedimentary record. The tracking of sea‐level patterns by INPEFA led to the detection of four to five third‐order sequences, supported by DYNOT median values. Obliquity evolution has been recorded in the Asmari Formation, with ~1.2 Myr cycles modulating cyclicities. These cycles were one of the main factors conducive to the development of third‐order sequences. Sedimentation rate estimates, obtained through the integration of eCOCO and SM methods indicated the highest rate during the Chattian stage and the lowest in the Burdigalian. The study suggests that sediments in the Asmari successions were influenced by astronomically forced climate changes. However, the paleogeographic events related to the closuring of the Neo‐Tethys Ocean were another inducing factor, especially in the Burdigalian.