Statistical Assessment of Sedimentation Cycle from Borehole Logs in Ib Valley Coalfield, Odisha

Pardeep Malik

Indian Institute of Technology, Roorkee, Uttarakhand - 247667

Dr. Vijay Prasad Dimri

CSIR - National Geophysical Research Institute, Hyderabad, Telangana - 500007

Abstract

In this project, a statistical approach is applied for the study of cyclicity in sedimentation of Barakar formation of Ib Valley coalfield, Odisha. Well-log data from ten boreholes in the study area was used for the identification of sedimentary litho-facies and their statistical analysis. On the basis of interpretation of density and gamma ray log readings, four litho-facies were identified and verified with the core data. The lithologies for analysis were grouped under four facies viz., coarse to medium grained sandstone, fine grained sandstone-shale, shale and coal. In order to study the cyclicity in a well, we carried out Markov chain and Entropy analysis of the litho-facies on the basis of a lithological transition matrix computed for the well bore. The Markov analysis indicates a fining upward cyclic succession: coarse to medium grained sandstone → fine grained sandstone-shale → shale → coal. Within a cyclic succession, the randomness in the occurrence of various litho-facies was judged based on the Entropy. The pre entropy and post entropy values for litho-facies were computed. Based on these computations, the normalized entropy and total system entropy were computed. All the Entropy plots were prepared and compared with the idealized entropy plots published in the literature. Interpretation was done and the observed cyclicity of the Barakar formation is Symmetric in nature which is explained by the rapid and frequent lateral shifting of channel course.

Keywords: Markov Chain Analysis, Chi-square test, Entropy Analysis, Barakar Formation, Ib Valley Coalfield