Assessment of sedimentation cycle and depositional history of coal bearing Barakar Formation in Mahanadi valley basin, India: A statistical approach

Abstract

The Mahanadi Basin of the Gondwana age, situated on the eastern coast of India, largely remained unexplored for its hydrocarbon potential. It is widely accepted that the deposition of Gondwana sediments in India was 'fluvial' until recently when some authors produced geochemical evidence for a marine deposition. Many researchers used statistical approaches and demonstrated that the 'entropy of the deposition diagram' computed for lower Gondwana sediments fits into the fluvial deposition category. Markov-Chain Analysis, Chi-square test, and Entropy method are commonly used statistical methods to understand the cyclicity of the sedimentation cycle and their depositional history. Thus in this paper, we first review these methods and then apply them to study the cyclicity in the sedimentation of Coal/CBM bearing Barakar Formation of Mahanadi Valley basin, India, using geophysical well-logs, core data, and the lithofacies. In this study, we observe that the Markov-chain analysis indicates a finning upward cyclic succession in the Barakar Formation. We also computed the entropy diagram for the formation, which was compared with the standard entropy diagram available in the literature. Our study confirms symmetric cyclic, however, probabilistically disordered sedimentation of the Barakar Formation of the Mahanadi Valley basin.Nevertheless, the results of the Entropy study indicate the possibility of both fluvial and shallow-marine deposition. The gamma-ray signatures from geophysical well logs, although non-uniquely, also suggest shallow marine sedimentary facies in an overall prograding deltaic setting. We, therefore, discuss the importance of re‐examining the widely accepted "continental fluvial" model for coal‐bearing Barakar Formation, which may be "shallow marine‐influenced," too, by using geological and geochemical methods, in the light of organic matter present in the deeper formations.