Geochemical characterization of shales of the Eocene Disang Group, Kohima Syncline, India: Inferences to hydrocarbon potential and depositional environment

Abstract

The Kohima Syncline exposes the Eocene Upper Disang Formation in an extensive region that is composed of grey shales with subordinate sandstone and siltstone alterations. The shales contain low to fair amount of organic matters represented by Type IV/Type III terrestrial kerogens. The Biomarkers indicate presence of terrestrial angiosperms and microbially reworked organic matters with some marine phytoplankton. Rock-Eval maturity parameters indicate maturity within a wide range between immature to post mature wet gas window, while the biomarkers show maturity between immature to mid mature. The biomarkers indicate that the Disang shales deposited in an oxic deltaic-terrigenous environment. The trace elements characterize an oxic to suboxic warm and humid depositional environment. Few trace elements point to deposition in a low salinity continental rather than a marine environment. Thus, several biomarker and trace element parameters show that a continental milieu was already established during the deposition of the Upper Disang sediments. It indicates that either the Disang basin, that evolved as a marine basin, suffered rapid shallowing during the deposition of the Upper Disang sediments, or the Lower and Upper Disangs were coeval and Lower Disangs represented deep marine facies while the Upper Disangs represented peripheral shallow marine deltaic facies. Maturity parameters like Tmax and, sterane and hopane isomerization ratios indicate that the organic matters of the Disang Group were affected by sudden short duration thermal events followed by cooling. This is possible in an active tectonic region like the Kohima Syncline that bears many evidences of thrusting and folding.