Palynology, palynofacies and organic geochemistry analysis of the late Eocene shale from Meghalaya, Northeast India

Abstract

Here the depositional environment and hydrocarbon source rock potential of the Kopili Formation is investigated using palynological analysis and Rock-Eval pyrolysis on samples from a borehole section (Borehole BUM14) collected at Umphyrluh area in the Jaintia Hills, Meghalaya. In these Kopili shales, amorphous organic matter is often associated with structural terrestrial organic matter, biodegraded organic matter, charcoal, black carbon debris, dinoflagellate cysts, and spores. The palynotaxa are mainly composed of dinoflagellate cysts comprising eight genera and twelve referable species. Based on the palynological data, the sediments of the study area were deposited in a shallow marine setting under oxygen deficient conditions in an environment that received a continuous terrestrial influx throughout the succession. Rock-Eval pyrolysis and Total Organic Carbon (TOC) analysis determine the quantity, type, and thermal maturity of the associated organic matter. TOC values range from 0.03 to 0.45 wt.% (averaging 0.28 wt.%) and the Genetic Potential (GP) and Hydrogen Index (HI) values vary from 0.04 to 0.24 mg HC/g rock and 22–100 mg HC/g TOC, respectively. These values imply that all the shale samples have very low TOC values (< 0.5%), S1, S2, and Hydrogen Index (HI) values. Although most of the samples are in a mature stage as the average Tmax value is 428.16°C and the Production Index (average 0.16) indicates a potential for oil generation, low Genetic Potential (S1 + S2) and TOC concentrations suggest there is limited potential for oil generation. The HI vs. OI plot and HI vs. Tmax plot show that most of the shale samples fall in the predominantly gas prone domain (mostly Type III and Type IV), because the organic matter is generally derived from a terrestrial source. Thus, the source rock potential for the Kopili shales of the Umphyrluh area is considered to be poor for gaseous hydrocarbons.