• A BIJAYALAXMI DEVI

      Articles written in Journal of Earth System Science

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

      N RESHMA DEVI Y RAGHUMANI SINGH MARK B ABBOTT A BIJAYALAXMI DEVI

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      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 $T_{\rm{max}}$ value is 428.16$^{\circ}$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. $T_{max}$ 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.

      $\bf{Highlights}$

      $\bullet$ Depositional environment and hydrocarbon source rock potential for the late Eocene Kopili shales of the Umphyrluh area, Jaintia Hills, Meghalaya were investigated.

      $\bullet$ Palynofacies were recovered by using palynological analysis indicating the status of the depositional environment of the Kopili Formation as a shallow marine environment by influx of terrestrial matter under a proximal dysoxic-anoxic condition.

      $\bullet$ Rock-Eval pyrolysis and Total Organic Carbon (TOC) analysis suggested the quantity, type, and thermal maturity of the associated organic matter.

      $\bullet$ The studied samples are predominantly gas prone (mostly Type III and Type IV) indicating their potentially for gaseous hydrocarbon.

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