MOLECULAR SIGNATURES OF PALEOGENE BIOTAS FROM CRUDE OILS AND ROCKS, EASTERN INDIA

The Paleogene period has been pivotal in bringing major evolutionary changes in terrestrial and marine biota. This time was crucial in developing worldwide petroleum systems. The Assam-Arakan Basin represents a polyhistory basin, which records organic imprints in Paleogene rocks and crude oils, within a tropical fluvio-deltaic setting. Rock Eval Pyrolysis of organic rich rocks from Eocene, Langpar formation and Oligocene, Barail formation from Meghalaya and Assam, India, respectively, contain mixed type II/type III kerogen with significant yield of liquid and gaseous hydrocarbons upon maturation. Biomarker characterization of crude oils and rocks using GC-MS reveals the dominance of C₂₉ steranes which indicates terrestrial source of the organic matter. Broadly, the biomarker assemblages from early-Eocene sediments from Meghalaya, constitute aliphatic tetracyclic diterpenoid markers such as phyllocladane, kaurane, pimarane, beyerane, rimuane and aromatic diterpenoids like retene, bisnorsimonellite which are indicative of conifer dominant vegetation coming from Podocarpaceae and Araucariaceae families. However, at few localities, the Eocene sediments contain biomarkers such as bicadinanes, oleananes and cadalene which are prominent markers of angiosperms. Bicadinanes are important constituent in SE Asian fluvio-deltaic oils which are produced by the catagenetic depolymerisation of cadalene coming from resins of Dipterocarpaceae family. The Oligocene paleovegetation from Assam is entirely dominated by angiosperms, with notable contribution from Dipterocarpaceae and no signatures of gymnosperms were recorded. Molecular signatures of crude oils from Eocene and Oligocene reservoirs reveal high abundance of angiosperm markers. The presence of rimuane, first to be reported from Eocene oils suggests some gymnosperm contribution in the oils. Difference in ratios of bicadinane, oleanane and rearranged oleanane indices indicate that both the oils were generated from different organo-facies. The abundance and dominance of angiosperms, post-Eocene highlights the Eocene-Oligocene transition to be a possible vegetative change from gymnosperms to angiosperms.