Inerpretation of Maturity Parameters Evaluated from Various Biomarker Ratios and Diamondoids Ratios for Oil/Condensates of Cauvery Basin, India

Sunil R. Mangotra and Jitendra Prasad
Regional Lab, ONGC, Chennai, India

Presence of various diamondoids reported in oils/condensates of Cauvery basin, indicates maturity near/ beyond peak oil generation stage. Maturities generally in the range of 0.7-0.8 VRo had been reported for oils/condensates of various subbasins on the basis of Methyl Phenanthrene index and other biomarkers. Maturity evaluated, for gases of Ramnad subbasin, on the basis of carbon isotopic data are in the range of 0.9-1.1 VRo. Higher maturities calculated on the basis of diamondoids indicate maturity in the range of 1.3-1.6 VRo for condensates of Ramnad subbasin. Higher maturities are also observed for oils/condensates of Kuthalam-Kali area, when compared to maturity evaluated from other biomarkers. GCMS finger prints m/z 191 and m/z 217 of oils/condensates of Cauvery Basin do not show increased presence of tricyclics, diasteranes and more of Ts as compared to Tm, which are indicators generally associated with maturity range more than 0.9 VRo. These two observations probably indicate, mixing of hydrocarbon charges of two different maturities, in the reservoirs. One hydrocarbon charge from a source rocks within oil window and other from a more mature gas generating source rock, explain the increased quantities of diamondoids in some of the oils/ condensates and also their GCMS fingerprints. Mixing of hydrocarbon charges of two different maturities is also indicated from the composition of gases and liquid hydrocarbons in Kuthalam-Kali area and carbon isotopic data of gases in Ramnad area. There seems to be mixing of hydrocarbon charges from source rock of different maturities in Cauvery Basin.

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