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ABSTRACT: Studies on stable carbon isotope composition of crude oils from Upper Assam shelf, Eastern India

Datta, G. C., R. Sharma, A.K. Mittal, K.N. Misra, and Kuldeep Chandra , ONGC, Dehradun, India

The geochemistry of 32 oil samples from different structures and pay zones from Upper Assam shelf was evaluated through gas chromatographic and stable carbon isotopic analysis of different crude oil fractions. Results show that all these oils are derived predominantly from terrigenous source organics of coaly nature deposited in oxic to sub-oxic depositional environment. Studies on distillate yields, NSO and asphaltene contents vis-a- vis API gravity suggest that the basal sandstones and basement oils are paraffinic-naphthenic type whereas the Tipam and Barail groups are mainly naphthenic-aromatic type. On the basis of isotopic ratios, the oils of Upper Assam shelf can be divided into two groups i.e. Tipam and Barail (Miocene to Oligocene) oils in one group and Basal sandstone (Paleocene), basement (Pre-Tertiary) in another group. These conclusions are supported by Sofer plots and Stahl type curves of stable carbon isotopic composition of saturate and aromatics of the oils. These plots also separate isotopically heavier Tipam (TS-1 & TS-2) and Barail oils from the lighter Tipam (TS-5 & TS-6) and Barail oils. Additionally, the Sofer plot also provides clear evidence of the predominantly terrigenous origin of these oils and their waxy character.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia