Nimmi Singh¹, S P Singh², Mamta Bisht³, A K Jain³, K C Koshel³, Kuldeep Chandra⁴

(1) KDMIPE, ONGC, DEHRADUN, Dehradun, India
(2) ONGC, DEHRADUN, INDIA, Dehradun, India
(3) ONGC
(4) Oil & Natural Gas Corporation Ltd, Dehradun, India

ABSTRACT: Geochemical Characterization And Bacterial Alteration Of Petroleum In Nagapattinam Sub-Basin Of Cauvery Basin

Biomarkers and their ratios have been studied extensively worldwide for crude oil characterization and genetic correlation. Various biomarker ratios based on the distribution of normal alkanes/isoalkanes and hopanes/steranes cycloalkanes and carbon isotopic composition of oils in Nagapattinam sub-basin have been calculated to assess the source organics, environment of deposition and maturity. Depletion of n-alkanes and presence of H2S indicated involvement of secondary processes in shallow reservoirs. To understand the process of petroleum alterations and role of native bacteria in these reservoirs, microbial analysis of co-produced waters and biodegradation studies were conducted. The oils are characterized by low asphaltene, medium density, low pour point and are mostly paraffinic in nature. GC & GCMS based parameters and stable carbon isotopic data indicate that the oils are genetically correlatable, derived from similar source organics (mixed input) and deposited under mildly oxic conditions irrespective of their age and formation. Data also confirm moderate maturity level and mild biodegradation in some Paleocene oils from Kovilkallapal and Kamlapuram structure and Oligocene oils from Narimanam structure. Analysis of co-produced waters indicates presence of thermophillic and halophillic sulfate-reducing bacteria and Pseudomonas and Bacillus species of aerobic bacteria. Reservoir conditions are also conducive for microbial activity. In-vitro biodegradation studies conducted on Eocene oil by deploying Pseudomonas species isolated from associated water of Oligocene reservoir of the same well and Rhodococcus rhodochrous confirm that isolated bacteria are potent hydrocarbon degraders. Native bacteria are responsible for alteration of original sweet high gravity crude to sour and biodegraded crude in shallow reservoirs. An integrated approach of bulk and specific parameters has made these studies more specific and definitive in nature.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado