The early identification of biodegraded oil occurrence and distribution during exploration and appraisal is important, because increasing levels of biodegradation heavily impact on the quality of the oil and consequently on its value and producibility, due to lower API gravity and higher viscosity.

The required conditions for biodegradation occurrence are the presence of microorganisms, substrate (mainly hydrocarbons), nutrients and an oxidizing agent, while the timing of hydrocarbon charge and the post-charge temperature history of the reservoir can have major effects. The optimum conditions occur often at the oil-water contact, but pre-drill predictions are difficult.

Common geochemical markers of oil biodegradation are the relative proportions of saturate aliphatic isoprenoids (pristane and phytane) and chromatographically closely eluting n-alkanes (n-C17, n-C18). Indeed, since the straight chain n-alkanes are typically attacked by bacteria before branched saturates, the ratios Pristane/n-C17 and Phytane/n-C18 can be used as indicators of biodegradation, with higher ratios associated with higher biodegradation.

On this basis, a convenient solution to characterize oil quality is to analyse reservoir rock samples through Thermal Extraction Gas Chromatography (TE-GC). This technology can be employed directly at the well site, with the advantage of providing quasi-real time information on fluid quality along the whole reservoir profile in a cost-effective manner. This type of information represents a high value for quick decision making on well completion solutions, to exclude intervals showing the onset of heavy biodegradation from production, as well as for planning future development strategies.

The present case history deals with the on-site application of TE-GC to the analysis of cores and cuttings from the Fatehgarh Formation, in the Bhagyam field of Barmer Basin (India), where a zone with variable levels of biodegradation extends into the oil leg. TE-GC analysis results, confirmed by those obtained in the laboratory with Gas Chromatography Mass Spectrometry (GC-MS), prove the value of the described approach. To conclude this study, some samples affected by severe mud contamination or heavy biodegradation were successfully characterized by integrating standard field services with more selective GC-MS laboratory analyses.

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