Alan Z. Yu¹, G. Cole¹, G. Grubitz¹, F. Peel¹

(1) BHP Petroleum, Houston, TX

ABSTRACT: Bio-Degradation Index ( BDI ): A New Approach to Predict Bio-degradation Risk and Reservoir Fluid Quality Based on the Basin's Thermal and Hydrocarbon Charge Histories

Recent drilling results in the Gulf of Mexico and West Africa highlight the vital economic importance of hydrocarbon fluid quality and biodegradation, particularly in the deepwater areas where drilling and development costs are high, but pre-drill prediction of these factors remains a challenge.

Oil field data show a complex distribution of biodegraded oil occurrences in the deepwater areas. Some fields, in which the present-day reservoir temperature is as high as 80C, contain biodegraded oils; whereas in other fields, reservoirs which are cooler than 50C contain relatively fresh and unaltered oils.

Recent advances in basin modeling define rock permeability and migration timing from source to reservoir, allowing a new approach, Bio-Degradation Index (BDI), to be developed by BHP Petroleum, which predicts the biodegradation risk and fluid quality in reservoirs. It has been long recognized that temperature plays a key role in bio-degradation, as most biodegraded oils occur at temperatures of <80C. However, most studies have not integrated the burial and charge histories of the fields. Instead of using only the present day temperature, which can be misleading and usually overestimates bio-degradation, BDI considers the temperature history and residence time of hydrocarbons in the reservoir. Using the temperature versus residence time plot, the area below the critical temperature (Tc), above which bacterial activity is not significant, indicates the bio-degradation risk.

An empirical relationship between BDI and oil API gravity was developed based on dozens of oil discoveries worldwide. The very encouraging results have shown that BDI can be used as a tool for the pre-drill assessment for bio-degradation risk and reservoir fluid quality.

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