The study of 22 oils from sidewall cores taken at different depths in the Lower Lagunillas Member, well LSJ-AB, Tia Juana Field, Maracaibo Lake is presented, with the purpose of predicting the intervals that present the best crude oil quality. Differences were detected in the biodegradation levels of the studied samples, which are correlated with the depth at which the sidewall core was taken. The API gravity was considered for the oils from each sidewall core and it was found that toward the top of the sequence, the oils have an API gravity of 10.6-11.2degrees, while toward the base part of the sequence, the well produces extra heavy oils with an API gravity that varies between 8.2 and 8.7degrees. (C) 2002 Elsevier Science Ltd. All rights reserved.