In early 80's, while reassessing some old gravel pack sands from a well, we noticed some sludge in the samples. The sludge had plugged formation and the gravel pack pores. The permeability was so low that the owner of the oil property thought of abandoning the well and consequently stopped all workover measures. The owner stated that there was no more oil left in the reservoir. This observation and the previous ones in early 70's, in addition to the laboratory studies conducted at a later date, suggested the existence of bacteria in the oil. In the current study we implemented a novel, sensitive technique for bacterial DNA fingerprinting; that is, denaturing gradient gel electrophoresis of PCR amplified 16S rRNA genes. This approach allowed us to identify and classify bacteria found in the oil. The research enabled us to evaluate the application of our technique to evaluate the origin of oil in the reservoirs. To prove the hypothesis that bacteria were present in oil, we requested an operating company to provide us with 6 samples taken from 2 separate wells, but drilled in the same reservoir or at least the operator believed both wells were drilled in the same reservoir. To characterize the oil and subsequently the reservoir, we implemented our procedure of fingerprinting bacteria DNA as follows: (1) DNA extraction from oil, (2) DNA concentration, (3) Making 0.7% and 1% Agarose gel, (4) Electrophoresis of total DNA isolated from oil, (5) PCR amplification of 16S rRNA gene fragment from DNA isolated from oil, (6) Running gel on the PCR product, (7) Running Denaturing Gradient Gel Electrophoresis (DGGE), and (8) Cycle Sequencing DNA. The result from DNA concentration using our technique proved the existence of the bacteria DNA in all crude oil samples, which was not possible to find previously. Using the governmental open source data from offshore Louisiana and the results of researchers work available in the literature, we conducted a detailed geological and petroleum engineering study of the field in our study area. Furthermore, in order to find petroleum engineering applications for our discovery of DNA in crude oil we classified the bacteria and found the origin of the crude oil produced from each well in the reservoir. Based on our results, we found that Well B drained oil from a grassland, lacustrine, fresh water environment, whereas Well A drained oil from a very shallow marine environment. From the results, we concluded that, these two wells are not drilled in the same reservoir, implying that there may be enough oil left in both reservoirs to be drained by one or more wells.