In some fields of the Tertiary Niger delta of Nigeria, heavy, asphaltic oil is found in shallow reservoirs and light, waxy oil is found in deeper reservoirs in the same field. Both oils appear to have had the same source. The change with depth from heavy to light oil is usually abrupt and occurs at a reservoir temperature between 150° and 180°F (66° and 82°C). In other areas similar degradation of oil has been ascribed to bacteria brought into the oil reservoir by invading meteoric water. In Nigeria, meteoric water is low in dissolved solids and high in bicarbonate, whereas connate water is saltier and contains more chloride. Samples of both types of oil and associated water from six fields were analyzed and compared with previously acquired analyses f om these fields. No clear relationship between meteoric water and degraded oil was found. Degraded oil may occur with either meteoric or connate water, and undegraded oil is sometimes found with meteoric water. This suggests extensive secondary migration of oil from one structure to another as well as deep penetration of meteoric water.