Exploration and development experience show that there is obvious oil gravity difference between the southern part and northern part of the “M1”reservoir in the Fanny oil field in the slope of the Oriente Basin, Ecuador. The American Petroleum Institute Gravity (API) values of oils in the northern part are higher than the one in the southern part of the Fanny oil field, with the values of 20o and 10.0o–13.0o, respectively. So the primary purpose of this study was to analyze the heavy oil characteristics of biodegradation and the oil–oil correlation according to the biomarker data and the δ13C signature of oil samples from T block. The results of the hydrocarbon gas chromatography fingermark and the inversion attribute characteristics indicated that there are fluid compartments between the “M1” reservoir of Fanny south. Finally, the models of oil–gas accumulation under the control of multiple-activities of complicated fault systems, as well as the origin of heavy oil, are contended. The early stage oils from the western part of the basin were biodegraded heavily in varying degrees in the whole basin, and the later stage oils were derived from the southern part in a large scale and were mature and lighter. Generally, oil mixing is the primary control of net oil properties, such as API gravity in Oriente Basin. We therefore predicted that the API gravity variation of oil pools radically depends on the injection amount of the later stage oil. Because of the shale barrier in the “M1”reservoir of Fanny south, the later stage hydrocarbon could not pass through and contribute to increase the oil API value.