Early Detection Of Biodegraded Oil Using NMR Downhole Logs And Hydrocarbon GC-MS Analysis On Sidewall Cores: An Offshore West Africa Case History

Biodegraded oil detection at an early stage in exploration wells is quite important, due to the completely different characteristics of these oils with respect to good quality and less viscous oils. Traditional logs, such as Resistivity, Porosity and Gamma Ray, are only able to show the presence of the reservoir and its content in hydrocarbons but are unable to give any information regarding the quality of the hydrocarbon, it being sometimes difficult even to differentiate between liquid and gas hydrocarbon accumulations. During an acquisition of wireline logs in an exploration well in offshore West Africa, an NMR log was run, aimed at integrating the traditional set of logs to better define porosity, permeability and Sw irreducible. Due to the lithological complexity of the reservoir, sidewall cores were cut and petrophysical values such as porosity and permeability were measured. The presence of high viscosity biodegraded oil accumulations was suspected early on due to the "peculiar" signature of the NMR log in the hydrocarbon-bearing rocks. Very low mean T2 values that, using the traditional partitioning of the NMR T2 distribution showed anomalously high amounts of clay bound and capillary bound fluids, were detected in different layers within the reservoir section. A particular study was performed on the residual oil extracted from the sidewall cores showing different levels of biodegradation in different sections of the reservoir. The good relationship between the mean T2 value from the NMR log and the biodegradation level found through GC-MS analysis gave us confidence in NMR logs for early detection of these kinds of oils in reservoirs where no cores are extracted