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7th Middle East Geosciences Conference and Exhibition
Manama, Bahrain
March 27-29, 2006
Analysis
on Sidewall Cores
1 SPEF, ENI E&P, Via Emilia 1, 20097 San Donato Milanese, Italy, phone: +39 02 520 62069, fax: +39 02 520 61820,
[email protected]
2 GEBA, ENI E&P, Via Emilia 1, 20097 San Donato Milanese, Italy
Detection of biodegraded oil at an early stage in exploration wells is quite important due to their poor characteristics with
respect to good quality, less viscous oils. Traditional logs, such as resistivity, porosity and gamma ray logs, show the
position of the reservoir and its hydrocarbon content but are unable to provide information regarding the hydrocarbon
quality, it being sometimes difficult even to differentiate between liquid and gas accumulations. During an acquisition of
wireline logs in an offshore exploration well
, an NMR
log
was run, aimed at integrating the traditional set of logs to better
define porosity, permeability and Swi. Because of 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, which using the traditional partitioning of the NMR T2 distribution indicate high volumes of clayand
capillary-bound fluids, were measured in oil-bearing layers within the reservoir. A detailed molecular study was
performed on the residual oil extracted from the sidewall cores and showed different levels of biodegradation in various
sections of the reservoir depending on both depth and proximity to the OWC. 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 cores are not available.