Key to the development of a deep water, cretaceous oilfield offshore Angola is a good appreciation of reservoir lithology, fluid type, porosity and permeability. The stacked reservoir systems have varying degrees of clastic and carbonate mixtures and are saturated with differing hydrocarbon types.
In this complex environment Logging-While-Drilling (LWD) Nuclear Magnetic Resonance (NMR) measurements were acquired simultaneously with LWD elemental capture spectroscopy, density, neutron and resistivity to enhance the understanding of reservoir fluids and lithology.
Under certain conditions, (NMR) logs have identified oil and gas on the basis of diffusion, these NMR only techniques require complex operating procedures and signal processing with a high magnetic field gradient NMR tool. An alternative, much simplified approach to evaluate fluid types is to combine multiple log measurements such as LWD NMR, density, neutron and resistivity.
A unique benefit of LWD measurements is their ability to measure reservoir fluid properties before significant drilling fluid invasion has occurred, they respond primarily to in-situ reservoir fluids and rock matrix. Essentially they are not compromised by drilling mud invasion, an important consideration when attempting to characterize reservoir fluids from log measurements.
Based on the above measurements, a simple and robust integrated technique will be shown to differentiate gas and light oil. In particular, the technique was applied to several field examples in Angola where the identification of gas, oil and condensate layers has been problematic. An easy to use fluid identification cross-plot will also be presented.