The high prevalence of thin sand laminations in the Malay Basin is generally established and has the potential to unlock large resource volumes, not previously considered significant. Laminated sand intervals can sometimes have challenging fluid interpretation issues because of the masking of hydrocarbon response on resistivity logs. It is not uncommon to miss-out entirely on potentially producible hydrocarbon zones because of a failure to properly evaluate these laminated sands.
Fluid sampling may provide information on hydrocarbon types but there still remains the difficulty of establishing the interval limits of the hydrocarbon zones. The problem becomes more complicated when wells are drilled with oil based muds which has near similar response with the light formation oils in the Malay basin. In many cases, hydrocarbon zones are basically assumed to be in virgin state at the time of discovery, with single fluid contacts defining the reservoir fluid distribution. In fact, fluid sampling depths are often rationalised to one (1) or two (2) sample depths once the presence of hydrocarbon has been established. This simplistic assumption changes with turbidite systems, where laminated sends tend to compartmentalise gross reservoir sections with little indications on conventional logs.
In our case study, Nuclear Magnetic Resonance (NMR) 2D diffusion maps have been used to confront some of the fluid interpretation challenges. The NMR diffusion data acquired at selected station-stop depths were compared with results from the In-situ Fluid Analyzer (IFA) tool for defining distinct cluster patterns of different fluid types.
The results of the fluid interpretation were not only informative on the type of formation fluids but provided a commonly overlooked observation of free water occurrence within gross hydrocarbon intervals. This presence of free water within hydrocarbon sand package raises the possibility of postaccumulation flushing or prior migration. This has significant bearings on both the resource and reserves assessments of the hydrocarbon sands.