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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThinly bedded reservoirs are increasingly a target of offshore exploration in the Malay Basin. These reservoirs exhibit heterolithic interbedding with vertical heterogeneity and a wide range of layer flow properties. This paper describes methods of real-time and high-resolution formation evaluation and formation testing used to characterize such reservoirs. The formation evaluation of thinly bedded reservoirs has several objectives:• identify the layers that may contain hydrocarbons • verify productivity and fluid types with formation testing and sampling • calculate net pay thickness and uncertainty range. The evaluation is complex because of bed geometry and lithology. The reservoir beds are often thinner than the resolution of the formation evaluation logs. They exhibit a silty lithology and fine grain texture, and require high quality borehole resistivity images to characterize their geometry. The exploration well offers the best chance to evaluate the prospect, but operational and economical constraints do not permit to core or test every potential reservoir. Therefore, a real-time evaluation must identify with certainty the prospects to test. The sequence of real-time formation evaluation described here consists of the following steps. 1. Wellsite petrophysical analysis of:• porosity • volumes of clay, minerals and fluids • permeability based on the combination of GR spectroscopy elemental yields, nuclear and sonic porosity, and array resistivity logs. 2. Identification of thin beds, and computation of sand-count from an electrical borehole resistivity imager.3. Validation of the wellsite petrophysical analysis and the borehole image within two hours of recording the logs, and design of the wireline testing and sampling program. A modular wireline formation tester is used to measure accurate in-situ pressures in potential reservoir rock and to establish fluid gradients and identify contacts. Additionally, local formation permeability is estimated from analyses of pretests, and is used to verify the productivity of thin beds identified by formation evaluation techniques.

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High-frequency dielectric dispersion has been shown to be sensitive to the water volume, the water salinity, and the water tortuosity within the pore space. Dielectric dispersion data can be inverted to estimate water saturation in conditions where the interpretation of resistivity with traditional saturation equations is problematic, such as low and variable water salinities, complex texture, and unknown wettability. There is, however, a recognized limitation: salinity can be estimated only in the low to medium range. Above about 60 ppk, water salinity, which influences the estimated water volume, must be fixed in the dielectric interpretation. The thermal neutron capture cross-section (sigma) measurement primarily responds to the volume and salinity of the water that is present in the region of the formation seen by the tool. It provides an excellent complement to dielectric measurements for several reasons. Sigma is highly sensitive to the salinity of the water phase, particularly at high salinities. Both sigma and dielectric measurements have a similar volume of investigation and so can be interpreted in a joint inversion. We developed a joint inversion of dielectric dispersion and neutron sigma measurements that solves for the water volume, water salinity, and tortuosity of the water phase. The addition of neutron sigma to the dielectric dispersion measurements extends the interpretation to high-salinity conditions, and enables the capability of quantifying water salinity in conditions where this was previously not possible. We illustrate the benefits of this joint inversion on some log examples. In a well drilled with water-based mud and fully invaded, the inversion provides a robust estimate of filtrate salinity even for highly saline filtrate and reduces the uncertainty on the estimated residual hydrocarbon. In the case of oil-based mud invasion, the method quantifies the salinity of the formation water even for moderately to highly saline water.