We present a new method to derive continuous reservoir fluids properties (saturation, salinity, density and hydrogen index) in a complex siliciclastic brownfield. The complexity of our study field lies in the uncertainty of formation salinity, as the water-flooded sands contain an unknown mixture of the connate brine and injected water. In such environments, one cannot simply assume a fixed salinity value when calculating saturation logs with equations that rely on a good knowledge of salinity. Formation evaluation is further complicated by low resistivity contrast between wet and pay sands, where high volume of salty irreducible water lowers resistivity in hydrocarbon-bearing sands.

In this field, water production is a big concern for the operator. They would like to gain better understanding of the water flood encroachment to make smarter development plans in the future. To achieve this goal, they acquired advanced LWD logs in a number of development wells to characterize reservoir fluids. The use of LWD logs ensures that invasion of the drilling fluid into the reservoir sands is minimal.

Thermal neutron capture cross-section (Sigma) is sensitive to chlorine in the reservoir fluids and rocks and can be used to distinguish between water and hydrocarbon in a salty water environment. Since both Sigma and resistivity are dictated by water saturation (Sw) and salinity together, we can use these two measurements to simultaneously solve for Sw and salinity at each depth in a nonlinear least squares inversion routine. The resistivity-sigma workflow assumes total porosity is known but does not require a priori knowledge of salinity and outputs a continuous Sw and water salinity log that best honor the input Sigma and resistivity logs.

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