Tight unconventional reservoirs have become an increasingly common target for hydrocarbon production in Oman. Exploitation of these resources requires a comprehensive reservoir description and a characterization program to estimate reserves, identify properties that control production, and account for fracturability. It is becoming evident, however, that any single technology by itself is unable to address all the key challenges, and the integration of technologies is crucial to answer all the questions to reduce key subsurface uncertainties.
This paper discusses in detail a case study in which the integration of advanced petrophysical logs has enabled successful downhole sampling and provided a comprehensive reservoir and fluid characterization despite the very challenging lithologies and very tight formation. The comprehensive logging suite included advanced measurements of dielectric dispersion, nuclear magnetic resonance (NMR), and spectroscopy. The reservoir fluids and dynamic properties were also characterized by a series of formation testing measurements.
Dielectric dispersion logs clearly identified the hydrocarbon-bearing zones despite the characterless resistivity profile, taking advantage of its resistivity-independent saturation approach. The accuracy of the measurement was key to estimating water- filled porosity down to 0.5 p.u. regardless of the formation water salinity and changes in the rock electrical parameters. The integration of dielectric and NMR measurements, reflecting the pore structure, has played a major role in identifying the "best" reservoir intervals and indicating the type of fluid (hydrocarbon or water) filling the free pore space. The NMR unimodal and bimodal T2 distributions revealed the pore structure along with polarization effects on light hydrocarbons, helping to gain insight on the reservoir quality. The NMR was also combined with the microimaging measurements to indicate pore connectivity and formation heterogeneity. This integrated approach was applied to a deep tight-gas exploration well and has contributed to achieving successful formation sampling that provided an in-situ fluid characterization despite the tightness of the rocks, with only 4 p.u. average porosity. Integrated logging measurements along with fluid sampling resulted in both enhanced formation and fluid characterization in this exploratory well, shedding light on the hydrocarbon potential over the region.