Porosity, water saturation, and net-to-gross evaluation can be challenging in thinly bedded sands. The use of standard induction resistivity for formation evaluation can lead to the overestimation of water saturation. This work explores the following options to improve formation evaluation in these conditions: the use of high resolution density and nuclear magnetic resonance (NMR) data to improve porosity vertical resolution; the use of high-resolution resistivity from an oil-based-mud microresistivity imaging tool in improving the saturation computation (Sw); and the comparison of imaging tool resistivity-based sand count and NMR-based thin-bed fraction.
Using high-resolution porosity inputs from density and NMR provided a porosity curve with a better vertical resolution to match the high resolution resistivity from the imaging tool. It also identified additional productive thin beds compared to the standard resolution outputs and allowed computation of a high-resolution irreducible water saturation.
The induction-based Sw is strongly affected by shoulder bed effect and overestimates Sw by approximately 10 to 15%. The high-resolution curve from the imaging tool was used as an input into the Sw computation, which was made possible by shallow oil-based mud (OBM) invasion. This approach gave good results in beds thicker than 6 in., where Sw from the imaging tool matches the irreducible water saturation computed from NMR, giving 20 to 30% Sw.
A thin-bed fraction curve was computed from the NMR data. It shows a good match with the image-based high resolution- sand count and the image features, demonstrating that NMR and the imaging tool are equally able to identify and quantify thin beds, even though they have different vertical resolutions. This study showed that the microresistivity imaging tool and NMR are essential tools to characterize thinly bedded reservoirs.