Evaluation of Thin Bed Using Resistivity Borehole and NMR Imaging Techniques

Abstract

In the study of sedimentary units to evaluate thin beds, there lie a lot of uncertainties, primarily, sand, shale or their combination in terms of their petrophysical properties and lateral extant. Good thin beds are capable of yielding a reasonable amount of hydrocarbon if discovered and evaluated. Regrettably, most thin beds are usually bypassed if direct log interpretation techniques are applied in their evaluation due to their low vertical resolutions and ability to only characterize an average of multiple beds which often result to a significant under estimation of oil and gas in the sedimentary unit. To better characterize thin beds and maximize reservoir potential, the resistivity borehole imaging and NMR techniques were adopted in this research study. In addition, the Thomas Stieber equations using laminated sand-shale sequence were employed in calculating parameters such as sand porosity, hydrocarbon saturation and shale volume. Results show that at depths of about 4350m and 4500m (Fig. 2), thin beds were identified, whereas, the conventional logs identified the points as spikes of sand. The NMR imaging technique further confirms the presence of hydrocarbon in the thin beds. It is therefore suggested that the resistivity borehole imaging and NMR techniques should be employed in characterizing thin beds since their use in this study yielded results that are reliable and dependable.

Keywords: Thin bed, Borehole image, Sand laminae, Conventional logs, lateral resistivity, Vertical resistivity.