In permeable add to favorites sandstone saturated with uranium bearing formation water, the U+6 can be deoxidized into U+4 by oil and gas migrating through faults and stratigraphic unconformities and accumulates in the form of UO2 in sandstones.5. Log Evaluation Techniques for High GR Sandstone ReservoirsThe log evaluation for high GR sandstone reservoirs includes two aspects: reservoir recognition and physical property evaluation. In pioneering papers, the high GR sandstone reservoir can be recognized in two ways: (1) unconventional logs such as GR spectrum logs and elemental capture spectroscopy (ECS) logs can be used to recognize high GR sandstone reservoir [1, 5, 11]. But except a small amount of GR spectrum logs, there are no ECS logs in the study area. So the way of recognizing high GR reservoirs by unconventional logs is not feasible.

(2) Cross plot of compensated neutron log (CNL), density log (DEN), and interval transit time log (AC) are used to identify high GR reservoir [4]. But CNL, DEN, and AC are greatly affected by formations’ high water sensitivity and poor borehole quality and cannot be used to recognize high GR reservoir. Figure 4 shows that the high GR sandstone reservoir cannot be recognized by the cross plot of CNL, DEN, and AC, because there is little difference among high GR sandstone reservoir, conventional sandstone reservoir, and mudstone in this cross plot. In this study, central type nuclear magnetic resonance (NMR) log, which is slightly or not affected by borehole quality [21], is proposed to recognize reservoirs and evaluate physical properties.

Figure 4Cross plot of CNL, DEN, and AC for mudstone, conventional sandstone reservoir, and high GR sandstone reservoir.It can be seen from Figure 5 that the high GR reservoirs’ GR value exceeds 110API (track 2). The U curves in track 3 show that the uranium content of high GR sandstone reservoir is significantly higher than that Drug_discovery of the conventional reservoir and mudstone. The track 4 and track 6 display that in characters of CNL, DEN, and AC, there is no significant difference among high GR sandstone reservoir, conventional sandstone reservoir, and mudstone. The NMR log in track 7 shows that, compared to the short transverse relaxation time (T2) of mudstone, the sandstone reservoir has a long T2. Therefore, the NMR log provides an effective way to recognize high GR reservoir.Figure 5Conventional logs and NMR log responses of a well in study area.