Genesis Analysis of High Gamma Ray (GR) Sandstone Reservoir: Taking Toutunhe Formation of Junggar Basin, Northwest China, as an Example

Abstract

On the basis of GR log analysis alone, the high GR sandstone reservoirs would probably have been regarded as mudstone and interpreted to be non-reservoir, which has greatly affected the exploration and exploitation of oil and gas reservoir to some extent. Therefore, the principles and standards to distinguish high GR sandstone reservoir from conventional sandstone reservoir, genesis of high GR sandstone reservoir have been systematically studied. Studies show that sandstone reservoir with apparent shale content calculated by GR log value greater than 50%, can be regarded as high GR sandstone reservoir. In the study area, the specific criterion of GR log value for high GR sandstone reservoir is 110API. The high GR characteristic of sandstone reservoir is mainly and directly caused by an abnormally high uranium enrichment, but not the tuff, feldspar or clay mineral. The study area has favorable conditions of abundant uranium source, favorable paleoclimate, and favorable lithology and lithofacies combination for uranium enrichment. In the process of oxygen bearing formation water migrating and leaching uranium bearing tuff, the U+4 can be oxidized to U+6, and uranium element transports in the form of UO2+2. In permeable sandstone reservoir 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.