Geological implications of gamma ray (GR) anomalies in marine shales: A case study of the Ordovician-Silurian Wufeng-Longmaxi succession in the Sichuan Basin and its periphery, Southwest China

Abstract

Conventionally, high gamma ray (GR) values usually indicate high argillaceous contents in sedimentary rocks, while high-GR sections in marine shales have more siliceous minerals and fewer clay minerals. However, the origin of the anomalously high-GR peaks in marine shales has been less well studied. In addition, high-GR marine shale intervals are usually important sections for horizontal drilling and fracturing. The Wufeng-Longmaxi shales in the Sichuan Basin and its periphery were investigated to study the sedimentary characteristics and genesis of anomalously high-GR peaks based on elemental and natural GR spectral logging data, sedimentary observations and geochemical analysis. Uranium enrichment is concluded to control the GR peak occurrences in the Wufeng-Longmaxi shales, and uranium enrichment is affected by a hypoxic reducing environment, high organic matter content, well-developed tuff beds, and the adsorption of phosphorus-bearing minerals and iron-bearing minerals. The anomalously high-GR peaks are manifestations of volcanic activity and rapid transgression. Rutile, uranium-bearing minerals, apatite, barium feldspar and tuff beds are petrological evidence of volcanic activity, and Fe, K and U/Th ratio anomalies also show geochemical evidence of volcanic activity. The degree of water stagnation, disorderly mineral distribution, and reducing environment are evidences of rapid transgression. The uranium enrichment indicates that volcanic eruptions provided large numbers of uranium ions for the seawater uranium pool, forming a material basis for uranium shale deposition. De-glaciation transgression provided a sedimentary environment and pathways for uranium enrichment. Two uranium enrichment events are identified in the Wufeng-Longmaxi shales. The high-GR shale section controlled by notable uranium enrichment is characterized by an anoxic reducing environment, high TOC content, good pore structure and high brittleness. High-GR segment identification contributes to shale gas sweet spot detection. This study demonstrates that high-GR anomalies in marine shales are related to geological events and also provide certain time references for marine shales.