Calcareous Interlayer Causes and Logging Identification for the Shawan Formation of the Chunfeng Oilfield

Background: Many sets of calcareous interlayers are present in the Shawan Formation in the Chunfeng oilfield, which leads to scattered residual oil that is difficult to extract. Objective: Observations of core samples and thin sections, analyses of trace element compositions and interpretations of logging data were used to identify the calcareous interlayers and identify their possible origin. Method: First, the petrologic characteristics of the calcareous interlayers were identified; then, we developed a quantitative function for identifying the calcareous interlayers based on well log data. Finally, the possible origin of these calcareous interlayers was identified based on these findings. Conclusion: The results show that the quantitative function has a high accuracy of 91.8% and average uranium content of calcareous interlayers can reach as high as 27×10-6, indicating that the quantitative identification function provided accurate identification of the calcareous interlayers, and highly radioactive uranium is one explanation for the origin of the calcium cementation.