Conglomerate Reservoir Pore Evolution Characteristics and Favorable Area Prediction: A Case Study of the Lower Triassic Baikouquan Formation in the Northwest Margin of the Junggar Basin, China

This study examines the characteristics and pore evolution of the Baikouquan conglomerate reservoir in the Mahu sag of the Junggar Basin from original sedimentation and diagenesis. Analysis is based on core observation, thin section, X-ray diffraction, cathodoluminescence and image analysis, and combined with physical property and well log data. The results show that conglomerate reservoir in the Baikouquan Formation can be divided into three lithofacies types: Type I is argillaceous filling conglomerate facies, in which cementation and dissolution are not developed, and the interstitial material is mainly argillaceous; Type II is tuffaceous filling in fine conglomerate facies, in which volcanic rock debris, illite and dissolution are developed; Type III is sandstone filling conglomerate facies, in which cementation and dissolution are developed. The reservoir undergoes complex diagenesis, and the diagenetic sequence is: compaction→early chlorite film→early calcite cementation→detritus, feldspar and tuffaceous dissolution→quartz secondary enlargement→late calcite cementation→oil invasion→forming illite. Quantitative study of pore evolution shows that dissolution and calcite cementation are relatively developed in lithofacies Type III, and that compaction has a great influence on lithofacies Type I and II. According to comprehensive evaluation of lithofacies, diagenesis and pore structure characteristics, the reservoir space type is mainly the dissolution pore. It is mainly primarily mainly composed of lithofacies Type III, thickness of the gravel body is more than 25 m, porosity is generally more than 12%, which represents favorable conditions for the distribution of favorable reservoir.