Geneses and geological significance of sand bodies in tight sandstone of the Jurassic Badaowan Formation, Central Junggar Basin

A breakthrough in oil and gas exploration has been achieved in Jurassic Badaowan Formation， central Junggar Basin; however， an unclear understanding of characteristics and distribution patterns for sand body development hinder further exploration efforts. This work integrates sedimentology with both logging and experimental analyses to investigate the petrological and sedimentary characteristics of the Badaowan Formation， aiming to understand the development of favorable sand bodies and their relationships to hydrocarbon reservoirs. Lithic sandstone and feldspar lithic sandstone were developed in the study area by core observations and thin section petrography. The results based on logging and sieve analyses indicate that a total of 11 typical centimeter-scale lithofacies and four meter-scale sand body sequences are developed in the study area. Thin section petrography and mercury intrusion porosimetry (MIP) supplemented with fractal interpretation show that type III reservoirs， corresponding to the estuary dam microfacies， are the most favorable ones， characterized by a reverse grading from fine at the base to coarse at the top， with a thickness ranging from 1.5-3 m. The sieve curves exhibit a two-stage pattern with the sizing coefficient Φ value of about 2-4. In addition， pore diameters from MIP analyses range from 2 to 5 μm， predominated by at 2 μm as mesopores. Lower values of fractal dimension correlate with simpler porosity and better physical properties. There is a negative correlation between porosity and content of plastic rock fragments. Sand bodies are longitudinally stacked in the Junggar Central Depression across eight sections， with Types II and III being widely developed. Optimal targets are identified to be located in the slope-basin transition areas， spreading as a ring-like pattern in the vertical source direction.