Prediction of fracture distribution in the Lower Jurassic Da'anzhai Member on the eastern slope of the Western Sichuan Depression, Sichuan Basin

The Da'anzhai Member of the Lower Jurassic on the eastern slope of the Western Sichuan Depression in the Sichuan Basin is a main target for the development of tight oil and gas. Fractures are essential for achieving high production in the Da'anzhai Member reservoirs. Due to their complex lithology, traditional methods for fracture prediction and evaluation have limited applicability and low prediction accuracy. Based on core fracture investigations and thin-section identification data, combined with geological statistics and numerical simulations, the lithological characteristics of various reservoirs were clarified, and the fracture development characteristics in the Da'anzhai Member were revealed. Considering both the fracture foundation and external fracturing forces, three fracture evaluation factors were proposed and constructed to predict and evaluate the planar distribution of fractures in each sub-member by distinguishing lithology and sub-members. The results showed that: (1) The Da'anzhai Member reservoirs had complex lithologies with interbedded (shell) limestone, sandstone, and shale. (2) Fully filled fractures were mainly developed, and (shell) limestone exhibited mainly structural fractures, while sandstone and shale mainly showed interlayer fractures with dissolution features on the fracture surfaces, which was beneficial for enhancing oil and gas flow. (3) Three fracture evaluation factors were proposed and constructed, which included lithologic thickness, tectonic deformation intensity, and fracture rupture intensity. A quantitative model for comprehensive prediction and evaluation of fractures was established to predict and evaluate the planar distribution of fractures in each sub-member comprehensively. The predicted fracture density aligned well with the fracture development index identified in individual wells, indicating the reliability of the prediction results. This method for fracture planar distribution prediction and evaluation offers a reference for similar oil and gas reservoirs.