Longmaxi-Wufeng Shale lithofacies identification and 3-D modeling in the northern Fuling Gas Field, Sichuan Basin

Mineral composition and total organic carbon (TOC) content of shale is related with rock brittleness and gas content, respectively. Shale lithofacies defined by them can effectively describe the heterogeneity of shale gas reservoirs. Therefore, a 3-D model of shale lithofacies was constructed for the Longmaxi-Wufeng Shale in Fuling Gas Field, Sichuan Basin. Firstly, three criteria were proposed to define ten shale lithofacies in Longmaxi-Wufeng Shale. The core- and elementary capture spectroscopy (ECS)-defined shale lithofacies with the wireline logs at their location consisted of a training dataset with 1044 sets of data for predicting shale lithofacies by conventional logs. And, artificial neural network and hierarchical decomposition algorithms were developed to achieve this prediction. Due to the instability of horizontal wellbore, a process of quality control was completed to remove all the low-quality data. To construct the 3-D model using data primarily from horizontal wells, it is critical to fix at least two special issues which are not common for the cases using data from only vertical wells. The relationship between horizontal wells and formation surfaces become much more complex, and data from wells was biased and failed to represent the distribution feature of the study area. Methods, including establishing pseudo vertical wells to provide more formation data and constructing 3-D probability trend model, were developed in this research to overcome these issues. The 3-D modeling of shale lithofacies is very helpful for optimizing the design of horizontal well trajectories and hydraulic fracture stimulation strategies.