Three-Dimensional Plane Fault Reconstruction and Geometric Feature of Shale Reservoirs Faults—A Case Study of the Ordovician Wufeng Formation-Silurian Longmaxi Formation in Southern Sichuan Shale Gas Region

ObjectiveThe three-dimensional refinement and characterization of faults in shale reservoirs are closely linked to the investigation of shale reservoir accumulation and preservation conditions, and are pivotal in revealing the mechanisms governing engineering casing deformation.MethodsThis study presents a method for precise fault geometric characterization through fine-scale reconstruction and analysis of fault data points, with the following key components: 1) Fault plane equation fitting using Singular Value Decomposition (SVD), evaluated by the R-squared coefficient of determination; 2) Geometric parameter calculations including fault dip angle, dip direction, relief degree (RDFS) defined as the average distance from scattered points to the fitted plane; 3) Boundary determination through Graham Scan and Alpha Shape algorithms for convex and non-convex boundaries respectively, with the Alpha Shape algorithm recommended for more precise boundary delineation; 4) Dimensional characterization using minimum bounding rectangles to quantify fault length, width, and height. The integrated methodology enables comprehensive geometric analysis of 3D fault surfaces, providing robust technical support for shale gas exploration and development in the southern Sichuan Basin.Results and discussion In the shale gas development area of southern Sichuan Basin, a total of 86 faults have been identified, primarily categorized into two dominant groups: one set dipping southeast (SE) and the other northwest (NW). These faults predominantly exhibit moderate-to-low dip angles, with high-angle faults being relatively scarce. Notably, four major faults adjacent to shale gas production zones demonstrate distinct geometric evolution: initiating from gently dipping detachment layers at the shale base, they traverse through reservoir formations with progressively increasing dip angles, ultimately developing steeply dipping segments within the Permian Maokou Formation. The geometric characteristics reveal an inverse correlation between fault scale and dip angle. Larger-scale faults typically display gentler dips (lower angles) and greater curvature, whereas smaller-scale faults tend to exhibit steeper dips (higher angles) and smoother geometries. Regional fault lengths reach up to 42.84 km, though most measure less than 10 km. Both fault length and area distributions follow power-law patterns with long-tail characteristics. Vertically, faults generally extend approximately 2 km, while horizontal spans range from typical 2 km to maximum 5 km. This distribution pattern reflects the self-similarity and multi-scale characteristics of fault systems, suggesting scale-invariant growth mechanisms during tectonic deformation. The observed geometric relationships provide critical constraints for understanding fault connectivity and fluid migration pathways in shale reservoirs.ConclusionThe reconstruction results of the planar fault are helpful for optimizing the horizontal well technology in shale gas operations. They also assist in gaining a clearer understanding of fault slip and the mechanism of crustal deformation, and are of great significance for predicting future fault activity and its development trends. Furthermore, it helps in understanding the development patterns of faults at different scales.