Elastic anisotropy differentiation of thin shale beds and fractures using a novel hybrid rock physics model

Abstract. Elastic anisotropy is frequently used to characterize fracture distribution. However, sets of parallel horizontal fractures and thin shale beds in tight sand can both cause vertical transverse isotropy. Here, we are not referring to shale layers on the logging scale but rather to very thin shale beds, a few centimeters thick, within tight sand. To accurately differentiate the anisotropy caused by horizontal fractures or thin shale beds, we propose a hybrid rock physics model. This new model combines the Hudson model and the shale-compacting orientation distribution function (ODF) model, based on the anisotropic self-consistent approximation (SCA) and differential effective medium (DEM) theory. The new model's reliability is demonstrated by comparison to the well logs. The proposed model can characterize the elastic properties of both thin shale beds and horizontal fractures. Based on this model, the rock physical analysis reveals that thin shale beds and horizontal fractures exhibit distinct elastic anisotropy characteristics. Furthermore, we analyze the seismic response differences between horizontal fractures and thin shale beds using the anisotropic Ruger's approximation formula. The analysis indicates that the seismic response of tight sand containing thin shale beds interferes with the fracture's identification. On the other hand, there are identifiable differences between the fractured tight sand and the tight sand containing thin shale beds. Based on this difference, we develop a new seismic attribute to characterize the fracture distribution. These difference-based attributes can effectively eliminate the interference from thin shale beds, making the distribution of horizontal fractures more apparent.