The role of cross-bed related anisotropy in the formation and orientation of compaction bands and joints in aeolian sandstone in 3D and the implications for fluid flow [electronic resource]
Keywords: Geological and Environmental Sciences.

Compaction bands (CBs) are closing mode structures, characterized by reduced porosity and represent one kinematic end-member of a family of deformation bands, which form by localization of volumetric strain into narrow tabular bands. We report the occurrence of bed-parallel CBs and high-angle CBs in the aeolian Aztec Sandstone exposed throughout the Valley of Fire State Park, Nevada. We distinguish three categories based upon depositional domains (dune units characterized by cross-beds therein) and structural domains (CBs of different orientations): 1) cross-beds with bed-parallel CBs 2) cross-beds with high-angle CBs, and 3) cross-beds with both bed-parallel and high-angle CBs overlapping in a relatively narrow transition zone. The field data demonstrate that the orientation of the cross-beds for each of these domains falls into a certain range. The hypothesis for this phenomenon is based on the strength anisotropy of localized compaction in anisotropic sandstones. We used a quadratic failure criterion to describe the strength anisotropy of localized compaction and compared the results with the field data. The results show a clear relationship among the cross-beds with (or without) CBs of certain orientations and the cross-beds with relatively lower (or higher) calculated strength of localized compaction. The field data on the opening mode joints demonstrate that the cross-bed package confined joints occur at high-angle to bedding and trend roughly parallel to the dip direction of the cross-beds. In comparison, the roughly N-S trending joint zones appear not to be influenced by the cross-beds in any significant way but are frequently truncated by the dune boundaries. In the laboratory, we determined an average P-wave anisotropy of Aztec Sandstone as slightly larger than 13%. Based on this result, a model based on the generalized Hooke's law for anisotropic materials is used to analyze deformation of cross-bedded sandstone as a transversely isotropic material. We find qualitative agreements between most of the model results and the observed field relations between joint sets and cross-beds. We conclude that different categories of joint sets formed in response to the variation of the boundary conditions and the interplay with the rock anisotropy is significant. We measured the permeability of multiple sets of compaction bands (CBs) and adjacent host rocks using a core-flooding laboratory technique as well as the image-based permeability calculations. The results show that the permeability within the high-angle CBs (three sets) is consistently three orders of magnitude lower than that of the host rocks. For the bed-parallel CBs, the measured permeability reduction with respect to the rock matrix is about half an order to three orders of magnitude. Significant differences exist between the lab-based and image-based permeability and porosity measurements of CBs and host rocks. Possible factors causing these differences are different sample sizes and heterogeneities within the host rocks, calibration on the image segmentation, incomplete characterization of clay minerals and fines migration during lab-based experiments. We performed flow simulations to investigate the upscaled permeability of the compartmentalized distribution of compaction bands (CBs). The results suggest that the upscaled permeability in the direction normal to the dune trend is controlled primarily by the high-angle CB domain, whereas the upscaled permeability in the vertical direction is controlled primarily by the bed-parallel CB domains. The orientations (plunge direction and plunge) of different principal permeability components have differing degrees of dependences on the orientations and permeability of CB sets: The major principal permeability component remains almost unchanged by the variation in permeability of CB sets whereas the minor principal permeability component is controlled by the interplay between the orientation of CB sets and their permeability.