| Description |
This dissertation examines mudstone heterogeneity in sedimentary features and geomechanical behavior. Mudstone, classically considered homogeneous and isotropic, is now known to contain complex internal heterogeneity. Herein, examination of heterogeneous stacking patterns and interpretations of depositional complexity led to the hypothesis that heterogeneities affect constitutive mechanical properties of mudstone. The Juana Lopez Member of the Mancos Shale preserves organic-carbon rich mudstone interbedded at high frequency with sandstone. Field and subsurface correlation, organic and inorganic geochemistry, and stratigraphic analyses were completed in order to determine depositional processes and environments that resulted in preservation of this strata. A depositional model is presented that suggests the Juana Lopez Member represents a balance between active, current driven deposition and quiescence. Interpretation of overlapping sediment dispersal processes acting parallel, perpendicular, and oblique to the paleo-shoreline pushes models of mudstone deposition into three-dimensions and subdivides the offshore realm into various depositional environments. Hydrodynamic, eventbed deposition and preservation of organic-carbon challenges conceptions of preservation of mudstone solely in suspension-dominated, anoxic environments. Recognition of sedimentary complexity in mudstone led to examination of the impact heterogeneities have on geomechanical behavior. Laboratory geomechanics analyses were completed on mudstone with varying degrees of heterogeneity from the Mancos Shale and the Agrio Fm. Heterogeneity indices were developed to describe variation in sample's texture, fabric, and composition, and they were used in addition to lithofacies and mechanofacies classifications. A variety of deformation and failure parameters were calculated that describe failure in tension and compression. Results revealed that percent saturation and anisotropy exhibit primary controls on rock deformation and failure. Saturation highlights anisotropy, heterogeneity, and affects fracture morphology and evolution. Failure in homogeneous, dry samples results in discrete, simpler fractures than in heterogeneous, saturated samples. Crack initiation was differentiated as a predictive parameter for correlating mechanics to heterogeneity, which may make it a powerful parameter for informing how fractures form in heterogeneous mudstone. Heterogeneity indices are a quantitative measure of sample variation and have predictive relationships with mechanics parameters; however, a lithofacies based approach allows for the most wholistic view of sample description, is more predictive, and is the most readily upscaled. |
