INVESTIGATING GRAVITY-DRIVEN SHALE TECTONICS: RESULTS FROM CLAY MODELS

Shale tectonics are critical to understanding the deformation and structures on gravitational spreading passive margins for shale dominated systems. Updip extension and downdip compression are linked through the deformation of mobile and overpressured shales, as observed in areas such as the Niger Delta, and the Port Isabel and Mexican Ridges fold belts in the Western Gulf of Mexico. Notably, these margins are also proven hydrocarbon basins, with the diapirs, faults, and folds in the compressional domain acting as traps and seals. We use analog modelling to investigate the mechanics and structures formed in shale systems. While modeling of salt tectonics has been extensive, few studies have attempted to recreate shale tectonics with analogue models. Scaled analog models using wet kaolin allows for both qualitative and quantitative observations of mobile shales in offshore gravity-driven margins.

Models are frequently created using dry sand and wet clay as their properties can be scaled to model brittle deformation in the crust (Cooke and van der Elst, 2012). Clay slurries of about 50-55% water content by mass were used to represent the upper crust & delta, with the slurry inherently including water as a pore fluid. In all experiments, linked extension expressed through listric normal faults occurred in and below the delta layers, and compressional folding occurred beneath and beyond the toe of the delta. The delta loading patterns form listric normal faults with regional dip, and some antithetic normal faults with counter-regional dip. In the compressional zones, anticlines and pop-up structures formed at and just beyond the toe of the delta. Pre-delta layers beneath the delta thicken in the compressional zone as they move out from under the delta. In models with uniform strength throughout the stratigraphic section, thickness changes and deformation were concentrated the upper 2 pre-delta layers, with minimal deformation occurring at the bottom of the model due to increased friction with the base of the model. Models designed with a mechanically weaker layer showed a greater degree of deformation within this layer, moving from extensional domain to the compressional domain. These deformation patterns are similar to the structures observed in the Mexican Ridges Foldbelt, though our models differ slightly in the extent of deformation observed due to delta size limitations.