Pore pressure estimation in salt-sealed formations

Abstract

Abstract Many conventional methods for pore pressure estimation are challenged by conditions below salt, and this is especially true in isolated sediments. However, for Gulf of Mexico salt allochthons – and potentially for other settings where sediments were sealed by salt early in their burial histories – a simple consideration of seal capacity provides useful insight into possible pressures. Data from several such confined settings are presented here and compared to two simple sealing models. The resulting observations are also compared to regional mudrock trends and intra-salt data to evaluate the potential implications. Together, the data suggest the salt seal fails at a constant pressure below the base-salt minimum confining stress, and pressure in sediments below that point increases at a constant gradient. The field data shown here indicate a consistent behavior over a large area of the Gulf of Mexico, despite substantial variability in salt thickness, water depth, etc. The proposed model provides estimates that agree with observed pressures within salt, at base salt, and through salt-sealed sediments below salt. A simplified version of the model can also provide a map of pressure or effective stress across base salt; while not fully accounting for sub-salt structure, such maps can be valuable in assessing relative drill-site risk to guide more detailed site-specific work.