Quantifying Present-Day Overpressure and its Development Through Time in Active and Passive Sedimentary Basins

Abstract

Pore fluid pressure is a critical variable that needs to be assessed for many geological applications, including hydrocarbon exploration and exploitation, geothermal energy, carbon capture and storage, H2, and nuclear waste storage. To further understand and mitigate the uncertainty of unexpected overpressured intervals or areas, this research assesses the occurrence of and the controls on pore fluid pressure in three geologically distinct regions: the East Coast Basin of New Zealand (ECB), the Magnolia Field in the Gulf of Mexico, and an Atlantic margin of the West African basin. The factors that contribute to overpressure generation, maintenance, and dissipation are assessed, as are the geological and geomechanical approaches used to identify the overpressure-generating mechanisms that contributed to the present-day overpressure.
Simple approaches to overpressure evaluation, such as analytical equations and log-based well interpretations, were applied to the data; however, they are insufficient to explain the distribution of overpressure in areas with complex geological histories (e.g., erosive events, changes in sedimentation rates, tectonic compression, active tectonism). Geological data from each area were therefore used to construct 1D and 2D geomechanical (thermo-hydro-mechanical and hydro-mechanical) models that provide the evolution of porosity, stresses, and pore fluid pressure through time and facilitate the evaluation of each overpressure mechanism separately.
Present-day overpressure in the three sedimentary basins results from different mechanisms acting at different periods of time. Overpressure dissipation occurs during erosive events and where lateral pressure drainage is present; the most recent events (e.g., <2 Ma) have the greatest influence on porosity and pore fluid pressure. Overpressure maintenance occurs when thick (>1,000 m) intervals of shale or thin (e.g., <85 m) successions of exceptionally low-permeability intervals are present. In tectonic active settings, tectonic compression acting during the most recent geological events (e.g., <6 Ma) has the greatest impact on porosity and pore fluid pressure; in the context of salt tectonics, this impact depends on the shape of the developing salt wall. Early geological clay diagenesis can locally reduce porosity and generate high overpressures due to the reduction of sediment permeability caused by a combination of chemical compaction (smectite to illite transformation) and mechanical compaction (post-diagenesis sedimentation). Finally, low (200 m/Ma) and high (3,000 m/Ma) sedimentation rates can generate overpressure when low-permeability intervals are present, also resulting in the preservation of high porosities.
A detailed geological interpretation used as an input for the construction of geomechanical models can provide more realistic results, and consequently a better understanding of the present-day overpressure distribution within a basin. In addition, this information can be used to mitigate the level of uncertainty in seal failure of sites with the potential to store CO2, H2, and nuclear waste.