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Pore pressure prediction in tectonically active basins

complex pore pressure distribution, with high overpressures in the Cretaceous through Paleogene section and variable overpressure in the Neogene sections

Pore pressure prediction is important for both real-time drilling to optimize field safety conditions, and for paleo-modeling to provide a framework for development of the basin and petroleum systems. Pore pressure distribution affects migration of fluids and gases, seal capacity and hydrocarbon column heights, the kinetics of vitrinite and hydrocarbon source rock maturation, and the evolution of basin structure. Basin and petroleum system modeling provides a means for testing potential scenarios under which excess pressure above hydrostatic pressure, or overpressure, can develop, how overpressure may dissipate or be redistributed across the basin, and how overpressure impacts migration and seal capacity. Additionally, basin and petroleum system modeling tracks the pore pressure throughout geologic time, providing a robust framework to understand the porosity and permeability evolution of the sediments based on predictions of effective stress through time.

In basins where the distribution of overpressure is complex due to structure and lithostratigraphy, basin and petroleum system modeling provides an ideal playground for examining the development of overpressure in context with stratgraphic and structural basin evolution. Common causes of overpressure include disequilibrium compaction and hydrocarbon generation. Aquathermal expansion, cementation, and the smectite-illite conversion can also generate overpressure, although typically to a lesser degree. Typically, only vertical stresses are considered in basin and petroleum system models using a Terzaghi-modeling approach. However, in tectonically active basins, horizontal stresses should also be considered, and require a poroelastic or poroplastic modeling approach.

Blair Burgreen considers these topics in the occurrence and distribution of overpressure in the East Coast Basin of New Zealand, which is an active subduction wedge, by linking the stratigraphic and structural history of the basin to present day pore pressure through 2D basin and petroleum system modeling. The basin has a complex pore pressure distribution, with high overpressures in the Cretaceous through Paleogene section and variable overpressure in the Neogene sections (Fig. 1). Pore pressure is poorly predicted by depth. The model allows hypothesis testing to examine the role of mudstones as seals, faults in basin compartmentalization, the structural history of uplift and erosion, and horizontal tectonic forces in the development and distribution of overpressure. The findings of this research have important petroleum system implications regarding the presence of naturally hydraulically fractured shale resources in the basin.