Modeling Overpressure Development and the Mechanical Behavior of Sediments in a Complex, Tectonically Active Setting: East Coast Basin, New Zealand

Abstract

The East Coast Basin (ECB) is a complex, active convergent margin located on the North Island of New Zealand, where variable but commonly very high overpressures occur in Cretaceous to Pleistocene stratigraphy. Using a consistent set of sediment physical properties and a single value for tectonic compression, a multi-1D hydro-mechanical modeling approach was employed to evaluate the primary controls on overpressure and porosity across a 40 km section of the ECB. The 1D hydro-mechanical models simulated the geological histories of five key wells across the ECB and investigated overpressure generated by burial disequilibrium compaction and tectonic compression. Present-day overpressure is associated with tectonic compression within the last 3 million years due to the subduction of the Pacific Plate beneath the Australian Plate; this has generated up to 18 MPa of overpressure at a depth of 2,000 m. Burial disequilibrium compaction contributed up to 4 MPa at the same depth. Whilst erosive events can lead to the dissipation of overpressure, the effects of all but the most recent erosive event are masked and overprinted by subsequent sedimentation. In some areas and in specific parts of the stratigraphy, overpressure appears to dissipate through lateral fluid flow. Throughout a complex geological history including several episodes of burial, tectonic compression and erosion, critical state soil mechanics models suggest that mechanical changes in mudstones and fine-grained carbonates occur almost entirely through compaction rather than dilation (shear).