Pockmark formation by porewater expulsion during rapid progradation in the offshore Taranaki Basin, New Zealand

After the first discovery of seabed pockmarks in the 1960s and 1970s, many examples of both modern and ancient pockmarks have been reported from sedimentary basins around the world. The exact mechanisms and fluids involved in pockmark formation are still subject to debate with many studies inferring that pockmark formation is a direct indication of hydrocarbon expulsion. This study provides the first description of a buried Pliocene paleo-pockmark field in the offshore Taranaki Basin, New Zealand. The paleo-pockmarks are located approximately 1.2–1.3 km below the seabed and are well imaged by three-dimensional seismic data. Their spatial density and size increases towards the distal portions of the fan, and they vary in size between 45 and 580 m in diameter and 2–35 ms TWT. Pockmark size is inversely correlated with the fan thickness in the study area. Detailed analysis rules out any links with faulting, while the relationship with fan variations in the depositional thickness suggests porewater expulsion during overburden progradation as the most likely cause of the paleo-pockmarks. A model is proposed in which rapid sediment loading generated overpressure which was greatest on the proximal fan due to the lateral gradient in the sediment load imposed by clinoform progradation. Fluids were consequently forced towards the distal fan where, eventually, the pore pressure exceeded the fracture gradient of the seal. The implications of this study are that not all pockmarks can be used as evidence for hydrocarbon expulsion in frontier regions and care should be taken to examine the subsurface context, plumbing systems and any associated acoustic anomalies before concluding on the pockmark origin.