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Seismic stratigraphy and fluid flow in the Taranaki and Great South Basins offshore New Zealand

Chenrai, Piyaphong

[Thesis]. Manchester, UK: The University of Manchester; 2016.

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Abstract

This study utilises seismic data to improve understanding of the subsurface fluid flow behaviour in the Taranaki and Great South Basins offshore New Zealand. The aim of this study is to characterise fluid flow features and to investigate their genesis, fluid origins and implications for subsurface fluid plumbing system by integrating seismic interpretation and 3D petroleum systems modelling techniques. After an early phase studying Pliocene pockmarks in the Taranaki Basin, this study has been focused on the subsurface fluid plumbing system and on the fluid expulsion history in the Great South Basin. The Taranaki Basin lies on the west coast and offshore of the North Island, New Zealand. The seismic interpretation revealed that paleo-pockmark formation in the study area relates to fluid escape due to a rapid sediment loading environment in a distal fan setting. Seismic analysis rules out any links between the paleo-pockmarks and faulting. The relationship between paleo-pockmark occurrence and fan depositional thickness variations suggests that pore-water expulsion during overburden progradation is the most likely cause of the paleo-pockmarks. The rapid sediment loading generated overpressure which was greatest on the proximal fan due to a lateral gradient in overburden pressure. Fluids were consequently forced towards the fan distal parts where, eventually, the pore pressure exceeded the fracture gradient of the seal.The Great South Basin lies off the southern coast of the South Island of New Zealand and is located beneath the modern shelf area. Evidence for past and present subsurface fluid flow in this basin is manifested by the presence of numerous paleo-pockmarks, seabed pockmarks, polygonal fault systems, bright spots and bottom simulating reflections (BSR), all of which help constrain aspects of the overburden plumbing system and may provide clues to deeper hydrocarbon prospectivity in this frontier region. The various types of fluid flow features observed in this study are interpreted to be caused by different fluid origins and mechanisms based on evidences from seismic interpretation in the study area. The possible fluid origins which contribute to fluid flow features in the Great South Basin are compactional pore water as well as biogenic and thermogenic hydrocarbons. Using 3D seismic attribute analysis it was possible to highlight the occurrence of these features, particularly polygonal faults and pockmarks, which tend to be hosted within fine-grained sequences. Paleo- and present-day fluid flow features were investigated using 3D basin and petroleum systems modelling with varying heat flow scenarios. The models predict that thermogenic gas is currently being generated in mid-Cretaceous sedimentary sequences and possibly migrates along tectonic faults and polygonal faults feeding present-day pockmarks at the seabed. The models suggest that biogenic gas was the main fluid source for the Middle Eocene paleo-pockmarks and compactional pore fluid may be the main fluid contributor to the Late Eocene paleo-pockmarks. Different heat flow scenarios show that only mid-Cretaceous source rocks have reached thermal maturity in the basin, whilst Late Cretaceous and Paleocene source rocks would be largely immature. The observations and interpretations provided here contribute to the ongoing discussion on basin de-watering and de-gassing and the fluid contributors involved in pockmark formation and the use of pockmarks as a potential indicator of hydrocarbon expulsion. It is clear from this study that seismically-defined fluid flow features should be integrated into petroleum systems modelling of frontier and mature exploration areas in order to improve our understanding on fluid phases, their migration routes, timings and eventual expulsion history.