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Basement Terranes, Basement Composition and Present-Day Heat Flow Models for the New Zealand Extended Continental Shelf

Abstract

In 2012, FROGTECH completed a structural study for New Zealand Petroleum and Minerals that resulted in a depth-to-basement model, a basement terranes map, and maps of total sediment, basement and crustal thickness, and depth to Moho of the New Zealand Extended Continental Shelf. This work, based on the systematic integration and interpretation of both non-seismic and seismic datasets, set the foundation for a New Zealand basement composition map and present-day basement heat flow models, including the depth of theoretical oil and gas maturation windows. In a follow-up project for NZPM during 2014, a basement composition map was generated by integrating public domain potential field data, basement wells and outcrop data. Heat flow models were generated following the integrated FROGTECH ArcGIS-based workflow, combining the 2012 study datasets, basement composition and local values of the radiogenic heat production derived from the GNS Science PETLAB geochemical database that allowed for generation of different models of radiogenic heat flow. Models consider the impact of past tectono-thermal events. The highest heat flow values are modelled in the continental arc terranes of the Western Province. The generally thinner and less radiogenic crust of the Eastern Province displays lower values. This trend is punctuated by lateral variations in basement composition, such as the presence of high heat productive felsic intrusives or low heat productive mafic lithologies. Modelling of the transient thermal impact of rifting events that occurred during and after the Early Cretaceous separation of the New Zealand terranes from Australia and Antarctica, suggests that presently remaining thermal anomalies are negligible in the Eastern Province. The strongest observed and modelled positive thermal anomalies are interpreted to be related to the subduction and roll-back of the Pacific Plate under the North Island, which resulted in the development of Eocene to present-day volcanoes. Top and bottom maturity isotherm depths for oil and gas were calculated using the produced heat flow map, in addition to the calculation of the basin radiogenic heat flow, and assuming a linear temperature gradient. The modelled maturity windows are shallowest in the East Taranaki Basin, where they intersect in wells with the known Eocene to Miocene reservoir rocks of commercial oil and gas fields. Other theoretical shallow areas are identified in the Great South Basin and the Reinga Basin.