Abstract

The Taranaki Basin contains the only known commercial hydrocarbon reserves in New Zealand. The hydrocarbons were derived principally from Late Cretaceous and Paleocene-Eocene coals. An average temperature gradient of 29 degrees C/km characterizes much of the basin, but gradients range geographically from 22 to 33 degrees C/km. Thermal and hydrocarbon generation histories were simulated for selected wells that characterize the different regions of the basin. Modeling results show that predepositional and syndepositional Mesozoic crustal thickening, erosion, and rifting resulted in high heat flow during the early stages of deposition. The early high heat flow affected only the deepest source rocks, especially where they are thick and were buried to depths greater than 2.5 km prior to 60 Ma; hydrocarbon generation and expulsion may have been as early as the early Paleocene in these areas. For wells in the Western Platform region, most potential source rocks are immature or have just reached expulsion maturity. However, in areas where initial burial was rapid and more than 1 km of Cretaceous-early Tertiary sediments accumulated, generation amounts sufficient for expulsion may have been reached in the last 1 m.y. for much of the source section, and possibly as early as the Eocene for the deepest source rocks. In the southern Taranaki region, temperatures and generation rates were greatest about 5-10 Ma. About 5 Ma, generation rates decreased and expulsion terminated due to cooling related o structural inversion; temperatures generally are too low for significant oil expulsion (less than 120 degrees C) at present. In the eastern Taranaki region, the combination of tectonic (rapid sedimentation and erosion) and magmatic effects caused variations in burial depths and geothermal gradients that resulted in oil generation and expulsion that were more spatially and temporally variable than in other regions. The bottom of the Cretaceous section (Rakopi Formation) is overmature in most of the eastern region, with present temperatures at the bottom of this section commonly greater than 180 degrees C; expulsion generally began in the Paleocene-Eocene. Paleocene source rocks began expelling hydrocarbons in the Eocene-Oligocene in some areas, but not until the late Miocene in others. Eocene source rocks began expelling as early as 10 Ma in areas where early Miocene deposition was relatively thick (4 km). Oil generation and expulsion on the northern Taranaki peninsula probably was enhanced in the last 1 m.y. by heat for Quaternary igneous intrusions associated with the Taranaki volcanic chain.

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