Publication Name: Eastern Australian Basins Symposium 2001
Authors: V. Stagpoole, R. Funnell and D. Darby
Date Published: November 2001
Number of Pages: 31
Reference Type: Magazine Article
Abstract:Volcanic activity and the effects of magmatism have often been considered detrimental to the prospectivity of petroleum basins. In the northern Taranaki Basin, Middle to Late Miocene andesitic arc magmatism associated with the Mohakatino Volcanic Centre has added more complexity to the petroleum system, but there also appear to be aspects that enhance prospectivity. Magmatic intrusions in the basement beneath the numerous (20+) volcanic cones have an estimated total volume of 1,000 to 2,000 km3. These intrusions altered the maturity of nearby source rocks and the timing of hydrocarbon expulsion, and also domed overlying sediments to create structural traps.
Thermal modelling of the basin history predicts two periods of hydrocarbon expulsion: the first during and immediately after magmatism (14 Ma to about 8 Ma), and the second following renewed burial since about 4 Ma. Late Cretaceous terrestrial source rocks lie at depths compatible with the present day oil window. However, modelling predicts that close to large magmatic intrusions, these source rocks became fully mature (Transformation Ratio = 1.0) during magmatism and their petroleum potential is now fully depleted. Overlying marine source rocks, not thermally affected by magmatism, are modelled to be generating and expelling hydrocarbons to the present-day (Transformation Ratio = 0.4 to 0.8). Thus, models indicate that the
widespread volcanism and associated intrusion of magma in the northern Taranaki Basin govern the charge history and the nature of hydrocarbons in areas that are close to large magmatic intrusions.
Magmatism is also responsible for the deposition of volcaniclastic reservoir rocks, alteration of the basin-scale permeability structure and the creation of giant structures due to doming of overlying sediments. In the past, parts of the northern Taranaki Basin have been deemed poorly prospective. However, an improved understanding of the effects of magmatism suggests that parts of this basin, and similar intra-arc basins in the southwest Pacific, may be highly prospective.