Abstract: Thermal Modeling and Hydrocarbon Generation in an Active Margin Basin: the Taranaki Basin, New Zealand

Phillip A. Armstrong, David S. Chapman, Robert H. Funnell, Richard G. Allis, Peter J.J. Kamp

The Taranaki basin is an active-margin basin located adjacent to the Hikurangi trench and above the subducting Pacific plate. The tectonic history includes pre-Cenozoic crustal thickening, erosion, and rifting, Cenozoic subsidence, and Miocene to recent extension, volcanism, and thrusting; all of these affect temperatures in Late Cretaceous-Recent basin sediments.

Corrected bottom-hole temperatures (BHTs) from 104 wells show that thermal gradients vary spatially from 22 to 32°C/km. BHTs, well-determined formation thermal conductivities, and robust porosity-depth functions were used to determine that heat flow presently is 50 to 70 mWm^-2 throughout the basin.

Various subcrustal heat flow and geodynamic conditions affect the thermal state and hydrocarbon generation. We have investigated models that reflect these conditions and satisfy heat flow, vitrinite reflectance, and apatite fission track age constraints. Our models include the transient effects of sedimentation, erosion, crustal thinning, uplift, and thrusting and simulate hydrocarbon generation using an Arrhenius parallel reaction approach. Constant subcrustal heat flow models, with no predepositional geodynamic history, suggest that oil generation rates in the Late Cretaceous/Paleocene source rocks were greatest in late Miocene to Recent times, coincident with rapid subsidence. With 12 km of predepositional erosion, to expose 115 Ma granites that locally underlie basin sediments, an 50% stretching during initial stages of basin formation, heat flow was 25-30% higher early in the basin history resulting in high generation rates in the early Cenozoic for deeper source rocks in some wells. The modelling shows the importance of incorporating long-lived predepositional geodynamic thermal effects.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994