Seismic Attributes to Constrain the Distribution of Rakopi Formation Coals in the Southwest Offshore Taranaki Basin, New Zealand
The Rakopi Formation, in the Taranaki Basin of New Zealand, is a Late Cretaceous (Haumurian) sequence of coal measures interbedded with siltstone and sparse sandstone. It is the lowest widespread stratigraphic unit in the Taranaki Basin, and has a syn-rift deposition which is concentrated in isolated grabens and sub-basins. It was deposited during the extensional tectonic regime linked to the breakup of Gondwana. The Rakopi is one of the major hydrocarbon source rocks in the Taranaki Basin; New Zealand’s only currently-producing basin. However, there are very few well penetrations of the Cretaceous interval within the basin, and this fact – coupled with the non-continuous deposition – means that the Rakopi’s distribution is poorly constrained further away from wells. In petroleum systems models, the entire Rakopi Formation interval is commonly represented entirely as source rock facies. By observing the limited well penetrations available, it is known that this is not the case. As such, it is likely that the total hydrocarbons generated within the basin are overestimated when modelled. Improved constraint of the distribution of the coal within the Rakopi Formation will improve the accuracy of these models. This study presents the results of the extraction and analysis of a suite of seismic attributes from 3D seismic reflection surveys in the southwest offshore Taranaki Basin, for the purpose of constraining the distribution of coal within the Rakopi Formation. The attribute outputs were temporally averaged, with the resulting frequency distributions analysed for significant patterns or variations between the coaly- versus non-coaly vertical intervals of the formation. Within the Maari 3D seismic reflection volume, six attributes were identified which correlated with the presence of coals observed in the Maui-4 well. These attributes were temporally averaged within the Rakopi Formation interval in both the Maari 3D and Pipeline 3D volumes, with the areas of high average values identified as being more likely to contain coal. Using this attribute analysis method, the distribution of coal within the Rakopi Formation has been better constrained. Attribute analysis is easily transferable, and has the potential to be utilised elsewhere for the identification of hydrocarbon source rocks.