Dual Component Estimation; a probabilistic approach to estimate undiscovered oil and gas in an underexplored basin, Taranaki Basin, New Zealand
New Zealand has relied on the supply of oil and gas from the Maui Field in the Taranaki Basin for 40 years. As this field nears depletion, there is considerable governmental encouragement for increased investment in exploration to ensure continued oil and gas production for domestic use and the export market. The Taranaki Basin covers a 100,000 km² area and has a 10 km-thick Cretaceous to Holocene sediment fill, which hosts only 200 exploration wells with 33 oil and gas discoveries ≥ 1 million barrels of oil equivalent (mmboe). Discoveries have been made throughout Paleocene to Holocene sequences in multiple reservoirs within onshore and offshore parts of the basin. A review of the hydrocarbon potential of the Taranaki Basin suggests that more than one working petroleum system operates in the basin, based on the distribution of oil and gas-condensate. As part of this study the most productive and prospective reservoir intervals have been studied to ascertain the working petroleum systems. The reservoir intervals are grouped into four plays and are referred to as the Cretaceous, Paleocene, Eocene and Miocene plays. The Dual Component Estimation (DCE) is a novel way of combining a modified, existing, size distribution-based discovery-sequence sampling method with a Geographical Information Systems (GIS)-based spatial method to estimate the amount and likely location of undiscovered oil and gas in an underexplored basin. In particular, the DCE uses an inverse sampling method to ensure the total number of all accumulations in the basin is not constrained by the size distributions of discoveries, which typically represent a very small proportion of accumulations in an underexplored basin. Furthermore, it is a probabilistic approach that captures ranges in uncertainties that result from using regional scale data and assumptions used to simplify the process of generating and trapping hydrocarbons. Given the underexplored character of the Taranaki Basin, this study has included potential discoveries to define the size distribution of the original population of all accumulations in a basin, which is used to derive the undiscovered volume. Potential discoveries are based on basin modelling and mapped structural traps. This approach increases the dataset of accumulations (discovered and modelled) from 33 to 338 and generates an original parent population that includes petroleum systems information from explored and unexplored areas of the basin. DCE modelling results suggest that the basin has an undiscovered oil and gas resource potential of ~1500 oil and gas accumulations totalling 8210–10800 mmboe. The mean discovery size is 328.7 mmboe; however the next discovery could be as large as 550–900 mmboe (with a 10% probability). More likely, the next discovery is estimated to be at least 50 mmboe (with a 90% probability) which is a commercially significant size in the Taranaki Basin. New discoveries in the Palaeocene play have been modelled in the Manaia anticline area, Western Platform and onshore eastern margin of the basin. It is most likely that a discovery in this play will be at least 40 mmboe, with a 90% probability; however it may be closer to 470 mmboe, with a 10% probability. The Eocene play is the most prospective and future discoveries will most likely be located along the eastern margin of the basin, nearshore and onshore of the western peninsula, and offshore, east of the Maui Field. There is a 10% probability that a new discovery in this play may be as big as 200–500 mmboe. More likely, with a 90% probability, a new discovery may be at least 35 mmboe. Discoveries in the Miocene play are most likely in the onshore peninsular area and in the offshore Northern Graben. There is a 90% probability that the next discovery may be at least 55 mmboe and a 10% probability that it may be much larger (310–800 mmboe). A discovery is yet to be made in the Cretaceous play. This study indicates that a new discovery in this play is most likely to be at least 50 mmboe (with a 90% probability) but may be greater and be at least 500 mmboe (with 10% probability). The strength of the DCE is in the use of additional geological data to include unexplored areas of the underexplored Taranaki Basin in the estimation. This estimation should also be applicable to other geologically similar, underexplored, sedimentary basins of New Zealand.