Evaluation of Moki sands prospectivity in Maui PML

摘要

This paper describes the rise and fall of a Miocene exploration prospect in the Maui PML offshore New Zealand. Strong positive indications of moveable hydrocarbons in Whaarangi were finally shown by a RCI program not to relate to mobile hydrocarbons. Petrophysical evaluation indicated moderate quality sands with medium hydrocarbon saturations (around 50%) in the Moki B3 sand in the Maui B area. Positive oil indications were observed while drilling. The B3 sand showed a rather clear oil pressure gradient. Moreover, a sample containing 1.5 litres of oil (40? API) had been recovered in an earlier MDT program. In addition, the inferred contact from logs and pressures fitted a DHI on 3D seismic that also coincided with a structural 4-way dip closure. However, given the history of other Moki sands (low productivity) and the small potential oil column (transitional), commercial produceability was considered the main risk. With the development infra-structure already in place for the deeper Eocene reservoirs, and given the potential size of the accumulation, only relatively low oil rates from the Moki sands would be required for commercial development. Consequently a recent infill well targeting the deeper reservoirs provided the opportunity to sample and potentially test the Moki sands. With facilities lifetime reducing, a definitive result on Moki development potential was essential. Several RCI samples at a single reservoir point were recovered at increasing pump-out. The proportion of mud filtrate versus formation water was analysed by using a tracer added to the mud system. At the end of the pump-out, mainly formation water was recovered with only traces of oil. This conclusively demonstrated that the oil was immobile. Consequently, by low cost means, a definite result on the production potential was obtained, thereby avoiding expensive production testing. In addition, it downgraded further Moki prospectivity in the wider Maui PML.