Seismic interpretation, reservoir characterization, gas origin and entrapment of the Miocene-Pliocene Mangaa C sandstone, Karewa Gas Field, North Taranaki Basin, New Zealand

The Karewa gas discovery highlighted the potential for commercial gas accumulations in the shallow MiocenePliocene stratigraphic horizons, thus changing the exploration strategy in Taranaki Basin, New Zealand. The gas origin, its entrapment mechanism, and the petrophysical characteristics of the Miocene-Pliocene Mangaa C sandstone reservoir have been evaluated. Organic geochemical analyses of Mangaa and Giant Foresets shales bounding the sandstone reservoir point to their immaturity (Tmax 426 degrees C) and poor capability for thermogenic hydrocarbon generation. Karewa gas displays an isotopic pattern of 813C1 >> 813C3 813C2 reflecting its biogenic origin with no signs of thermogenic input. Additionally, the depleted 813C1 (of range -61.32% to -61.02%) and 8DC1 values (of range -183.5% to -182.3%) support its microbial origin likely by bacterial carbonate reduction and degradation of the organically-bound CO2 to form methane. Shales of the Giant Foresets and Mangaa formations contain, and therefore contain abundant organically-bound CO2. Karewa prospect as a roll-over structure bounded by listric faults forming a four-way dip closure. These faults are the gas migration paths from Mangaa and Giant Forests shales to the Mangaa C sandstone reservoir. Excellent petrophysical reservoir characteristics of Mangaa C sandstone (visual porosity av. 20.7%, helium porosity from core av. 30.2%, shale volume av. 6.4%, and water saturation av. 34.5%) prompted its gas potential with high hydrocarbon saturation (av. 65.5%). Mild mechanical compaction and low content of authigenic mineral phases aided in the preservation of the primary intergranular porosity. Seal capacity analysis of the Mangaa C system confirmed the efficiency of the overlying mudstones to seal a thick gas column (up to 150 m) with a minimal risk of leakage.

Automated summary

The Karewa gas discovery in the Taranaki Basin of New Zealand revealed commercial gas potential in the Miocene-Pliocene stratigraphic horizons, originating from microbial carbonate reduction processes. The reservoir characteristics of the Mangaa C sandstone and the seal capacity analysis of the system confirm its gas potential and efficiency in sealing a thick gas column.