Abstract:
There has been low interest in petroleum exploration in the Wanganui Basin
as it lacks known hydrocarbon source rock of sufficient age or burial depth.
However, the onshore Southeast Wanganui Basin has many occurrences of
methane-rich biogenic gas found in shallow water wells. This project used
three studies across the Horowhenua area to examine the faulting style in
the Southeast Wanganui Basin where it is bounded by the Tararua range-
front, and how this faulting relates to the accumulation of gas deposits in
the shallow sedimentary section.
South of Levin the Tararua range front steps laterally near Muhunoa East
Road. A previous seismic reflection line identified a deep intra-basement
arrival, which could have been either a low-angle thrust fault or side-swipe
from a pull-apart basin at the step in the Tararua range front. Two seismic
lines and a gravity survey found no sub-vertical drops in basement depth
which would indicate the presence of a pull-apart basin or a favourable surface
off which a laterally travelling seismic wave could reflect. The intra-basement
arrival on the previous seismic line was therefore interpreted to be from an
intra-basement low-angle thrust fault.
Also two biogenic gas sites also were surveyed. A shallow gas reservoir
east of Levin on Wallace Road, abutting the Tararua range front, had been
discovered when a water well was drilled; and a potential reservoir southwest
of Sanson was located when an aerial survey identified a domed structure
with high resistivity. In both areas biogenic gas was thought to be trapped
in buried sand dunes at a depth of approximately 20 m. Shallow seismic
refraction and reflection methods and amplitude variation with offset analysis
were used to map both reservoir bodies and confirm the presence of biogenic
gas.
