FRANCIS, DAVE, Consultant Geologist, Lower Hutt, New Zealand

ABSTRACT: Tertiary Reservoir Formations in the Prospective East Coast Basin, New Zealand

New Zealand's extensive East Coast Basin has dozens of oil seeps and impregnations, hundreds of gas seeps, good structure, but no production from the 45 mostly old wells drilled.

Lack of reservoir has in the past been cited as a discouraging feature of the basin, but new work indicates this is unlikely to be a problem. Although earlier wells targeted Cretaceous or Pliocene formations, the Paleocene to Miocene has not been seriously tested.

Paleocene prospects include fractured shale in contact with the Waipawa Black Shale, the most likely source in the basin, and associated greensands.

The Oligocene consists dominantly of muddy calcareous marl, but includes quartzo-feldspathic sandstone over 25 m thick in places, and overlie source sequences unconformably.

Within the 3 to 5 km of Miocene deep-water sediments are several intervals of thick-bedded sandstone with reservoir potential. They reach up to several hundred metres in the basal Miocene, Lower Miocene, mid-Miocene, and Upper Miocene in different parts of the basin.

The Miocene sandstones in the basin axis are deep water lobe-fans, generally fine grained, quartzo-feldspathic with variable lithic content derived from metasedimentary and rhyolitic terranes. Glauconite is present in some older sandstones. The mid-Miocene sandstones are the thickest and most extensive in outcrop, and have been recognised in seismic.

All of the sandstones discussed are associated with oil seeps, or are oil impregnated in some part of the basin, but are underplayed. Exploration in the East Coast should also consider these formations as viable targets.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.