CAMPBELL, KATHLEEN A., Geology Department, University of Auckland, Private Bag 92019, Auckland, New Zealand; DAVID A. FRANCIS, Geological Research Ltd, P.O. Box 30-819, Lower Hutt, New Zealand; and MIKE COLLINS, Geology Department, University of Auckland, Private Bag 92019, Auckland, New Zealand

Abstract: Hydrocarbon Seepage in the East Coast Basin Convergent Margin (Miocene-Recent), New Zealand

Hydrocarbon seepage has occurred in the forearc-convergent/wrench Pacific-Australian margin (Eastern North Island, New Zealand) for the past ~20 m.y. Evidence for substantial fluid expulsion along this 500 km+ margin includes 13 Lower-Upper Miocene methane-derived carbonate deposits, up to 13 modern offshore seeps, and >350 modern onshore oil, gas and brine seeps. Onshore seeps vary from C1-C4 hydrocarbons to abundant marine-sourced oil, formed by thermogenic and biogenic processes linked geochemically to Cretaceous-Pliocene formations known in outcrop.

Miocene sites are characterized by authigenic carbonates of variable dimensions (boulders to 300 ´ 100 ´ 40 m mounds). Northern deposits are mussel-dominated and isotopically depleted in carbonate carbon to d¹³C -51.7 ‰ PDB. Clear contact relationships with bathyal mudstone are evident, e.g., basal plumbing features (carbonate nodules, "pipes," Paramoudra-like concretions), or mound-top corrosion and caryophylliid coral communities. Focused, localized fluid flow produced chimneys, cemented cylindrical conduits, brecciated veins of fibrous aragonite, and/or micrite nodules precipitated along coarse sediment horizons and burrows. Southern localities are vesicomyid clam-dominated, with sandy carbonates more enriched in ¹³C.

Preliminary research reported here on NZ East Coast Basin paleoseep carbonate deposits has aided the understanding of fluid flow, hydrocarbon generation, timing, and facies associations in this formerly frontier southwest Pacific basin.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California