HYDROCARBON POTENTIAL OF THE NORTH WANGANUI BASIN, NEW ZEALAND

Abstract

The proximity of the North Wanganui Basin to the onshore Kapuni and offshore Maui gas/condensate fields of the adjacent Taranaki Basin has attracted the interest of oil companies for some time. Exploration during the late 1950's and early 1960's delineated several prospective traps. Some of these were subsequently drilled and, apart from minor gas indications, were dry holes. Data from the sixteen wells drilled are on open file and these together with other available information are incorporated in a series of maps representing stages in the evolution of the basin.

The sedimentary history of the North Wanganui Basin began early in the Oligocene when a shallow north to south marine transgression saw the infilling of structurally defined northeast-southwest trending depressions in the folded Mesozoic basement. As the relief of the peripheral landmass was reduced the former basin irregularities were smoothed out and the way was paved for the spread of carbonate rich seas from which a suite of carbonate grainstones and packstones was deposited during the middle Oligocene. Late Oligocene time saw the renewed influx of clastic sediments as movements on the dominant wrench fault basement structure brought revived relief to areas of sediment supply. At that time the southern margin of the basin possibly merged westwards with the Taranaki Basin. Punctuated by periods of vulcanicity centred west of the present coastline, a thick sequence of mud, silt and sand, dominantly marine but locally including terrestrial coal measures, makes up the Miocene succession. Sedimentation controlled by growth faulting is characteristic, and the east-west barrier of the Pipiriki High persisted in restricting the southern extension of the basin during that time. The Miocene closed with a tilting movement hinged on the Pipiriki High such that subsequent Pliocene and Pleistocene sedimentation followed a south-east migrating depocentre in the quite separate South Wanganui Basin.

Hydrocarbon indications in the basin itself are few. The presence there of potential reservoir and source rocks in addition to the proven production in the adjacent Taranaki Basin are reason for some optimism in the further evaluation of the hydrocarbon prospects of the North Wanganui Basin.