FLORES, R.M., U.S. Geological Survey, Denver, CO; and J. M. BEGGS, and P. R. KING, Institute of Geological and Nuclear Sciences, Ltd., Lower Hutt, New Zealand

ABSTRACT: Sedimentology of Tide-Dominated Reservoir Sandstones in the Eocene Kapuni Group, Taranaki Basin, New Zealand

Sedimentology of the Kapuni Group (Eocene) reservoir system of the Taranaki Basin, New Zealand, was studied using almost 1,000 m of core from nine wells in the Maui and Kapuni fields, and the Toru-1 well. The Maui and Kapuni fields have combined recoverable reserves of more than 4 TCF of gas and 175MM barrels of condensate; Toru 1 is a gas discovery under appraisal. Reservoir sandstones formed in tidal inlet channel, tidal creek, shoreface, and fluviotidal channel environments. They are sealed by marine shales and by mixed-tidal and low- to high-marsh fine-grained facies.

In the Maui field, reservoir sandstones occur in two stratigraphic intervals (C and D Sands). The upper interval (C Sands) comprises lower and upper shoreface sandstones both containing hummocky cross stratification; stacked tidal-inlet-channel sandstones that are extensively reworked by Ophiomorpha burrows; and tidal creek sandstones with heterolithic stratification, tidal bundles, spring-neap tidal couplets, double mud drapes, and Teredolites. The lower interval (D Sands, below the marine D Shale) is composed of stacked tidal-inlet-channel and tidal-creek sandstones. In the Kapuni field, reservoir sandstones equivalent to the C Sands interval were mainly deposited in stacked tidal-inlet, tidal-creek and fluviotidal channels (the latter two most common). In the Toru-1 well, reservoir sandstones partly equivalent to the D Sands, D Shale, and C Sands, were deposited in multiscoured fluviotidal channels with tidally deposited flat pebble conglomerates, associated with mudstone tidal-channel-plug and mixed tidal-flat deposits.

During deposition, these fields straddled a fluctuating paleoshoreline, with Maui located farthest seaward (west), and Toru 1 farthest landward. Tidal processes influenced sedimentation over a >70-km-wide tract where paleovalleys were incised into the upper to lower coastal plain. Major transgressions across this tide-dominated coastal plain are recorded in ravinement lags and glauconitic sandstones at the base of the C and D Shales; smaller-order, relative sea-level changes controlled sandstone reservoir architecture through repeated incision and backfilling of paleovalleys. The predominant reservoirs in the Maui field are storm-dominated, shoreface reservoir facies of landward-stepping parasequences, whereas stacked fluviotidal reservoir facies, bounded by major truncations, are ommon in the Kapuni field and the Toru-1 well.

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