Abstract

The Eocene-Miocene Te Kuiti Group is a succession of cool-water carbonate and siliciclastic rocks on the North Island of New Zealand. The limestones are subtidal, bioclastic, and largely composed of bryozoans, echinoderms, and benthic foraminifers. These rocks can be divided into a horizontally bedded facies assemblage and a cross-bedded facies assemblage. The horizontally bedded facies assemblage represents deposition on a wave- (storm-) dominated shelf and is composed of grainstones and packstones containing textures resulting from biologic processes. The cross-bedded facies assemblage represents deposition on a current-dominated shelf and consists of grainstones with textures formed chiefly by physical processes. The Te Kuiti Group can be used as an outcrop analog for cross-bedded, calcite-dominated hydrocarbon reservoirs. Porosity, permeability, and reservoir potential of the horizontally bedded facies assemblage decreased rapidly during late shallow burial owing to mechanical compaction and to the fine grain size, close packing, and ineffective early cementation of the rock. Coarser grain size, open framework, and earlier cementation within the cross-bedded facies assemblage resulted in significant heterogeneity and good reservoir potential before moderate burial. Tabular, lens-shaped, and sigmoidal reservoir units are proposed. Primary porosity and early developed traps are essential to reservoirs formed of cool-water limestones and those carbonates from "calcite seas." The influence of depositional setting extends into the eventual bedding style, texture, physical properties, and reservoir potential of the rock.

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