ABSTRACT: Pore-Level Reservoir Heterogeneity in Carbonates: Lisburne Reservoir, Prudhoe Bay Field, North Slope, Alaska

John G. Kaldi, David L. Boyer, David A. Budd

The heterogeneity of the pore system in the Lisburne reservoir at Prudhoe Bay, Alaska, can be related to the variability of reservoir properties in this giant oil field. The reservoir consists of four petrographically defined pore types. The most common, pore type 1, is microporosity occurring within limestone grains. A characteristic of these microporous rocks is their high porosities and low permeabilities. Due to the great hydrocarbon column at Prudhoe Bay, much of this microporosity is oil filled, but unproducible in the absence of fractures. Pore type 2 consists of moldic and/or intraparticle pores in limestones that commonly result in pay intervals with good porosity and fair permeability. Pore type 3 comprises intercrystalline pore networks in dolomite that, depend ng on pore throat size, also contribute to pay. Pore type 4 is an intercrystalline pore network in dolomite, with more than 10% of the total porosity in the form of molds or vugs. Pore type 4 commonly has the most favorable reservoir properties and, as indicated by production tests, contributes the majority of the reservoir's flow. Lisburne pore types owe their origin to a combination of depositional and diagenetic processes. Early diagenetic dolomitization of mudstones and wackestones created pore type 3 layers that acted as permeable conduits for later diagenetic solutions. These solutions dissolved limestone grains within the dolomites to produce pore type 4, as well as leaching adjacent lime packstones and grainstones to produce pore types 1 and 2. The geometry of pore types 1 and 2, which form halos around pore types 3 and 4, is the basis for defining flow units within the Lisburne reservoir.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990