Predicting Reservoir Rock Geometry and Continuity in Pennsylvanian Reservoirs, Elk City Field, Oklahoma

Abstract

A geological-petrophysical engineering study of sandstone and conglomerate reservoirs was made to characterize and map pore-space distribution and to help predict fluid flow response of a waterflood. Floodable sand volume is believed to be represented accurately by net permeable sand isopach maps prepared from sand genesis knowledge. prepared from sand genesis knowledge. Introduction Detailed knowledge of the distribution of pore space and barriers influencing fluid flow combined with reservoir engineering data is critical for selecting, planning, and implementing operationally sound supplementary recovery projects. Reservoir engineers have recognized for many yearn that sandstone reservoirs are not homogeneous or simply layered. However, reservoir analyses commonly do not account adequately for the inhomogeneities or variations in reservoir and nonreservoir rock properties because these variations are not always defined accurately. The increasing understanding of sand-body genesis - how sands are deposited - makes possible accurate delineation of the spatial distribution of possible accurate delineation of the spatial distribution of pore space and barriers and helps significantly in pore space and barriers and helps significantly in predicting performance. This paper shows how the knowledge predicting performance. This paper shows how the knowledge of sand genesis can be used to provide an accurate picture of the reservoir rock system and insight into reservoir performance. This paper is based on a detailed performance. This paper is based on a detailed geological and petrophysical study of sandstone and conglomerate reservoirs and associated nonreservoir rocks in a plus or minus 500-ft-thick interval in the Elk City field, a 100-million-bbl field in the Anadarko basin in south-western Oklahoma (Fig. 1). The paper describes (1) the reservoir rock characteristics of the genetic sand-body types that make up the reservoirs, (2) the geometry and continuity of the pore space and barriers to fluid flow, and (3) most useful maps for portraying floodable pay and predicting performance. The study was conducted during the time of declining primary production and while a waterflood was being planned. The Elk City field produces from Pennsylvanian sandstones and conglomerates interbedded with nonproductive siltstones, shales, and carbonates. The interval studied includes the L and M zones, the two major reservoirs (Fig. 2). The field is an anticline developed by 3 10 wells on 40-acre spacing covering about 25 sq miles. Fig. 3 is a structure map on the dense marine limestone marker, Marker M, that separates the L and M zones. Data Control Most of the 310 wells were logged with a combination of SP log, 8- and 16-in. normal resistivity log, 24-ft lateral log, and microlog. In 16 wells, a gamma-ray neutron log was run instead of the microlog. Cores representing more than 1,700 ft of interval from 26 wells were studied. Approach and Methods of Study The study was conducted in four interrelated parts:lithology and petrophysical properties of the cores were studied, and the logs were calibrated with lithology in the cored intervals;genesis of the reservoir and associated rocks was interpreted;correlation was established; andthe interval was divided into subzones, and the net sand and sequences of rock types were mapped for each subzone. P. 851