Introduction to Geologic Models
Authors: Richardson J.G. , Sangree J.B. , Sneider R.M. 
Published: December 2007

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose: To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology.
Interspersed within the Technology Today Series, JPT will present severalshort papers during the coming year describing the characteristics of differenttypes of sandstone reservoirs. Two appear in this issue. These papers willfeature conceptual geologic models that illustrate how depositionalenvironments affect the quality and continuity of pay and nonpay in sandstonereservoirs. Subsequent papers will discuss engineering use of the geologicinformation in mathematical and computer models to study reservoir performance. performance. The purpose of these papers is to communicate to our engineeringsociety some of the tremendous advances during the last 10 or 15 years ingeologists' ability to describe reservoirs. It is hoped that these papers willenhance the understanding of the need for papers will enhance the understandingof the need for teamwork among engineers and geologists on projects to increaserecovery of oil and gas. The recent geologic literature describes numerousstudies of Recent sediments and outcrops deposited by rivers, in oceans, or bywinds on land. Descriptions and measurements on cores from these deposits haveprovided evidence of how the environments and the provided evidence of how theenvironments and the energy levels and variations of the currents presentduring their formation before burial affect their reservoir characteristics. Byreservoir characteristics, we mean the quality of the payi.e., level anddistribution of porosity and permeabilityand of the lateral and verticalcontinuity of the pay and nonpay formations. The conceptual geologic modelsthat have evolved from studies of the data on Recent sediments, outcrops, andreservoirs are the best foundations on which to build descriptions of reservoircharacteristics between wells. The geologic models, combined with data fromwell tests, cores, logs, and seismic surveys, provide a basis for averagingpermeabilities and porosities and for developing the reservoir framework."Reservoir framework" means the structure of the reservoir and the distributionof impermeable barriers and faults within the reservoir. The geologic models, combined with seismic data on the aquifers, also provide the best basis ofextrapolation of reservoir data for describing the characteristics of theaquifers. While such diagenetic processes as compaction, formation ofpore-filling minerals (quartz overgrowths, calcite, pyrite, or clays), andleaching can greatly alter the rocks after they are deposited, discussion ofthese effects is deferred to future papers. papers. This series will includepapers on geologic models of sandstone deposits in various types of land, river, delta, and deep marine environments (see Fig. 1). The impact of modelproperties on well location, perforating policies, and reservoir performancewill perforating policies, and reservoir performance will be discussed. A briefpaper describes application of geophysical data in reservoir description. Shortengineering papers will describe how the geologic models can be used to assessthe potential importance of gravity drainage, viscous crossflow, and coning inoil recovery.
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