ABSTRACT: GEOMODEL: A 3-D Model of Reservoir Continuity

Donna Neuhart, Dave Foster, Greg Benson, Mike Lukk, Jeremy Jameson

GEOMODEL is a three-dimensional (3-D) geologic framework into which reservoir properties can be modeled. It is based on detailed log and lithology correlations and is built entirely in SAS. GEOMODEL is currently used to assess the Wahoo Formation of the Lisburne oil field, Prudhoe Bay, Alaska.

The Wahoo Formation consists of cyclic shallow-water carbonate facies, alternating between lagoonal muds and grain shoals. Three distinct episodes of diagenesis have altered the original rocks to form moldic porosity in limestones and intercrystalline porosity in dolomites. The resulting complex reservoir contains 83 individual pay layers with porosity greater than 10%. These layers are 2-25 ft thick, are of variable lateral extent, and commonly are interbedded with nonpay layers.

To model reservoir continuity, pay intervals were correlated among 63 wells using FDC/CNL and gamma-ray logs and core. All 3-D space was filled by creating nonpay layers between pay. Isopach maps and average porosity values at each well for both pay and non-pay layers were then gridded into 800-ft. × 800-ft. cells of variable height. Merging these grids with the top Wahoo structure grid resulted in a 3-D GEOMODEL containing 650,000 cells. A search logic scanned from cell to cell in all directions to tag units meeting pay cutoff criteria vertically or across faults.

GEOMODEL output depicts continuous units on various types of maps, cross sections, fault-plane profiles, and synthetic logs. Because GEOMODEL is a SAS program, it is suitable for statistical and volumetric analyses. GEOMODEL also serves as an extensive database into which virtually any attribute derivable from log or core can be stored and analyzed in three dimensions.

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