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AAPG Bulletin


AAPG Bulletin, V. 86, No. 5 (May 2002), P. 779-795.

Copyright ©2002. The American Association of Petroleum Geologists. All rights reserved.

Outcrop-based reservoir characterization: A composite phylloid-algal mound, western Orogrande basin (New Mexico)

Patrick D. Doherty,1 Gerilyn S. Soreghan,2 John P. Castagna3

1School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma, 73019; email: [email protected]
2School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma, 73019; email: [email protected]
3School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma, 73019; email: [email protected]


Patrick Doherty received his B.S. (1996) and M.S. (2000) degrees in geology from the University of Oklahoma, Norman. He is currently a geophysicist with Phillips Petroleum in Bartlesville, Oklahoma.

G. S. (Lynn) Soreghan received her B.S. degree in geology (1986) from the University of California, Los Angeles, and her Ph.D. in geology (1992) from the University of Arizona. She worked for Amoco Exploration and Production Company (1992-1996) before joining the faculty in geology and geophysics at the University of Oklahoma, where she is an associate professor. Her research focuses on sedimentary systems of the late Paleozoic as archives of Earth processes and reservoirs for key resources.

John Castagna is a professor in the School of Geology and Geophysics and director of the Institute for Exploration and Development Geosciences at the University of Oklahoma. He earned B.S. and M.S. degrees in geology in 1976 and 1981 from Brooklyn College, New York, and his Ph.D. in exploration geophysics at the University of Texas-Austin in 1983. He has served the Society of Exploration Geophysicists as chairman of the Leading Edge editorial board and first vice president and was Distinguished Lecturer in 1995.


Acknowledgment is made to the donors of The Petroleum Research Fund (ACS-PRF#: 31893-G8; 35217-AC) and to Conoco (Young Professor Award to G. S. Soreghan) for partial support of this research. The AAPG and Geological Society of America (GSA) Grants-in-Aid programs provided additional support to Patrick Doherty. We gratefully acknowledge the help of R. Myers (geologist, Environmental Services Division), T. A. Ladd, and D. Downs of the U.S. Army White Sands Missile Range for access and photographic permissions for the range. Many thanks to H. Liu and D. Sissom at the University of Oklahoma for assistance with thin section preparation. S. Undan and P. Mohab were extremely helpful with computer modeling. Critical petrophysical measurements were obtained through the help of M. Grammer (Texaco), A. Saller (Unocal), D. Larese (Amoco), and the Geophysical Reservoir Characterization Consortium of the Institute for Exploration and Development Geosciences at the University of Oklahoma. Rock physics measurements were made at the University of Oklahoma Integrated Core Characterization Center in Tulsa. Thanks to B. Spears, C. Sondergeld, C. Rai, and F. Gallice for rock physics measurements and analysis. M. Murphy and S. Seals were invaluable field assistants. Additionally, the following individuals supplied valuable insight and/or reviews that significantly improved the article: J. Pigott, C. Kerans, J. Lucia, and S. Ruppel.


Algal mounds of the western Orogrande basin present an excellent opportunity for studying reservoir behavior because these mounds are superbly exposed and of reservoir scale. Upper Paleozoic algal bioherms form significant reservoirs both domestically and internationally but are difficult to exploit owing to characteristically pronounced facies and diagenetic heterogeneity. Establishing predictive relationships between petrophysical properties and stratigraphy is key to enhancing exploitation success in these systems. This article focuses on a well-exposed, reservoir-scale algal mound complex of the western Orogrande basin (New Mexico) to construct an outcrop-based reservoir model for this and analogous systems.

An accurate sequence stratigraphic framework is key for meaningful modeling but is difficult to establish in the subsurface. The studied mound consists of multiple high-frequency sequences with elevated porosity and dolomitization proximal to sequence boundaries, particularly in mound-flank positions. The porosity model indicates that the mound flanks may form a stratified reservoir, whereas the mound core is tight and, therefore, acts as a lateral seal. Early marine cement occluded primary porosity in outcrop mound-core facies.

Forward seismic modeling shows that the internal stratigraphic architecture of the mound complex is below conventional seismic resolution. Rock-physics measurements on 20 outcrop samples, however, demonstrate a relationship between dolomite content and the ratio of compressional-wave to shear-wave velocities at in-situ conditions. For a given dolomite percentage, seismic impedance decreases as porosity increases. Accordingly, surface seismic data could potentially be used to estimate gross reservoir properties.

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