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AAPG Bulletin

Abstract

AAPG Bulletin, V. 87, No. 9 (September 2003), P. 1509-1529.

Copyright copy2003. The American Association of Petroleum Geologists. All rights reserved.

Discrimination of effective from ineffective porosity in heterogeneous Cretaceous carbonates, Al Ghubar field, Oman

Langhorne B. Smith,1 Gregor P. Eberli,2 Jose Luis Masaferro,3 Salah Al-Dhahab4

1Reservoir Characterization Group, New York State Museum, Room 3140 CEC Albany, New York; email: [email protected]
2Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida; email: [email protected]
3Shell International Exploration amp Production B.V., Volmerlaan 8, 2280 AB Rijswijk, The Netherlands
4Shell International Exploration amp Production B.V., Volmerlaan 8, 2280 AB Rijswijk, The Netherlands

AUTHORS

Langhorne Smith currently heads the Reservoir Characterization Group at the New York State Museum. He holds a B.S. degree from Temple University, a Ph.D. from Virginia Tech, and did postdoctoral work at the University of Miami. He also worked for Chevron as a development geologist for two years. His current research interests are in carbonate reservoir characterization and hydrothermal alteration of carbonate reservoirs.

Gregor Eberli received his Ph.D. from the Swiss Institute of Technology (ETH), Zuumlrich, Switzerland in 1985. He joined the University of Miami in 1991. His research focuses on sedimentology, stratigraphy, and petrophysics of modern and ancient carbonates. He uses a combination of laboratory experiments and outcrop work to better understand the seismic and log signatures of sedimentary strata. In 2000, he spent a sabbatical year with the Carbonate Team at the Shell research offices in Rijswijk (Netherlands).

Jose Massaferro has worked at Shell Technology Applications and Research Center in the Netherlands since 1998. He received his Ph.D. from University of Miami in 1997 and was a Fulbright Fellow. He also worked for Texaco Petrolera Argentina. While at Shell, he has worked on 3-D seismic visualization/interpretation, high-resolution sequence stratigraphy, and kinematic modeling. Jose's research interest is to integrate 3-D seismic with borehole data for carbonate reservoir characterization.

Salah Al-Dhahab is a graduate of Aberdeen University, class of 1995. After graduation, he went to work for Petroleum Development of Oman, where his first job was as a wellsite petroleum engineer. He subsequently worked as a production geologist for numerous carbonate fields in Oman. In 2002, he took a position as a structural geologist with the Shell Carbonate Research and Development Team in the Netherlands.

ACKNOWLEDGMENTS

Many thanks to Jeff Dravis, Bob Loucks, and Chris Kendall for their constructive reviews of this manuscipt. We thank the Carbonate Development Team for asking us to participate in the Al Ghubar Field Study. We would also like to thank Neil Frewin and Henk Droste from Petroleum Development Oman for their background information and logistical support and Tom Mauduit, Pascal Richard, and David Peacock for their camaraderie during the outcrop study. We would also like to thank Hildegard Westphal and the SEM laboratory at the University of Kiel for their SEM analyses.

ABSTRACT

The Natih E heavy-oil reservoir (21deg API) at Al Ghubar field, Oman has produced less than 5% of the calculated oil in place. Porosity logs used to calculate reserves show high porosity throughout the reservoir, but further analysis of the only continuous core taken from the field indicates that much of the porosity is ineffective.

There are four heavily oil-stained, high-permeability skeletal-pelletal grainstone units with interparticle porosity in the core that probably contributed most of the production. The four permeable grainstone units occur at the top of small-scale accommodation cycles that have wackestone and packstone bases. These grainstones make up about 20% of the total thickness of the porous Natih E reservoir. The other 80% is composed of packstone and wackestone with ineffective microporosity, interparticle porosity in burrows, and isolated moldic and intraskeletal porosity. The small-scale reservoir-bearing cycles can be correlated across the field using the separation between the medium and deep induction curves as a guide.

Resistivity logs are the most reliable tool to distinguish effective from ineffective porosity. Most high-permeability grainstone units have deep induction values more than 100 ohm m and separation of more than 10 ohm m between the medium and deep induction curves. The ineffective intervals with microporosity, burrow porosity, and moldic porosity have lower resistivity and little separation between the medium and deep induction curves.

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