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E & P NOTES |
1 Statoil, N-4035 Stavanger, Norway; sne{at}statoil.com
2 Statoil, N-4035 Stavanger, Norway; phn{at}statoil.com
3 Statoil, N-4035 Stavanger, Norway; present address: Statoil Arabian Gulf, Samarqand Street, Hai Al-Ssalam, Al-Rabia, Amman, Jordan; aamaq{at}statoil.com
Steve has a Ph.D. from the University of California at Los Angeles. He works on sandstone and carbonate reservoir studies for exploration and production projects.
Joining Statoil in 1986, Paul now serves as a specialist in global exploration working on basin evaluation and petroleum systems analysis. Originating from Maine, Paul received a B.S. degree from Boston College and a Ph.D. from Dartmouth College. He received the Schlumberger Medal from the Mineralogical Society and the Brindley Award from the Clay Minerals Society.
Adnan works as a business development manager for international exploration and production. He has more than 25 years of international geological experience in the Middle East, southeast Asia, and northwest Europe, with various geological research institutes and oil companies. He has published many articles and has recently completed a book, together with J. Goff, A. Horbury, and F. Sadooni, on the petroleum geology of Iraq. Adnan received his B.Sc. and M.Sc. degrees from the University of Baghdad and his Ph.D. and D.I.C. from Imperial College, London. He is a member of the AAPG, the International Association of Sedimentologists, the Society of Petroleum Engineers, the Iraqi Geological Society, and the Norwegian Petroleum Society.
ABSTRACT
A compilation of average porosity and permeability data for petroleum reservoirs in the Permian–Triassic Khuff Formation and the Jurassic Arab Formation shows that most Khuff reservoirs have an average porosity of less than 12%, whereas most Arab reservoirs have an average porosity of 12–26%. Higher porosity correlates with shallower depth, suggesting that burial diagenesis is the main cause of the overall porosity difference between these units. Deeper burial of Khuff reservoirs is inferred to have resulted in greater porosity loss by chemical compaction and associated cementation. A broad correlation also exists between average porosity and average permeability, suggesting that deeper burial and the resulting porosity decrease are also a primary cause of the lower permeabilities of the Khuff reservoirs. In addition to greater burial depth, however, a combination of depositional and early diagenetic factors is also reflected in the lower average porosity and permeability values of the Khuff reservoirs. Khuff strata were deposited on an extensive, poorly circulated, very low-relief shelf and consist in large part of interbedded mudstones and grainstones having relatively fine grain size, with major amounts of depositional calcium sulfate present. Arab reservoirs were deposited under better circulated conditions near platform margins facing deep, intracratonic basins and, thus, have coarser, more grain-dominated fabrics and lesser overall content of chemically precipitated grains, calcium sulfate, and dolomite. Khuff deposits were likely composed of less stable mineralogy than Arab sediments because the Late Permian was a time of aragonite seas, whereas the Late Jurassic was a time of calcite seas. The combined result of these factors is that Arab reservoirs are characterized by greater preservation of primary depositional pore types, more coarsely crystalline dolomite fabrics, and lesser plugging by anhydrite. Finally, a possible factor affecting the average porosity and permeability values is petroleum composition, which is gas in most Khuff reservoirs and oil in Arab reservoirs. Lower economic cutoff values for gas production would favor inclusion of low-permeability zones in Khuff reservoirs, thus reducing average reservoir values.
Two main aspects of these results are innovative. This is the first time that porosity and permeability values for either Khuff or Arab reservoirs have been examined regionally. Second, the conclusion that thermal exposure is the primary control on average porosity and permeability in these units is consistent with previous work from other carbonates, but is new for the Middle East.
This article has been cited by other articles:
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  S. N. Ehrenberg, P. H. Nadeau, and O. Steen Petroleum reservoir porosity versus depth: Influence of geological age AAPG Bulletin, October 1, 2009; 93(10): 1281 - 1296. [Abstract] [Full Text] [PDF]
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 S. N. Ehrenberg, A. A. M. Aqrawi, and P. H. Nadeau An overview of reservoir quality in producing Cretaceous strata of the Middle East Petroleum Geoscience, November 1, 2008; 14(4): 307 - 318. [Abstract] [Full Text] [PDF]
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S. N. Ehrenberg, T. A. Svana, and P. K. Swart Uranium depletion across the Permian-Triassic boundary in Middle East carbonates: Signature of oceanic anoxia AAPG Bulletin, June 1, 2008; 92(6): 691 - 707. [Abstract] [Full Text] [PDF]
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