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Steep Microbial-Dominated Platform Margins – Examples and Implications*
By
Paul M. (Mitch) Harris1
and J.A.M. Kenter2
Search and Discovery Article #40298 (2008)
Posted August 27, 2008
*Adapted from oral presentation at AAPG Annual Convention, Salt Lake City, Utah, May 11-14, 2003
1ChevronTexaco E & P Technology Company, San Ramon, CA ([email protected])
2Earth Sciences Dept., Vrije Universiteit, Amsterdam, Netherlands; currently ETC, Chevron, Voorburg, Netherlands ([email protected])
Seaward progradation of several km’s or more has been documented mostly for low-angle carbonate platform slope systems with a dominant bank top sediment source. But steep and high-relief margins fronting deep basins can also prograde and as such are somewhat perplexing. Characteristics of prograding ancient examples, Permian and Carboniferous in age, provide a model, which may apply elsewhere.
The Capitan margin rimmed the Delaware Basin of west Texas and southeastern New Mexico during the Late Guadalupian (Capitanian). Information on the morphology and progradational history of this margin comes from subsurface seismic and well data, and also superb outcrops in the nearby Guadalupe Mountains. Although the margin was high-relief (300 to 550 m high) and steep (30 to over 70° dip), progradation extended the margin from 5 to 19 km. Outcrops of steep, high relief Serpukhovian to Moscovian margins in northern Spain serve as important analogs for subsurface hydrocarbon reservoirs in steep-sided isolated platforms of the Pricaspian Basin in western Kazakhstan. Seismic and well data corroborate the outcrop pattern, showing progradation of, respectively, more than 10 and up to 5 km despite the high-relief (up to 600 m) and steep (~20-32°) nature of these margins.
The examples share a highly productive microbial cement boundstone factory extending from the platform break to nearly 300 m depth and a lower slope dominated by (mega)breccias and grain flow deposits derived from the margin and slope itself. The broad depth range of microbial cement boundstone increases the potential for production during both lowstands and highstands of sea level and thereby facilitates progradation. Rapid in situ lithification of the boundstone provides stability to the steep slopes, but also leads to readjustment through shearing and avalanching. What controls the microbial cement boundstone formation remains a debate but its presence is a key factor in the progradational geometry of these and possibly other older, and younger, margins.
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Cemented Microbial Boundstone Platform Margin Caveats
What localizes these factors along certain margins?
Common themes from the examples provide a “model” for prograding steep, high relief carbonate platform margins to be considered for other areas. But many aspects of the microbial boundstone, cementation, and slope processes remain poorly understood.
Bahamonde, Juan R., Jeroen A. M. Kenter, Giovanna Della Porta, Lorenz Keim, Adrian Immenhauser, and John J.G. Reijmer, 2004, Lithofacies and depositional processes on a high, steep-margined Carboniferous (Bashkirian–Moscovian) carbonate platform slope, Sierra del Cuera, NW Spain: Sedimentary Geology, v. 166, p. 145-156. Cook, H.E., et al., 2000, Paleozoic carbonate platforms in the Bolshoi Karatau of southern Kazakstan; outcrop analogs for coeval carbonate reservoirs in the North Caspian Basin: AAPG Annual Convention Program New Orleans, Louisiana, Expanded Abstracts, p. 30. Cook, H.E., W. G. Zempolich, V.G. Zhemchuzhnikov, and J.J. Corboy, 1997, Inside Kazakstan: Cooperative oil and gas research: Geotimes, v. 42, no. 11, p. 16-20. Della Porta, G., J.A.M. Kenter, J. Bahamonde, A. Immenhauser, and E. Villa, 2003, Microbial boundstone dominated carbonate slope (Upper Carboniferous, N. Spain); microfacies, lithofacies distribution and stratal geometry: Facies, v. 49/1, p. 175-207. Garber, R.A., G.A. Grover, P.M. Harris, 1989, Geology of the Capitan shelf margin; subsurface data from the northern Delaware Basin, Subsurface and Outcrop Examination of the Capitan Shelf Margin, Northern Delaware Basin: SEPM Core Workshop, 13, p. 3-269. Harris, P.M., 2001, Geologic framework for the Tengiz and Korolev fields, Kazakhstan; Carboniferous isolated carbonate platforms: AAPG Bulletin, v. 85/4, p. 762-763. Harris, P.M., and A.H. Saller, 1999, Subsurface expression of the Capitan depositional system and implications for hydrocarbon reservoirs, northeastern Delaware Basin, in Geologic Framework of the Capitan Reef: SEPM Special Publication 65, p. 37-49. Kenter, J. A. M., P. M. Harris, J. F. Collins, L. J. Weber, G. Kuanysheva, and D. J. Fischer, 2006, Late Visean to Bashkirian platform cyclicity in the central Tengiz buildup, Precaspian Basin, Kazakhstan: Depositional evolution and reservoir development, in P. M. Harris and L. J. Weber, eds., Giant hydrocarbon reservoirs of the world; From rocks to reservoir characterization and modeling: AAPG Memoir 88/SEPM Special Publication, p. 7-54. Kenter, J.A.M., P.M. Harris, and G. Della-Porta, 2005, Steep microbial boundstone-dominated platform margins; examples and implications: Sedimentary Geology, v. 178/1-2, p. 5-30. Kenter, J.A.M., G.G. Drijkoningen, A. Filippidou, and H. Braaksma, 2003, Geology and Geophysics; Groundtruthing through natural seismic laboratories: AAPG Annual International Exhibition and Program Barcelona, Spain, September 21-24, p. 49. Verwer, K., J.A.M. Kenter, B. Maathuis and G. Della Porta, 2004, Stratal patterns and lithofacies of an intact seismic-scale Carboniferous carbonate platform (Asturias, N.W. Spain); A virtual outcrop model, in Geological Prior Information; Informing Science and Engineering: Geological Society (London), Special Publication, 239, p. 29-41.
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