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PSMicrobial Boundstone Slope Shedding – A Model for Carbonate Platform Growth*

By

Jeroen A.M. Kenter1, Paul M. (Mitch) Harris2, and Giovanna Della Porta3

 

Search and Discovery Article #40296 (2008)

Posted August 14, 2008

 

*Adapted from poster presentation at AAPG International Conference & Exhibition, Paris, France, September 11-14, 2005. See companion article, “Microbial and Cement Boundstone-Dominated Flanks (and Reservoirs) of an Isolated Carbonate Platform,” Search and Discovery Article #40297 (2008).

Click to view list of articles adapted from presentations by P.M. (Mitch) Harris or by his co-workers and him at AAPG meetings from 2000 to 2008.

 

1 Vrije Universiteit, De Boelelaan 1085, Amsterdam, 1081 HV Netherlands; currently ETC, Chevron, Voorburg, Netherlands ([email protected])

2 Chevron Energy Technology Company, San Ramon, California, USA ([email protected])

3 Universität Potsdam, Potsdam, Germany; currently Cardiff University, Cardiff, UK ([email protected])

 

Abstract

Characteristics of two prograding steep, high-relief margins fronting deep basins provide a depositional model which may apply elsewhere. Seismic and well data from Tengiz, one of the larger fields in the Pricaspian Basin characterized by Latest Visean and Serpukhovian progradation, corroborate outcrop patterns of Serpukhovian to Moscovian progradation in Asturias of northern Spain. These margins show progradation of up to 5 km and more than 10 km, respectively, despite the high-relief (up to 600 m) and their steep (~20-32°) nature.

Both examples share a highly productive microbial boundstone slope extending from the platform break to nearly 300 m (or more) 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 and cement boundstone “factory” 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. Remarkable observations are the contrasts with the Bahamian highstand shedding depositional model, little control by fluctuations in sea level or by paleo-wind directions due to their self-nourishing nature, and the accretion rates of in-situ boundstone.

This new model of “slope” shedding has implications for slope readjustment, slope architecture, sequence stratigraphic models, reservoir characterization, and reservoir modeling, especially given that the isotropic character of microbial boundstone will reduce the potential for coherent seismic reflections to develop and possibly invoke, under certain stress regimes, shattering and fracturing, thereby generating significant non-matrix permeability.

 

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Selected Figures

Tengiz field, western Kazahstan. The Late Visean and Serpukhovian is characterized by several kilometers of platform progradation, followed by drowning and termination of the platform in the early Bashkirian. Platform to basin relief approached 1.5 km. One of the uncertainties of the Tengiz buildup was the nature of the high rising Late Visean to Serpukhovian flanks or slopes (0-700 m below the platform break) that comprise some 25% of the reservoir volume. Detailed observations and analog studies in northern Spain confirmed the presence of a “deep” microbial cement “reef” that extends from sea level down to more than 300 m below sea level.

Asturias outcrops, northern Spain, as an analog. Upper to middle slope lithofacies are dominated by: 1) massive "cabbage-shaped" to "wave laminated" micrite-cement boundstone with dominant marine cements (25-60%) in primary cm-dm irregular-shaped voids with minor skeletal grains; 2) micrite-cement boundstone with minor "growth" structures, few primary voids, minor cement (<25%) and abundant platform-derived skeletal grains and; 3) mosaic breccia of in-place collapsed boundstone. Microfilamental fossils are abundant and interpreted as cyanobacterial species. It is most likely that microbes have played a role in the genesis of the peloidal and accretionary fabrics and mediated the growth of marine cements.

Slope shedding model versus highstand shedding model (modified after Della Porta et al., 2003).

 

Implications of Microbial Boundstone Slopes

In conclusion, the role of microbes in the evolution of “reefal“ margins has been largely neglected but may present a depositional system with different rules controlling its spatial distribution and response to climatic and eustatic sea level changes and resulting reservoir properties. Both Tengiz and Asturias share a highly productive microbial cement boundstone factory extending from the platform break to nearly 300 m of water 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. This contrasts sharply with the Bahamian highstand shedding concept that is based on domination of sediment supply during highstands of sea level only. Rapid in-situ lithification of the boundstone provides stability to the steep slopes, but also leads to readjustment through shearing and avalanching. Remarkable observations to both Tengiz and Asturias are the rates of in-situ boundstone growth (and as result progradation rates) that equal those of Recent coralgal (skeletal) reef systems and the asymmetric distribution not related to paleo-wind directions. 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 many other older, and younger, margins.

In conclusion, the role of microbes in the evolution of “reefal“ margins has been largely neglected but may present a depositional system with different rules controlling its spatial distribution and response to climatic and eustatic sea level changes and resulting reservoir properties.

 

References

Della Porta, D., 2003, Microbial boundstone dominated carbonate slope (Upper Carboniferous, N. Spain); Microfacies, lithofacies distribution and stratal geometry: Facies, v. 49/1, p.175-207.

Della Porta, D., J.A.M. Kenter, and J.R. Bahamonde, 2004, Depositional facies and stratal geometry of an Upper Carboniferous prograding and aggrading high-relief carbonate platform (Cantabrian Mountains, N. Spain): Sedimentology, v. 51, p. 267-295.

Eberli, G.P., C.G. Kendall, P. Moore, G.L.Whittle, R. Cannon, 1994. Testing a seismic interpretation of Great Bahama Bank with a computer simulation. AAPG Bulletin, v.78, p. 981-1004.

Gradstein, F.M., J.G. Ogg, and A.G. Smith, 2004, A geologic time scale: International Commission on Stratigraphy (ICS) under: www.stratigraphy.org.

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.

Kerans, C., and S.W. Tinker, 1999, Extrinsic stratigraphic controls on development of the Capitan reef complex: in Geologic Framework of the Capitan Reef, SEPM Special Publication 65, p. 15-36.

Lidz, B.H., C.D. Reich, and E.A. Shinn, 2003. Regional Quaternary submarine geomorphology in the Florida Keys. GSA Bulletin, v. 115, p. 45–866.

Schlager, W., 2000, Sedimentation rates and growth potential of tropical, cool-water and mud-mound carbonate systems: Carbonate Platform Systems-Components and Interactions, GS (London) Special Publication 178, p. 217-227.

Tinker, S.W., 1998, Shelf-to-basin facies distributions and sequence stratigraphy of a steep-rimmed carbonate margin; Capitan depositional system, McKittrick Canyon, New Mexico and Texas: Journal of Sedimentary Research, v. 68, p. 1146-1174.

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