GEOHORIZONS AUTHORS Erwin W. Adams Department of Earth, At- mospheric, and Planetary Sciences, Massachu- Digital characterization of setts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139; present address: Shell International Exploration and Pro- thrombolite-stromatolite reef duction B.V., Kessler Park 1,2288 GS Rijswijk, Netherlands; [email protected] distribution in a carbonate ramp Erwin Adams received his M.Sc. degree (1996) and his Ph.D. (2001) in geology from the Vrije Univer- system (terminal Proterozoic, siteit Amsterdam, Netherlands. He worked for three years at the Massachusetts Institute of Technology (MIT), deploying digital methods for mapping and Nama Group, Namibia) modeling reservoir-scale carbonate outcrops in the terminal Proterozoic of the Nama Group, Namibia, Erwin W. Adams, John P. Grotzinger, Wesley A. Watters, and the Devonian of the Canning basin, Western Stefan Schro¨ der, David S. McCormick, and Australia. Erwin joined the Carbonate Team at Shell Hisham A. Al-Siyabi in 2004. John P. Grotzinger DepartmentofEarth, Atmospheric, and Planetary Sciences, Massachu- setts Institute of Technology, 77 Massachusetts ABSTRACT Avenue, Cambridge, Massachusetts 02139; [email protected] The stratigraphic architecture of a terminal Proterozoic carbonate ramp system (ca. 550 Ma, Nama Group, Namibia) was mapped John Grotzinger is the Robert Shrock Professor of Geology at MIT. He received degrees in geology quantitatively with digital surveying technologies. The carbonate from Hobart College (B.Sc.), the University of ramp consists of a shoaling-upward ramp sequence in which Montana (M.Sc.), and Virginia Tech (Ph.D.). His re- thrombolite-stromatolite reefs developed at several stratigraphic search focuses on field-based outcrop studies of levels. The reefs are associated with grainstone and heterolithic reservoir-scale heterogeneity, evaluation of biogeo- facies and exhibit diverse geometries and dimensions related to the chemical events at the Precambrian–Cambrian position in the sequence-stratigraphic framework. Laterally exten- boundary, and robotic investigations of the strat- igraphic record of Mars. sive reefs with a tabular geometry developed when accommodation was relatively low, whereas discontinuous oblate dome-shaped reefs Wesley A. Watters Department of Earth, developed during times when accommodation space was relatively Atmospheric, and Planetary Sciences, Massachu- high. Collecting sedimentological and stratigraphic data digitally in setts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139; an extensive canyon system allowed a comprehensive documenta- [email protected] tion of the three-dimensional (3-D) architecture and dimensions Wesley A. Watters is a graduate student in geophys- of the reefal buildups. Both deterministic and stochastic methods ics at MIT. He studies the effects of large impacts were used to extend outcrop observations to construct 3-D models on planetary evolution and is a member of the Mars that honor the observed stratigraphy. In particular, the accuracy Exploration Rover Athena Science Team. He also with which dimensions of reefal buildups can be measured is criti- works on problems relating to the morphometry cally important in the statistical modeling of the dome-shaped and morphogenesis of stromatolites and early skele- togenous metazoa. buildups. Calculations and corrections can be applied directly to the digital data set and serve as input during model building. The final Stefan Schro¨der Department of Earth, Atmo- 3-D model faithfully reproduces the outcrop distribution of facies spheric, and Planetary Sciences, Massachusetts and geological objects and has a high spatial resolution, compared Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139; present address: Department of Geology, University of Johannesburg, Auckland Park 2006, P.O. Box 524, Johannesburg, Republic of South Africa; [email protected] Copyright #2005. The American Association of Petroleum Geologists. All rights reserved. Stefan Schro¨der received geology degrees from Manuscript received January 7, 2005; provisional acceptance March 16, 2005; revised manuscript the Universities of Wu¨rzburg, Germany (M.Sc.), received June 14, 2005; final acceptance June 16, 2005. and Bern, Switzerland (Ph.D.). Since then, he has DOI:10.1306/06160505005 AAPG Bulletin, v. 89, no. 10 (October 2005), pp. 1293–1318 1293 worked at MIT on Neoproterozoic reservoir rocks with petroleum industry reservoir models. The organization of the in Namibia and Oman. He is currently taking a post- reefal buildups in the stratigraphic framework has direct impli- doctoral study at the University of Johannesburg cations for reservoir continuity and connectivity in analogous set- and studies Paleoproterozoic carbonates together with Nicolas Beukes. His research focuses on sedi- tings. The digital characterization and 3-D outcrop models present- mentary processes and environmental factors gov- ed in this article can be subsequently used to condition dynamic erning sedimentation in the Precambrian. reservoir-simulation modeling of geologically similar areas. David S. McCormick Schlumberger-Doll Re- search, 320 Bent Street, Cambridge, Massachusetts 02141; [email protected] INTRODUCTION David McCormick is a senior research scientist and program manager at Schlumberger-Doll Research. To better understand the spatial heterogeneity in the properties He received geology degrees in sedimentology from that determine fluid flow performance of petroleum and ground- Dartmouth College (B.A.), Columbia University water reservoirs, it is instructive to create three-dimensional (3-D) (M.A.), and MIT (Ph.D.). Before Schlumberger, he geologic models and to use these static models as input for dy- worked at Chevron Petroleum Technology Company. His main interests lie in digital mapping and quan- namic reservoir modeling (Weber, 1986; Kerans and Tinker, 1997; tification of geology and outcrop analogs for reser- Gro¨tsch and Mercadier, 1999; Lucia, 1999; Gro¨tsch et al., 2003; voir characterization and modeling. Larue and Legarre, 2004). Outcrop models are of particular inter- est because they include observations on many scales and in 3-D Hisham A. Al-Siyabi Shell Exploration and Production Company, One Shell Square, P.O. Box (White and Barton, 1999; Willis and White, 2000; Li and White, 61933, New Orleans, Louisiana 70161; 2003). However, it is commonly difficult or impossible to obtain [email protected] 3-D quantitative data from mountain-size outcrops. Carbonate Hisham A. Al-Siyabi holds an M.Sc. (1994) degree ramps are a good example. They have received a great deal of at- and a Ph.D. (1998) from the Colorado School of tention in the last decade, and much is known about their tectonic Mines. Hisham joined Petroleum Development Oman setting, sequence-stratigraphic development, hydrodynamics, and in 1999, and since 2001 has worked as a geologist sedimentary facies organization (Burchette and Wright, 1992; Tucker and seismic interpreter on the South Oman Explo- et al., 1993; Wright and Burchette, 1998). However, with widths ration Team, working exclusively on the terminal Proterozoic intrasalt Ara stringers. In 2005, Hisham of approximately 50–200 km (31–124 mi), lengths of easily more joined Shell Exploration and Production Company than 200 km (124 mi), but a thickness of a few hundreds of meters, as an exploration geologist. carbonate ramps commonly are too extensive and relatively thin for quantitative geometric analyses (Burchette and Wright, 1992). However, geologic outcrop studies can drastically be improved ACKNOWLEDGEMENTS using digital data acquisition methods such as differential global We are grateful to Marianne and Rob Field for their positioning systems or terrestrial scanning lidar to map quantita- hospitality and access to the Zebra River farm. tively large-scale outcrops (e.g., Adams et al., 2004; Verwer et al., We express many thanks to the digital geology 2004; Bellian et al., 2005). Two of the advantages to digital data mapping crew of Zebra River: Abdullah Al-Habsy, Rashid Al-Hashimi, Rashid Al-Hinai, Omar Al- collection (Kramer, 1998; Thurmond et al., 1999; Wolf, 2002) are Ja’aidi, Issa Al-Mazroui, Hamad Al-Shuaily, Aus the ability to rapidly visualize and analyze digital data in 3-D in the Al-Tawil (including photography), Joachim Amthor, field. In addition, the collected spatial coordinates can be tagged Tony Dickson (including photography), Leon Hoff- with additional geologic information (e.g., lithology or sedimentary mann, Marc Newell, Mia van Steenwinkel (in- facies, fossil type, stratigraphic-surface type, etc.), which facilitates cluding photography), and Rachel Wood (including photography). Logistical support was provided by building 3-D data sets with geological significance. For geologists the Geological Survey of Namibia. We also thank who work with subsurface data and want to hierarchically model Wim Dewulf for logistical support. Support for this geological bodies in large-scale reservoir models (Dreyer et al., project was provided by National Science Foundation 1993; Geehan and Underwood, 1993; White and Willis, 2000), Grants EAR-9904298 and EAR-0001018, Schlum- digital data collection allows one to quantify accurately and rapidly berger Oilfield Research, and Petroleum Develop- length scales of, for example, sedimentary bodies. Furthermore, ment Oman. Reviews from Paul M. Harris, William C. Parcell, and Peter D. Warwick helped to improve