Balanced cross sections

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Predicting the occurrence of oil and gas traps
Chapter	Exploring for structural traps
Author	R.A. Nelson, T.L. Patton, S. Serra
Link	Web page
Store	AAPG Store

Definition

Balanced cross sections honor all available data and are constructed and analyzed to ensure they are geometrically possible and geologically admissible, given reasonable assumptions about the predeformation setting of rocks and how rocks behave during deformation in a particular tectonic environment. “Balanced” refers to the basic assumption made in constructing these sections—that rock area (rock volume) does not change substantially as a result of deformation. Balanced cross sections are restorable. This means that, while maintaining constant area, the deformation displayed in a balanced cross section can be incrementally removed to yield a geologically plausible predeformation configuration.

The constant area (constant volume) assumption is generally valid for deformation that has occurred in upper crustal, nonmetamorphic settings, but there are important exceptions. For example, in some settings syntectonic deposition and compaction can result in substantial rock volume changes throughout the course of deformation. In these cases, approximations of the volume changes must be incorporated in the balancing process.

Balancing is an interative, trial-and-error process. If done manually, it is tedious and very time consuming. Computer programs greatly simplify the measurement and drafting aspects of cross section balancing. Some of these programs also incorporate functions and algorithms that permit some rudimentary 3-D balancing of structures.

Value of balancing

Balanced cross sections are not necessarily correct. However, the methodical scrutiny imposed by the balancing process highlights discrepancies in interpretations, points to the types of data or alternative schemes needed to resolve the discrepancies, and generally results in more carefully constructed, defensible, and explainable cross sections.

Further reading

• Balanced cross sections short course from AGU

Examples of use

• Buchanan, P. G., Nieuwland, D. A., eds., 1996, Modern Developments in Structural Interpretation, Validation and Modelling: Geological Society of London Special Publication 99, p. 369.
• Dahlstrom, C. D. A., 1969, Balanced cross sections: Canadian Journal of Earth Sciences, vol. 6, p. 743–757, DOI: 10.1139/e69-069.
• Mount, V. S., Suppe, J., Hook, S. C., 1990, A forward modeling strategy for balancing cross sections: AAPG Bulletin, vol. 74, p. 521–531.
• Moffat, D. T., Henage, L. F., Brash, R. A., Tauer, R. W., Harahap, B. H., 1991, Lengguru, Irian Jaya: prospect selection using field mapping, balanced cross sections, and gravity modeling: Proceedings of the Indonesian Petroleum Association's 20th Annual Convention, p. 85–106.
• White, N., Yielding, G., 1991, Calculating normal faults geometries at depth: theory and examples, in Roberts, A., M., Yielding, G., Freeman, B., eds., The Geometry of Normal Faults: Geological Society of London Special Publication 56, p. 251–260.
• Groshong, R. H., Jr., 1990, Unique determination of normal fault shape from hanging-wall bed geometry in detached half grabens: Ecologae Geologicae Helvetiae, vol. 83, p. 455–471.
• Gibbs, A. D., 1983, Balanced cross-section construction from seismic sections in areas of extensional tectonics: Journal of Structural Geology, vol. 5, p. 153–160, DOI: 10.1016/0191-8141(83)90040-8.
• Geiser, P. A., 1988, The role of kinematics in the construction and analysis of geological cross sections in deformed terranes, in Mitra, G., Wojtal, S., eds., Geometries and Mechanisms of Thrusting, with Special Reference to the Appalachians: Geological Society of America Special Paper 222, p. 47–76.
• Diegel, V. A., Karlo, J. F., Schuster, D. C., Shoup, R. C., Tauvers, P. R., 1995, Cenozoic structural evolution and tectono-stratigraphic framework of the northern Gulf Coast continental margin, in Jackson, M. P. A., Roberts, D. G., Snelson, S., eds., Salt Tectonics: A Global Perspective: AAPG Memoir 65, p. 109–151.
• Mitra, S., Namsom, J., 1989, Equal-area balancing: American Journal of Science, vol. 289, p. 563–599, DOI: 10.2475/ajs.289.5.563.
• Mitra, S., 1992, Balanced structural interpretations in fold and thrust belts, in Mitra, S., Fisher, G., W., eds., Structural Geology of Fold and Thrust Belts: Johns Hopkins University Press, Maryland, p. 53–77.
• Baby, P., Herail, G., Salinas, R., Sempere, T., 1992, Geometry and kinematic evolution of passive roof duplexes deduced from cross section balancing: example from the foreland thrust system of the southern Bolivian Subandean zone: Tectonics, vol. 11, p. 523–536, DOI: 10.1029/91TC03090.
• Buchanan, P. G., Bishop, D. J., Hood, D. N., 1996, Development of salt-related structures in the central North Sea: results from section balancing, in Alsop, G. I., Blun-dell, D. J., Davison, I., eds., Salt Tectonics: Geological Society of London Special Publication 100, p. 111–128.
• Rowan, M. G., Kligfield, R., 1989, Cross section restoration and balancing as aid to seismic interpretation in extensional terranes: AAPG Bulletin, vol. 73, p. 955–966.
• Rowan, M. G., 1993, A systematic technique for the sequential restoration of salt structures: Tectonophysics, vol. 228, p. 331–348, DOI: 10.1016/0040-1951(93)90347-M.
• Marshak, S., Woodward, N., 1988, Introduction to cross-section balancing in Marshak, S., Mitra, G., eds., Basic Methods of Structural Geology: Englewood Cliffs, New Jersey, Prentice Hall, p. 303–302.
• Hossack, J., 1995, Geometric rules of section balancing for salt structures, in Jackson, M., P., A., Roberts, D., G., Snelson, S., eds., Salt Tectonics: A Global Perspective: AAPG Memoir 65, p. 29–40.
• Rouby, D., Fossen, H., Cobbold, P. R., 1996, Extension, displacement, and block rotation in the larger Gullfaks area, northern North Sea: determined from map view restoration: AAPG Bulletin, vol. 80, p. 875–890.

See also

• Seismic data
• Tectonic modeling
• Dipmeter analysis
• Petrofabrics
• Fracture analysis
• Cross section

External links

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