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Mechanics of low-relief detachment folding in the Bajiaochang field, Sichuan Basin, China

¹ Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, Massachusetts 02138; andreas_plesch{at}harvard.edu
² Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, Massachusetts 02138
³ Burlington Resources, ConocoPhillips, 600 North Dairy Ashford, Houston, Texas 77079

Andreas Plesch is a research associate in the Structural Geology and Earth Resources Group in the Earth and Planetary Science Department at Harvard University. He received an M.S. degree in geology from State University of New York, Albany (1994), and his Ph.D. from the Free University Berlin, Germany, (1999). His research interests involve three-dimensional modeling and analysis of contractional structures on the reservoir to mountain belt scale using a variety of methods with a focus on quantitative aspects and interpretation of seismic reflection data. He has been able to work on structures worldwide, commonly in cooperation with industry partners.

John H. Shaw is the Harry C. Dudley Professor of Structural and Economic Geology at Harvard University and leads an active research program in structural geology and geophysics, with emphasis on petroleum exploration and production methods. He received a Ph.D. from Princeton University in structural geology and applied geophysics and was employed as a senior research geoscientist at Texaco's Exploration and Production Technology Department in Houston, Texas. Shaw's research interests include complex trap and reservoir characterization in fold and thrust belts and deep-water passive margins. He heads the Structural Geology and Earth Resources Program at Harvard, an industry-academic partnership that supports student research in petroleum systems.

David Kronman is a principal geologist with ConocoPhillips Global New Ventures Exploration in Houston, Texas. He earned his B.S. degree in geology from the State University of New York at Cortland (1980) and his M.S. degree in geology from Vanderbilt University (1982). David has more than 25 years domestic and international experience with Shell Oil Company, EOG Resources, and Burlington Resources (prior to its merger with ConocoPhillips). For the past 17 years, his primary focus has been on integrated exploitation analysis, synthesis of basins in compressional regimes, unconventional resources, and stratigraphic modeling.

Using three-dimensional (3-D) seismic reflection data, we apply new methods of two-dimensional (2-D) and 3-D structural restoration based on mechanical constraints to gain insights into the development of the Bajiaochang anticline, Sichuan Basin, China. This structure forms the trap for the Bajiaochang field, which is reported to contain substantial gas reserves within thick, Upper Triassic delta-front and lakeshore facies siliciclastics. The basal detachment lies in a Middle Triassic evaporite unit, above which layers are folded and faulted to various intensities based on their mechanical strengths. The new restoration methods enable us to test a variety of fault-displacement directions and to resolve that the Bajiaochang structure grew primarily by dip slip and contractional folding. Moreover, we quantify the styles and amounts of shortening within various tectonic levels of the structure. Large amounts of shortening and ductile thickening at the basal detachment level are consistent with general detachment fold models. However, we demonstrate that slip on the basal detachment is much larger than would be predicted by these models, implying that substantial slip on the basal detachment extends beyond the structure into the foreland. Moreover, structural thickening in the core of the fold is localized by a series of small ramps in the basal detachment that produce structural imbrication. This implies that the Bajiaochang detachment fold grew by a mechanism akin to duplexes, which has important implications for the manner in which shortening and strain are partitioned within different mechanical layers of the fold.