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AAPG Bulletin, V.
Near-salt deformation in La Popa basin, Mexico, and the northern Gulf of Mexico: A general model for passive diapirism
1Rowan Consulting, Inc., 850 8th Street, Boulder, Colorado 80302; email: [email protected]
2Institute for Tectonic Studies, New Mexico State University, Las Cruces, New Mexico 88003
3Institute for Tectonic Studies, New Mexico State University, Las Cruces, New Mexico 88003
4Geo-Logic Systems, LLC, Boulder, Colorado
Mark Rowan has degrees from Caltech, Berkeley, and the University of Colorado. He was an exploration geologist with Sohio, a consultant with Geo-Logic Systems and Alastair Beach Associates, and a research professor at the University of Colorado. He is now an independent consultant specializing in structural geology and salt tectonics and is the instructor for AAPG's salt tectonics school.
Tim Lawton received degrees from the University of California, Santa Cruz (1973), Stanford (1980), and the University of Arizona (1983). He worked as a development geologist and research sedimentologist for Sohio Petroleum and is currently a professor in the Department of Geological Sciences at New Mexico State University. He specializes in clastic depositional systems, stratigraphy, and tectonic analysis.
Kate Giles graduated from the University of Wisconsin, Madison, the University of Iowa, and the University of Arizona. She worked at Exxon Production Research and is currently an associate professor at New Mexico State University and director of the Institute of Tectonic Studies. She specializes in carbonate depositional systems, sequence stratigraphy, and sedimentation as it relates to tectonics.
Bob Ratliff received his Ph.D. from the University of Colorado. He worked for Geo-Logic Systems (GLS) and CogniSeis Development, where he was responsible for the technical content and marketing of the GeoSec and GeoStrat software packages. He rejoined GLS and is currently director of Product Development and active in research on the geometry, kinematics, and interpretation of rock deformation.
We thank the sponsors of the La Popa basin Joint Industry Consortium for funding: Amerada Hess, Anadarko, BP, ChevronTexaco, Conoco, Devon, ENI-Agip, Enterprise, ExxonMobil, Phillips, and Shell. Discussions with their staff as well as B. Vendeville, F. Vega-Vera, I. Davison, and D. Schultz-Ela were very helpful, and M. Jackson, D. Schultz-Ela, B. Trudgill, and K. Bowker suggested revisions that helped clarify our message. This research could not have been completed without the hard work of graduate students at New Mexico State University: J. Aschoff, S. Furgal, K. Graff, K. Hon, L. Hunnicutt, D. Mercer, D. Shelley, and A. Weislogel. F. Diegel (Shell) and R. Hobbs and L. Liro (Veritas) assisted with permission to show seismic data, and WesternGeco and Veritas kindly allowed the data to be shown. Finally, we thank Geo-Logic Systems, LLC for use of the LithoTect restoration software.
Strata adjacent to exposed diapirs in La Popa basin, northeastern Mexico, comprise stacked halokinetic sequences consisting of unconformity-bounded packages of thinned and rotated strata cut by radial faults. Deformation results from shallow drape folding over the flanks of the rising diapirs and not from deep drag folding in diapir-peripheral shear zones. Subsurface analogs from the Gulf of Mexico have diapir-flanking geometries ranging from similar, wide zones of upturned and thinned strata to undeformed, constant-thickness strata. Subhorizontal salt tongues display little subsalt deformation and thinning.
We propose a general model for passive diapirism and flank deformation that includes (1) gradually varying salt-flow rates, (2) superposed episodic sedimentation that results in changing bathymetric relief, (3) rotation of near-surface strata as salt inflates relative to the adjacent basin, (4) failure and erosion of strata in the steepening bathymetric halo, and (5) bedding-parallel slip surfaces that converge on unconformities and onlap surfaces. A primary factor influencing flank geometries is the width of the bathymetric high extending beyond the diapir edge. This is largely dependent on the thickness of the halokinetic sequence onlapping the diapir, which in turn is controlled mostly by the interplay between salt inflation/deflation rates and sedimentation rates. Other factors include the amount of concurrent shortening, which produces a wider but less intense zone of deformation, and the position on the scarp of salt breakout and extrusion.
Our model is important for exploration and production in diapir-flank and subsalt settings because of its implications for trap size and geometry, reservoir distribution, trap compartmentalization and pressure seals, and hydrocarbon charge. It can help in explaining complex and enigmatic well data and in better assessing risk in areas of poor seismic imaging.
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