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AAPG Bulletin, V.
Fault-seal analysis South Marsh Island 36 field, Gulf of Mexico
1Rock Deformation Research, USA Inc., P.O. Box 2998, McKinney, Texas, 75070-8998; email: rdaviesrdrusa.com
23708 Silverleaf Court, Flower Mound, Texas, 75018
3BP Exploration, 501 West Lake Park Blvd., Houston, Texas, 77079-2696
4Exxon Mobil Corporation, Houston, Texas
5BP Exploration, 501 West Lake Park Blvd., Houston, Texas, 77079-2696
Russell Davies is the U.S. operations manager and consulting structural geologist for Rock Deformation Research, Dallas, Texas. He received a Ph.D. in structural geology from Texas A&M University and has worked in the oil and gas industry for over 10 years in exploration and production with Shell and research on fault interpretation methods, structural styles, and seal analysis with ARCO.
Linji An is a structural geologist specializing in fault network studies, fault-seal analysis, and 3-D computer modeling of geological structures and has worked for System Technology Associates and ARCO. He has a Ph.D. in geophysics from the University of Southern California and is currently a senior software engineer for Sterling Commerce.
Paul Jones received a B.A. degree in geology from Northeastern University in 1980, and M.S. degree in geographics from Purdue University in 1982. He is currently a geologist in the Gulf of Mexico Deepwater Production Business Unit of BP working the Na Kika development project. He has worked as an exploration and development geophysicist for ARCO/Vastar/BP in Alaska, Gulf of Mexico Shelf, and Deepwater Gulf of Mexico.
Anne LeNoir Mathis earned her M.S. degree in geology from Louisiana State University in 1987. She worked for ARCO Oil and Gas and then Vastar Resources as an exploration/exploitation geologist working in south Texas for ExxonMobil Production located in Houston, Texas.
Carol Cornette is a reservoir engineer for BP America Production in Houston. She has also held various positions with ARCO Oil and Gas and Vastar Resources including operations/analytical engineering and reservoir log analysis. Carol holds a B.S. degree in chemistry from the University of Florida and a B.S. degree in chemical engineering from the University of Louisiana in Lafayette.
We thank the original partners in the field, Vastar, ARCO, Walter Oil and Gas, Santa Fe Snyder, IP Petroleum, and Howell Group, for access to data and permission to publish. Some of the company names have since changed as a result of mergers and acquisitions. Thanks to Alton Brown for helpful discussions and James Handschy and Tim Needham for their reviews.
SMI 36 is a predominantly gas field on the Gulf of Mexico shelf. The principal trap is updip fault closure in the hanging wall of a large down-to-the-north normal fault. A second normal fault, the intrareservoir fault, separates the reservoir into two compartments. Juxtaposition (Allan) diagrams of the stratigraphy projected onto this intrareservoir fault surface show several intervals of sand-on-sand juxtaposition. In the lowermost part of the section, the MA sand in the footwall is juxtaposed against the LN and LP sands in the hanging wall. Differential fluid contacts and pressures in these sands across this fault suggest that the fault is sealing. We hypothesize shale gouge as the mechanism for fault seal. Estimates of shale percent along the fault plane between the sand intervals range between 40 and 80%. Mud weights and limited reservoir pressure data show that the deep section is overpressured with a water pressure differential as much as 600 psi (4.13 106 Pa) across the fault. The cross-fault differences in the aquifer pressure partially seal the hydrocarbons in the hanging wall. The hydrocarbons in the footwall reservoir support a capillary pressure as much as 80 psi (5.51 105 Pa) for a shale gouge of 45%. Production pressure profiles record a cross-fault pressure difference of 3000 psi (2.06 107 Pa) across the fault before pressure support between the reservoirs suggesting seal “breakdown.”
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