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Russell Davies1, Linji An2, Anne Mathis3, Paul Jones4, Carol Cornette3
(1) ARCO, Plano, TX
(2) ARCO
(3) Vastar, Houston, TX
(4) Vastar

Abstract: Fault seal analysis, SMI 36 field, Gulf of Mexico

SMI 36 is a predominantly gas field on the Gulf of Mexico shelf. The principal trap is up-dip fault closure in the hangingwall of a large down-to-the north normal fault. The reservoir is cut by a second normal fault that separates the reservoir into two compartments.

Juxtaposition (Allan) diagrams of the stratigraphy projected onto this intra-reservoir fault surface show several intervals of sand-on-sand juxtaposition. In the lower-most part of the section the MA sand in the footwall is juxtaposed against the LN and LP sands in the hangingwall. Differential fluid contacts and pressures in these sands across this fault suggest that the fault is sealing. We hypothesize shale smear or gouge as the mechanism for fault seal. Estimates of shale percent along the fault plane between the sand intervals range between 45 and 90%.

Mudweights and limited reservoir pressure data show that the deep section is abnormally pressured and that a water pressure differential of approximately 500 psi is recorded across the fault. The greatest reservoir pressure difference across the fault occurs between the MA sand in the footwall and the upper LN sand in the hangingwall. A production pressure difference of 3000 psi is recorded across the fault for seal break-down between these sands. Shale along the fault is a baffle to water flow and a capillary seal to hydrocarbons. The shale percentages on the faults between these sands and the estimated pressure differences serve as a calibration for fault seal capacity.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana