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Tectonics, fluid migration, and fluid pressure in a deformed forearc basin, Cook Inlet, Alaska

¹ Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA

Faulting controls fluid migration within transpressive fault-propagation folds in the Cook Inlet forearc basin of south-central Alaska. Na-Ca-Cl brine migrates out of Mesozoic rocks through reverse and oblique-slip faults into the cores of anticlines, where the fluid spreads laterally outward into lower Tertiary strata by flow through cross faults and permeable beds. Precipitation of zeolite and carbonate cement and veins reduces the permeability of folded bedding and faults. Zeolite minerals are formed by chemical reactions between Na-Ca-Cl brine and sedimentary rocks. Carbonate minerals are precipitated when Na-HCO₃ connate fluid in the Tertiary section reacts with rocks during diagenesis, and by mixing of migrated Na-Ca-Cl brine with the Na-HCO₃ pore fluid. Carbonate cement is also precipitated by fluctuations in PCO₂ during faulting and jointing.

High fluid pressure is encountered while drilling through lower Tertiary and Mesozoic strata in some anticlines. High-pressure fluid is contained within porous beds that are intercalated with strata cemented by carbonate and zeolite minerals. Zeolite and carbonate cemented beds retard the dissipation of high fluid pressure, and channel fluid flow parallel to bedding within the anticlines. High fluid pressure may be generated by several processes, acting either alone or together. The evidence for fault-controlled migration of fluid out of the basement suggests that volumetric strain related to deformation is most important, but may be augmented by dynamo-thermal metamorphism, sedimentary compaction, alteration of organic-rich rock and hydrocarbons, and possibly glacial loading.

Key Words: fluid pressure • forearc basins • petroleum geology • structural geology • tectonics

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