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

Faulting controls fluid migration within transpressive fault-propagation fo lds 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 int o the cores of anticlines, where the fluid spreads laterally outward into l ower Tertiary strata by now through cross faults and permeable beds. Precip itation of zeolite and carbonate cement and veins reduces the permeability of folded bedding and faults, Zeolite minerals are formed by chemical react ions between Na-Ca-Cl brine acid 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 an d Mesozoic strata in some anticlines. High-pressure fluid is contained with in porous beds that are intercalated with strata cemented by carbonate and zeolite minerals. Zeolite and carbonate cemented beds retard the dissipatio n 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 migrati on of fluid out of the basement suggests that volumetric strain related to deformation is most important, but may be augmented by dynamo-thermal metam orphism, sedimentary compaction, alteration of organic-rich rock and hydroc arbons, and possibly glacial loading.