Abstract: Diagenesis, Deformation, and Fluid Flow in the Monterey Formation of Coastal California

EICHHUBL, PETER, Stanford University, Stanford, CA; andRICHARD J. BEHL, California State University, Long Beach, CA

The Miocene Monterey Formation, a siliceous, organic-rich hemipelagic mudstone sequence deposited across the Neogene California margin, underwent complex sequences of regional and localized diagenetic alteration. Diagenesis accompanied multiple stages of pore fluid expulsion during burial and differential exhumation of basin flanks. The most significant alteration reactions are dissolution and reprecipitation of biogenic opal-A, metastable opal-CT, and stable quartz, the formation and recrystallization of authigenic dolomite, transformation of smectite to illite, and the diagenesis and catagenesis of organic matter. Alteration is controlled by initial sediment composition, permeability, and porosity, and by development of secondary fluid flow pathways. Diagenetic alteration involving silica and carbonate affects not only the matrix permeability but also the brittle response to deformation, thus focusing pore fluid expulsion along brecciated beds, faults, and fractures. An increase in fluid focusing with diagenetic maturation is accompanied by an increase in scale of mass transport. Modern formation fluid, sampled from offshore oil wells, is diagenetically altered connate water. The system locally evolved from 'closed system' or microscale mass transport in mudstone, diatomite, and porcelanite to bedding-scale flow during chert formation and deformation to large-scale intra- formational fluid expulsion during hydrocarbon maturation and migration. The latest stages of fluid expulsion during exhumation of the basin flanks produced massive carbonate cementation along faults and associated fracture systems. These faults channeled fluid, originating in structural lows and migrating parallel to the formation for several kilometers, across the stratigraphy to higher structural levels and to the surface. Fluid expulsion is apparently driven by porosity collapse which is linked to the dissolution/reprecipitation of loadsupporting silica phases, and to the catagenesis of organic matter.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California