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The Oligocene Sespe Formation has undergone differential burial resulting from complex tectonic evolution of the Ventura Basin. Samples collected from more than 5 km of burial depth allow documentation of the origin and depth-dependent diagenesis of clay minerals in texturally variable sandstones.

Clay minerals can be distinguished as depositional matrix, infiltrated clays, or authigenic clays on the basis of pétrographie and SEM criteria. "Clean" (<20% ductile components) and "dirty" (>20% ductile components) sandstones reacted differently during burial diagenesis, resulting in contrasting clay-mineral assemblages. In "clean" sandstones, the clay-mineral assemblage changes from smectite plus subordinate kaolinite in shallow samples to chlorite in deep samples, suggesting that the major clay-mineral reaction is the transformation of smectite to chlorite. In "dirty" sandstones, the clay-mineral assemblage changes from smectite plus minor kaolinite at shallow depths, to smectite plus chlorite at intermediate depths, to mixed-layer illite/smectite plus minor chlorite at greatest depths. Formation of illite at the expense of smectite appears to be the predominant clay-mineral reaction. The origin of chlorite and the role that it plays in illite formation are unclear. In both types of sandstones, the presence of kaolinite in shallow samples may reflect meteoric diagenesis following tectonic uplift.

Original texture significantly influenced clay-mineral diagenesis in Sespe sandstones. Low-permeability "dirty" sandstones behaved as closed diagenetic systems, whereas higher permeability "clean" sandstones behaved as relatively open diagenetic systems.

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