The Matilija Sandstone Member, exposed in the Santa Ynez Mountains, California, records a major regressive event in the Eocene Santa Ynez basin in which turbidites were deposited in the basin and subsequently covered by shallow-marine complexes. Despite thick favorable source beds and generally good initial reservoir characteristics, the Matilija sandstone is not a productive unit in the basin. Lowered reservoir rock permeability (< 1 md) and porosity (0-10%) are due to early compaction, cementation, and diagenesis.

The lower part of the Matilija sandstone is a 700-m-thick sand-rich deep-sea fan complex which overlies basin plain and turbidite deposits (Juncal shale). The Matilija sandstone consists of anastomosing outer-fan depositional lobes overlain by channelized middle- and inner-fan deposits. Cross-bedded sandstone, red-bed, and carbonate-evaporite sequences overlie the turbidites. Matilija sandstone deposition closed with rapid transgression which culminated in the deposition of basin plain and turbidite deposits (Cozy Dell shale).

The Matilija sandstone lower deep-sea fan complex has a high sandstone and shale ratio (4:1) and consists of submature arkoses of facies B. The average sandstone is medium grained, moderately sorted, subangular, massive, and contains 40% quartz, 35% feldspar (about equal amounts of potassium and plagioclase feldspars), 10% lithic fragments (mostly granitic and volcanic types), and smaller amounts of mica, chert, and heavy minerals. Early compaction, carbonate cementation, and authigenic pore-lining chlorite, albite, and quartz have reduced the initial porosity and permeability. Minor secondary fractures are the only effective porosity in these rocks now.

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