Sedimentology and Reservoir Potential of Matilija Sandstone: An Eocene Sand-Rich Deep-Sea Fan and Shallow-Marine Complex, California

Abstract

The Matilija Sandstone of the Santa Ynez Mountains of southern California records a major regressive event in the Eocene Santa Ynez basin in which turbidites were deposited and subsequently covered by shallow-marine deposits. Despite thick, favorable source beds and generally good initial reservoir characteristics, the Matilija Sandstone is not a productive unit in the basin. Low reservoir permeability (generally less than 1 md) and porosity (1 to 10%) are interpreted to be the result of early compaction, cementation, and diagenesis. The lower part of the Matilija Sandstone is a 1,300 ft (400 m) thick sand-rich deep-sea fan complex, which overlies basin plain turbidite deposits of the Juncal Formation. The Matilija Sandstone consists of anastomosing outer fan lobes, lobe fringe, and interlobe deposits overlain by channelized middle and inner fan deposits. The upper Matilija Sandstone consists of shallow-marine cross-bedded sandstone, red beds, and carbonate-evaporite-lignite sequences which overlie the turbidites. Deposition of the Matilija Sandstone was terminated by a rapid basin deepening event which culminated in deposition of basin plain, slope, and turbidite deposits of the Cozy Dell Formation. The lower Matilija Sandstone deep-sea fan deposits have a high sandstone to shale ratio (4:1 to 10:1) and consist of submature arkoses (Folk, 1980). The depositional lobes consist of facies B, C, and D turbidites and a channelized sequence of facies A and B turbidites. The average sandstone is medium grained, moderately sorted, subangular to subrounded, and massive, containing 40% quartz, 35% feldspar (about equal amounts of plagioclase and potassium feldspars), 10% lithic fragments (mostly volcanic and metamorphic types), and minor amounts of detrital clay (illite), mica, chert, and heavy minerals. Early compaction, carbonate cementation, and authigenic pore-lining and pore-filling chlorite, albite, potassium feldspar, quartz, and localized laumontite have reduced the initial porosit and permeability. The only remaining porosity is secondary, resulting from the dissolution of detrital feldspars, and minor fracture porosity.