A preliminary study of DSDP cores from deep-sea fan deposits off southern California and Baja California indicates that sandstone turbidites and interbedded shales can be rapidly and extensively altered at shallow burial depths. Their porosity and permeability are reduced or totally occluded by the early (commonly pre-compaction) formation of authigenic smectite and carbonate cements. As a result, many turbidite sands become tightly cemented at burial depths shallower than 400 m and thus lose their potential as petroleum reservoirs.

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Authigenic smectite, recently reported to occur over large areas of the deep sea, forms in fan deposits as young as 0.4 m.y. at temperatures less than 10°C and has been found at burial depths less than 10 m. Scanning electron microscopy shows that crystallization of authigenic smectite is related to the progressive downhole dissolution of biogenic silica and volcanic glass. Sandstone cemented with only a few percent smectite retains high porosities (25 to 30%) but permeabilities are greatly reduced (< 100 md).

Ancient sandstone turbidites commonly have their original interparticle pore space filled with carbonate cement. Textural evidence (few grain contacts, low grain density) indicates that, as in the DSDP cores, much carbonate cement in ancient sandstone turbidites is the product of an early pre-compaction cementation. At greater depths, these relatively uncompacted calcite-supported sandstones may once again become prospective petroleum reservoirs as decarbonatization generates secondary porosity.

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