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The AAPG/Datapages Combined Publications Database
Scanning electron microscopy shows that the formation of pore-filling authigenic minerals in the deep-water fan deposits known as the Stevens sandstone at Elk Hills field is related to the expulsion of ion-charged water from siliceous shale into adjacent sandstone bodies. Extensive authigenic clay, quartz, and carbonate cements occur in the sandstone reservoirs within the first few meters of the upper or lower contact with a thick shale. All detectable authigenic phases in the Stevens sandstone, including kaolinite, quartz, calcite, ferroan dolomite, iron-rich smectite, and albite, are common to both sandstone and shale units. Cementation is most rapid and extensive in the thin sandstone beds typical of outer fan, levee, or overbank deposits.
Although many low permeability sandstone samples studied appear to contain a high percentage of detrital matrix, SEM examination shows that the "matrix" is actually a dense aggregate of authigenic kaolinite (euhedral booklets up to 30µ). Authigenic kaolinite, which is commonly intermixed with authigenic quartz, calcite, and ferroan dolomite, occurs in both pore lining and pore bridging forms. Parts of the Stevens sandstone adjacent to thick shale, or below the oil-water contact, have authigenic albite and iron-rich smectite intermixed with the other cements. Permeabilities in the albite-smectite cemented zones are commonly less than 0.3 md.
Petroleum exploitation in zones with low sand to shale ratios and sandstone sequences less than 15 m thick may be difficult in the Stevens sandstone. Thin permeability zones, left open after the initial injection of pore fluids, commonly are occluded by later carbonate cementation. Pore-filling cements related to shale dewatering, if consistent across the basin, may limit significant finds to inner and middle fan depositional regimes in the Stevens sandstone.
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