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Synchronous sandstones associated with the lacustrine Eocene Green River Formation of Wyoming from three distinct provenance areas and two general depositional environments were studied to determine provenance and depositional controls on diagenesis.
Quartzitic fluvial sands in an area dominated by ground water flow were cemented early with calcite. Marginal lacustrine sands of the same detrital mineralogy became cemented with calcite and ankerite. Lacustrine sandstones with a micritic matrix contain no porosity, whereas fluvial sandstones with well developed secondary porosity registered porosities as high as 25% and permeabilities above 2,300 md.
Arkosic sands deposited from rivers meandering across a fluvial floodplain developed mostly authigenic clay cement from the dissolution of feldspars. In contrast, arkosic beach sands intercalated with lacustrine carbonates were cemented with ankerite.
Volcanogenic fluvial sands were cemented with zeolite and clay minerals, reducing the effective porosity to zero. On the other hand, environmentally controlled carbonate forms the principal cement of marginal lucustrine volcanogenic sandstones. Cementation by carbonate halted the degradation process of volcanic rock fragments to pore-filling clays and zeolites. Furthermore, some of these rocks exhibit excellent secondary porosity and permeability.
In conclusion, provenance set the stage for diagenesis by providing the raw materials for subsequent sandstone alteration. However, depositional environment can cause sands of similar detrital mineralogy to follow different diagenetic courses leading to the development of significantly different porosity and permeability characteristics. Diagenetically produced porosity differences in a marginal lacustrine setting can yield a wide variety of reservoirs, seals, and traps.
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