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Effects of Diagenesis on Petrophysical Properties of Reservoir Rocks, Washakie Basin
Almond Formation sandstones in the Washakie Basin were deposited in a barrier-island complex environment. In the present study, lithofacies recognized in Almond sandstones are tidal channel and upper, middle, and lower shoreface. Outcropping sandstones from all the lithofacies have similar porosity values (about 27%), but permeability values are considerably different, ranging from 2317 md in the tidal channel sandstones to 580 md in the lower shoreface sandstones. Sandstones in the subsurface also have similar porosity values but different permeability values. These variations in porosity and permeability between sandstones from different lithofacies are attributed to the various sandstone diagenetic pathways.
Tidal channel sandstones are richer in quartz grains and carbonate cement compared with shoreface sandstones. With burial, mechanical compaction has reduced porosity in these sandstones. Dissolution of carbonate cement has enhanced both porosity and permeability. Partial dissolution of the feldspar and lithic fragments has generally created micropores and precipitated clay minerals, enhancing porosity but not permeability. As the tidal channel sandstones have undergone more carbonate dissolution and less lithic dissolution than the shoreface sandstones, they have similar porosity values but higher permeability values than the shoreface sandstones at intermediate burial.
At great depths, the tidal channel sandstones still maintain more than 10% porosity and more than 0.1 md permeability, whereas the shoreface sandstones maintain 10% porosity and less than 0.1 md permeability. Therefore, the tidal channel sandstones have higher overall potential for exploration and production than the shoreface sandstones.
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