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Cored intervals from the Lower Cretaceous Hosston sandstone show sedimentary structures typical of fluvial and deltaic environments. The present depth of burial of the sandstone is 4,300 to 5,500 m (14,100 to 18,050 ft).
Selected samples of the sandstones were analyzed by petrographic, X-ray diffraction, scanning electron microscopy, and other methods to determine the composition and texture of the detrital and authigenic phases, diagenetic sequence, and the relation of facies, diagenesis, and reservoir rock properties.
The sandstones are fine grained, well sorted, and composed on the average of 68% quartz, 17% lithic fragments, 2% feldspars, 7% matrix, and 6% other minerals. Cements include silica and carbonate, which respectively constitute 8% and 9% of the bulk sample in general. Silica cement dominates in the fluvial facies, carbonate cement in the deltaic sandstones.
Alteration of rock fragments and feldspars results in clay authigenesis which accounts for practically all of the < 0.01 mm size fraction in the sandstones. Coarsely crystalline kaolinite makes up 51% of the clays, illite 42%, and chlorite 6%. Kaolinite alters to illite as a function of temperature increase. While kaolinite is pore-filling, illite and chlorite are pore-lining.
The sandstones have an overall average porosity of 4.2%; the fluvial facies generally has porosities below average, the deltaic facies above. Intergranular pores and oversized pores are the dominant porosity types; both have developed by dissolution of cement or detrital grains. The deltaic facies exhibits inverse relation between porosity and total cement content. Because of the persistent presence of authigenic clays in the pores, microporosity forms a significant portion of total porosity, especially in the fluvial facies.
Permeability of the Hosston sandstones ranges from less than 0.01 md to slightly more than 5 md, the average being between 0.1 and 0.2 md. Thin sandstones of fluvial origin, which have microporosity, show lowest permeability, whereas sandstones of deltaic origin in nearby areas have high permeabilities primarily because of dissolution of grains and carbonate cement. Future exploration in the Hosston should, therefore, be directed to the deltaic sandstone.
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