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Heterogeneities of Eolian Tensleep/Minnelusa Reservoirs in Wyoming

Yin, Peigui 1
1 EORI, University of Wyoming, Laramie, WY.

The majority of eolian Tensleep/Minnelusa reservoirs in Wyoming produce more than 95% of water with oil recovery factors less than 50%. The high water-cut and low recovery are attributed to the reservoir heterogeneity in matrix and well-developed fractures. The matrix heterogeneity is a result of variation in lithologies, depositional structures, and diagenetic modifications. The high-quality sandstone reservoir interval is often interrupted by tight dolostone, sandy dolostone, or dolomitic sandstone vertically or laterally. Large-scale cross-beddings composed of alternative coarse-grained and fine-grained laminae may cause reservoir permeability anisotropy. Early-formed, authigenic microcrystalline dolomite coats sand grains and infills pore space, greatly reducing both porosity and permeability. This dolomite cement is more concentrated in the fine-grained laminae than in the coarse-grained laminae, further increasing the permeability anisotropy in cross-bedded Tensleep/Minnelusa sandstones. Abundant anhydrite nodules, patches or stripes are usually distributed in the coarse-grained laminae, heavily damaging the reservoir quality. Natural fractures are commonly observed in the Tensleep structural reservoirs. Open fractures can serve as flow conduits in the tightly-cemented sandstones, forming thief zones in otherwise low-permeable intervals. Sealed fractures and related cemented bands demonstrate much lower permeabilities than the associated highly permeable matrix, potentially causing permeability heterogeneity, and even reservoir compartmentalization. It is crucial to have a comprehensive understanding of reservoir heterogeneity for effective oil recovery from Tensleep/Minnelusa reservoirs.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009