The Hrubetz Ellenburger field of Coleman County, Texas, produces from dolomite of the Ordovician Ellenburger Group. The discovery of commercial volumes of hydrocarbons in the Ellenburger has rejuvenated interest in areas formerly considered too mature to yield significant discoveries. The fractured nature of the reservoirs has required explorationists to reevaluate the criteria used in formation evaluation. Recognition of this type of reservoir requires conscientious sample examination as well as a knowledge of how fractures are manifested upon electric logs. Fractured intervals possessing porosities normally regarded as being below productive values are completed regularly with profitable results.

Productibilities vary, but estimates derived from decline-curve extrapolations suggest primary recoverabilities will be 50 to 100 bbl of oil/acre-ft of reservoir. The recoverabilities are a consequence of fractured reservoirs possessing a low volume of permeable pore space and a high degree of lenticularity. Accumulative production is expected to range from 15,000 to 40,000 bbl/well.

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