Predicting Reservoir Quality at the Development Scale: Methods for Quantifying Remaining Hydrocarbon Resource in Diagenetically Complex Carbonate Reservoirs

ABSTRACT The Jordan (San Andres) reservoir comprises 400 ft (120 m) of upward-shoaling subtidal to peritidal carbonate strata, which is now thoroughly dolomitized and partly cemented by sulfates. Subtidal facies include dominant pellet packstone/grainstone, with local bryozoans, algae, and coral bioherms and associated skeletal grainstone flanking beds. The lower part of the subtidal section is characterized by stratigraphically distinct zones in which permeability has been enhanced by a postburial carbonate-leaching event. These diagenetically altered (leached) zones crosscut subtidal depositional facies. Peritidal facies are nonporous mudstone and generally nonporous pisolite packstone characterized by abundant sulfate cement. The pisolitic rocks are locally porous and permeable where sulfate cement is either leached or absent from fenestrae. Cumulative production is 68 million stock tank barrels (MMSTB) of 218 MMSTB original oil in place, which is a recovery efficiency of 31%. A total of 47 MMSTB of remaining mobile oil occurs as bypassed oil in the contacted upper part of the reservoir, which has been penetrated by well bores; 12 MMSTB of mobile oil is in the uncontacted lower part, which has not been penetrated by well bores. The most prospective areas for increased production by waterflood profile modification in the contacted part of the reservoir are the southwest corner of the field, where low-permeability, diagenetically unaltered subtidal rocks are incompletely swept, and the eastern central part of the field, where heterogeneous permeability in peritidal rocks has resulted in an incomplete sweep. The most prospective areas for increased production End_Page 231------------------------ through well-bore deepening into the uncontacted part of the reservoir are the southeast corner of the field, where high-permeability, diagenetically altered subtidal rocks are uncontacted, and the central part of the field, where high-permeability, diagenetically altered subtidal rocks are uncontacted. An understanding of diagenetically controlled reservoir properties can be used to predict the locus of remaining resource and to design recovery strategies.