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Strategies for Optimizing Incremental Recovery from Mature Reservoirs in Oligocene Frio Fluvial-Deltaic Sandstones, Rincon Field, South Texas
Lee E. McRae (1*), Mark H. Holtz (2)
Fluvial-deltaic sandstones represent a large percentage of reservoirs in U. S. fields that face premature abandonment, despite having an estimated 30 billion barrels of unrecovered oil. These heterogeneous sandstones possess excellent potential for incremental recovery of significant oil volumes currently isolated in untapped and incompletely drained reservoir compartments. Results from geological and petrophysical characterization of selected Frio reservoirs are being used both to identify specific infill and recompletion opportunities and to guide incremental recovery strategies in mature reservoirs throughout South Texas that are now on the verge of being abandoned.
Reservoir mapping and stratigraphic facies analysis in Rincon Field were used to describe depositional styles present within a 150-ft sequence that has produced more than 24 million barrels of oil since 1940. The distribution and geometry of more than eight separate flow units in the Frio D-E reservoir interval record aggradational channel-fill sedimentation within an overall backstepping pattern, followed by development of strike-oriented shoreface bars, and a return to channel deposition in an overall progradational regime. Differences in sandstone architecture between retrogradational and progradational units result in different degrees of reservoir compartmentalization that directly impact recovery efficiency. In the retrogradational Frio E reservoir, sandstone thickness is greatest in an updip position relative to the crest of the structure, creating potential for increased flow communication between dominantly dip-oriented channel units that results in a relatively high recovery efficiency of 38%. In contrast, Frio D reservoir units exhibit more complex facies patterns within a net progradational sequence of narrower, less interconnected channel-fill deposits, with areas of maximum sediment thickness located downdip from the structural crest, thus limiting opportunities for lateral flow communication that has resulted in a lower (29%) recovery efficiency.
Petrophysical modeling based on evaluation of abundant core data and saturation estimates from special core analyses is being used to describe permeability structure, map remaining oil distribution, and target undeveloped reservoir compartments with large oil volumes. Better understanding of controls on the stratigraphic distribution of oil in these sandstones and its subsequent production has wide application to many other analogous reservoirs and is a key to optimizing incremental recovery in mature Frio reservoirs.
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