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GUEVARA, EDGAR H., Lagoven S.A., Caracas, Venezuela, and WILLIAM L. FISHER and NOEL TYLER, Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

ABSTRACT: Stratigraphic Control of Unrecovered Oil and Gas in Mature Fields

During the past decade, significant additional recovery of oil and gas has come from intensive, non-tertiary development of mature fields. The potential for additional recovery is related directly to reservoir heterogeneity. The more geologically complex a reservoir, the greater the potential for additional recovery. Stratigraphic and structural complexities segment the reservoirs into isolated or partly connected compartments. Commonly, inadequate distribution of wells mostly located according to grids that ignore geological variability in the interwell area -- results in uncontacted and poorly drained compartments.

Basic knowledge of lithological and petrophysical architecture, along with fundamental production attributes, is essential for delineating reservoirs with maximum additional production potential. Potential is low in reservoirs with relatively homogeneous facies architecture, such as wave-dominated deltas and barrier-bar reservoirs of the Woodbine Formation (Cretaceous) that show recoveries in excess of 80% in the extensively drilled East Texas field. In contrast, potential is high in complex reservoirs such as those formed by laterally discontinuous meanderbelt sandstones, restricted platform carbonates, or slope-basin fan channels.

Production increases in stratigraphically complex, basin-floor sandstone oil reservoirs in the Spraberry and Dean Formations (Permian), Midland Basin (West Texas), illustrate incremental recovery potential in complex, mature fields. Even 40 years after discovery, the attractiveness of large unrecovered volumes of oil (locally 95% of OOIP) result in these reservoirs being among the most intensely drilled, extended development targets in the U.S. Production increases in the 1980's reflect aggressive drilling programs comprising geologically targeted locations and completions that, guided by improved well-stimulation techniques, resulted in annual productions that surpassed peak production of the early 1950's. At the reservoir scale, similar results were obtained in 1991 in equivalent re ervoirs of the Benedum field, where a production increase of 20% was obtained through a comparable development strategy.

The key to realizing advanced recovery potential is the detailing of geologically complex reservoirs by integrating advanced geological-petrophysical modeling, advanced geophysical technology, and engineering data to build strategic drilling plans.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.