Approaches to Finding New Gas in Mature Fields: An Example from the Middle Frio, Onshore Texas Gulf Coast Basin

LEVEY, R. A., University of Texas at Austin, Austin, TX, R. R. RAY, R3 Exploration, Lakewood, CO, and R. SINGLE and R. J. FINLEY, University of Texas at Austin, Austin, TX

Refined depositional models of reservoir distribution applied to new and previously acquired data sets (reflection seismic, well logs, and core) can lead to potential infield opportunities. Conventional 2-D seismic and well logs are standard industry tools used as part of many exploration and field-development programs. Reflection seismic lines across a 50-year-old gas field along the Vicksburg fault trend of the Gulf Coast basin in a nonmarine part of the Oligocene Frio section were interpreted with emphasis on changes in waveform character. A comparison of velocity-only derived synthetics to velocity/density derived synthetics show a close similarity. Observed amplitude variations are nonfluid related but are lithology and thickness dependent. Synthetic seismograms constructed for w ll logs and tied to conventional reflection seismic lines were correlated with stratigraphic cross sections and facies maps. Standard resistivity logs, typically available for all well control, can be digitized to create psuedosynthetics for well-to-seismic correlations where velocity and density logs are unavailable.

Observed patterns of variation in seismic character were linked to geologic interpretation from well logs. Seismic variations range from single waveform peaks (15 ms) that correspond to isolated 45+ ft channel complexes to zones of 3000-ft-wide convex reflections (40 ms or 200 ft thick) that correspond to variable lithologic patterns across part of the field. Both vertical and lateral transitions from sandstone-rich amalgamated channel sequences to sandstone-poor nonchannel areas are the dominant explanation for reflection contrast. High-amplitude reflectors are mappable along a 2-D grid with an approximate 2000-ft spacing. Comparison of known gas-productive intervals at moderate depths (4000-7000 ft) from the existing well control with the detailed geologic and seismic interpretation reveals a combination of potentially untapped and partially drained parts of multiple reservoirs that warrant consideration for infield drilling or recompletion. Combined interpretation of well logs, reflection seismic data, and production information is a simple, cost-effective approach to using standard tools and information in a mature field to locate opportunities for incremental gas recovery.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)