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Integrating Core, Well Logs and Seismic Interpretation to Improve Understanding of Albian Patch Reefs, Maverick Basin, SW Texas *
By
Enzo S. Aconcha1
Search and Discovery Article #50104 (2008)
Posted October 22, 2008
*Adapted from oral presentation at AAPG Annual
Convention,
1Jackson School of Geosciences, University of Texas at Austin, Austin, TX. ([email protected])
Using the stratal slicing technique, a seismic sedimentologic analysis was performed on the mid-Albian carbonates of the Glen Rose Formation, Maverick Basin, SW Texas. The 3D seismic subset covers an area of 186 km2 and is integrated with 68 wells, 2 cored intervals and 40 thin sections. A retrogradational stacking pattern is documented from the sequential development of shallow platform buildups with an early phase in the south (basinward) followed by a younger phase in the north (landward). This suggests the mound growth occurred during the TST of the lower Glen Rose composite sequence. The external shape of the buildups on each stratal slice was reconstructed through the geometrical parameters computed from the best-fit ellipses. Buildup area decreases over time, with buildup cores migrating westward, and average external shape becoming NNW-SSE elongated. The repose angles of the patch reefs show very steep west margins, with values that in the SW direction frequently exceeds 50 degrees. East-facing flanks display more gradual 12-20 degree dips. The implications of these morphological patterns may be related to prevailing currents and winds during buildups growth. Patch reefs usually display a clean box-shape gamma-ray pattern (5-20 GAPI and low variance) and are associated with the highest negative seismic amplitudes. Positive seismic amplitudes pair with a jagged (mud-rich) GR (20-60 GAPI and high variance). Negative seismic amplitudes close to the zero crossing may show either a clean box or muddy jagged GR response. The stratal slices reveal that positive amplitudes define positive relief features (mud mounds) preceding the buildup facies proper. Before this result, buildups were thought to develop on top of the relatively flat, shallow platform.
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The Lower Glen Rose (Albian) buildups in the Maverick Basin: l Developed in three levels within the sequence 7. Two of the levels occurred within the TST. Buildups occurred as biostrome and bioherms with an arrangement of shoaling-reefal rock facies deposited mostly during 4th order decreasing accommodation cycles. l Are mounded features elongated NNW-SSE with steeper sides to the SW. This asymmetry may have resulted from paleocurrents directed NNE and E paleo-trade-winds. The implication is that better reservoir rock facies locate to the SW side of the buildups. l Are mounded features elongated NNW-SSE with steeper sides to the SW. This asymmetry
l Jackson School of Geosciences and GCAGS. l Charlie Kerans, Hongliu Zeng, William Fisher, Bob Loucks, Carlos Torres-Verdin. l TxCO: Bob Scott, Craig Cobb, Barbara Romans. l BEG: Dallas Dunlap, Lesli Wood. l RCRL: Jerry Lucia, Xavier Janson, Jerome Bellian, Jim Jennings, Chris Zham. l AGM & Landmark: Recon Stratal Slicing and Interpretation Software. l ChevronTexaco.
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