ABSTRACT: Using a hybrid seismic attribute to differentiate lithologies and facies in a deltaic system, Texas Gulf Coast, U.S.A.
A "hybrid seismic attribute" can be constructed by combining several physical and geometric seismic attributes such that the new hybrid enables the interpreter to differentiate lithology and facies. In our example, a hybrid attribute was constructed to distinguish lithofacies commonly encountered in deltaic depositional systems, where varying amounts of sandstone, mudstone and carbonaceous facies occur, and are intercalated in a variety of bedding styles (e.g., thin-bedded continuous; thick-bedded discontinuous; massive; etc.).
Isolated sandstone gas reservoirs (up to 50 feet thick) in the basal part of the Vicksburg Formation are encased in several hundred feet of marine shale. The reservoir sandstones were initially detected by examination of the hybrid attribute volume. After calibration with well logs and core information, we were able to map lithofacies and reservoir properties in detail.
The hybrid attribute map revealed a complex 'valley' representing a fluvio-distributary channel system that fed a coastal strandplain. Three seismic attribute facies were identified: (1) Sinuous patterns that meander from one side of the valley to the other. Interpretation: Abandoned channel mudstones and lignite; (2) 'Crescents' that are tucked within the concave portions of Facies 1. Interpretation: Point-bar and active distributary channel reservoir sandstones; (3) Valley-adjacent, mottled areas with subtle dip- and strike-oriented textures present locally. Interpretation: Composites of delta-plain mudstones and crevasse splay sandstones, plus older strandplain sandstones.
The hybrid seismic attribute enabled a high-resolution determination of lithofacies, including reservoir sandstone facies, that could not have been accomplished on the basis of standard seismic amplitude extractions and well logs.
AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia