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Impedance
Facies Map in Reservoir Interpretation
ADDY, SUNIT K. and JIM HALLIN , Reservoir Geophysics, CGG Americas Inc, Houston, TX
Three-dimensional seismic inversion was performed on a 3D survey in
central Lavaca County around Hallettsville, Texas in an area with several oil and gas
fields. The study shows low impedance layers in the upper Edwards limestone suggesting
porosity development.
The seismic inversion procedure uses a global optimization technique to derive a
framework of acoustic impedance layers from the 3D seismic data. The layered impedance
data show increased vertical resolution, increased coherency and less noise compared to
the original seismic data and provide a better understanding of porosity distribution in
Edwards limestone. Well logs were used to validate the inversion results but were not used
directly in the inversion. Impedance facies maps in the upper Edwards were generated by
combining several impedance layers using a neural net classification method.
Porosity development depends on karsting and fracturing near faults and is reflected by
lower impedance. A broad swath extending 2 to 3 km on both sides of the shelf edge shows a
distinct impedance facies assemblage where selectively the layers show lower impedance and
greater time thickness relative to normal limestone. Impedance and seismic facies maps,
various attribute maps and well information were used to study the reservoir
heterogeneity. It is suggested that karsting occurred after the deposition of the Edwards
limestone during a low sea level stand. The characterization of the reservoir using
impedance
facies map can be used in exploration as well as exploitation programs.
AAPG Search and Discovery Article #90908©2000 GCAGS, Houston, Texas