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