Seismic Characterization of Natural Fractures in the Buda Limestone of Zavala County, Texas

Abstract

The Buda Limestone is a naturally fractured Early Cretaceous carbonate formation in south Texas which is unconformably overlain by the Eagle Ford Shale. Matrix porosity of the Buda is less than 6%, therefore natural fractures improve the potential for commercial hydrocarbon production from this tight limestone formation. It is a challenge for producers to identify these zones using well log and poststack 3D seismic data which typically available to medium or small exploration companies. This project provides a workflow based on well log analysis tied to seismic acoustic impedance (AI) inversion to locate areas of probable natural fractures.

Acoustic impedance inversion was performed across a 40 square mile 3D seismic survey. The AI data shows low impedance shadow zones on the down thrown side of faults. Post stack geometric seismic attributes such as coherence, maximum and minimum curvature were analyzed in the anomalous AI areas, along with physical seismic attributes such as RMS amplitude and instantaneous frequency.

To map primary porosity, a relationship between acoustic impedance and porosity is established by crossplotting well log data. A linear fit to the Buda data in one well indicates a robust correlation between sonic porosity, density-porosity and AI. Sonic porosity is an indicator of the matrix porosity in the Buda Limestone, while density porosity represents both matrix and fracture porosity. Using the trend line equation for AI vs sonic porosity, the 3D seismic impedance volume was scaled to a matrix porosity volume.

In the downthrown faulted areas, the porosity volume indicates values greater than expected of matrix porosity. This has been reported elsewhere in carbonate reservoirs as an indicator of enhanced secondary (fracture) porosity.

This study indicates that a combination of acoustic impedance inversion and seismic attributes can identify areas of enhanced natural fracturing within the Buda Limestone interval.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018