Seismic-attribute identification of brittle and TOC-rich zones within the Eagle Ford Shale, Dimmit County, South Texas

Osareni Ogiesoba,Ursula Hammes

doi:10.1007/s13202-014-0106-1

Abstract

Analysis of 3D poststack seismic attributes can be used to identify areas of high exploration potential within shale resource plays. We integrated seismic attributes and acoustic impedance (AI) with wireline logs to determine total organic carbon (TOC) distribution within the Eagle Ford Shale in South Texas. We computed TOC from wireline logs using the Δ Log R method and then used seismic attributes to predict TOC and deep-resistivity log distribution, and identify brittle zones within the seismic survey. Our results show that high-TOC and high-resistivity zones are laterally more continuous in the south part of the survey. In the north, continuity of these properties is broken by NE–SW-trending faults having throws ranging from about 10 to 100 ft (3–30 m). High resistivity occurs in high-quality-factor (Q) attribute zones. Although the relationship is nonlinear, resistivity and TOC increase as Q increases. That is, both properties increase with increasing bed resistance suggesting increasing carbonate. Two high-resistivity zones, an upper resistive bed and a lower resistive bed, are identified within the Eagle Ford Shale. Additionally, because a strong positive linear relationship exists between AI and Q, Q can be used to identify brittle zones. Compared to other attributes used in identification of brittle zones, Q is faster and cheaper to compute from 3D poststack seismic data. Therefore, Q could serve as a quicker, alternate method of identifying brittle zones within the Eagle Ford Shale.

Highlights

IntroductionSince the Eagle Ford Shale was recognized as a major gas shale play in 2008, exploration there has been geared toward delineating fault-related fracture zones, as well as stratigraphic layers having abundant minor faults and brittle zones, to find the most productive intervals (Kuich 1989; Treadgold et al 2011)
The Cretaceous Eagle Ford Shale in South Texas is one of the most prolific shale resource plays in the United States
Compared to other attributes used in identification of brittle zones, Q is faster and cheaper to compute from 3D poststack seismic data

Summary

Introduction

Since the Eagle Ford Shale was recognized as a major gas shale play in 2008, exploration there has been geared toward delineating fault-related fracture zones, as well as stratigraphic layers having abundant minor faults and brittle zones, to find the most productive intervals (Kuich 1989; Treadgold et al 2011). The success or failure of hydraulic stimulation, depends on our ability to accurately identify and delineate brittle and ductile zones (Laubach et al 2009) within the shale-gas interval Brittle zones are those rock layers characterized by high velocity—that is, high Young’s modulus—whereas ductile zones are layers characterized by low velocity—that is, low Young’s modulus (Koesoemadinata et al 2011; Sena et al 2011). Seismic technology has been employed successfully through amplitude-versus-offset (AVO) inversion, which computes elastic properties—the kl and lq attributes (where k and l are Lame’s constants and q is density) used to determine brittleness of the rock

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