A Detailed Diagenetic Characterization of the Cretaceous Eagle Ford Reservoir, South Texas

Seth J. Workman and G. Michael Grammer
Western Michigan University, Kalamazoo, MI,
[email protected], [email protected]

The Mid- to- Late Cretaceous (Cenomanian-Turonian) Eagle Ford Shale of South Texas emerged as a mixed siliciclastic/carbonate unconventional resource play and is representative of similar resource plays including the Marcellus, Barnett, Bakken and Woodford Shales. Previous work on the Eagle Ford has emphasized macro-scaled lithofacies, source-rock characteristics, microfacies, and biostratigraphic heterogeneities in order to improve geologic understandings of variable reservoir quality. However, this work is inadequate in that it is limited to the macro- scale. It is the micro- to nano- scale heterogeneities that most significantly govern key reservoir properties in these mudstones. This warrants additional work focused specifically on defining the role of diagenesis, and its implications on pore system development as a means to better understand the spatial variability of overall reservoir quality.

The ongoing study is primarily a detailed petrographic investigation of the Eagle Ford. The fundamental goal is to provide a geologic understanding of the controls on local and regional variability in reservoir quality. Thin- and ultra-thin sections will be utilized in conjunction with supplementary core, standard core analysis, X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) data when available. These data should enable: ¹⁾ characterization of diagenetic alterations, ²⁾ identification of specific facies susceptible to diagenetic modification based on dominant mineralogy, and ³⁾ the evaluation of the role diagenetic processes (mechanical and chemical) play in pore system development. Among others, diagenetic modifications may include microstylolitization, pyritization, recrystallization, dolomitization, cementation, compaction, and dissolution. These will be characterized in the context of overall reservoir quality by relating the relative degree of diagenetic alteration to well performance.

The ongoing investigation will evaluate a set of three wells which form a representative group of Eagle Ford wells within the current play area. Wells were selected because each has: 1) a complete Eagle Ford core, 2) a full suite of conventional and mechanical wireline logs (i.e. Young’s Modulus, Poisson’s Ratio, Gamma Ray, and Resistivity), and 3) each is dissimilar in mineralogy, well performance, or geophysical attributes. This is important because it enables direct comparisons and relationships between diagenetic alterations and reservoir quality across the play area. Methods of identifying, characterizing, and tying diagenetic modifications to both specific facies, and log signatures should not only enhance current understandings of variable reservoir quality but also increase regional predictability of reservoir quality. Insight gained from this work on the Eagle Ford may also be applied to aid in exploration and reservoir development of similar unconventional resource plays.

AAPG Search and Discovery Article #90154©2012 AAPG Eastern Section Meeting, Cleveland, Ohio, 22-26 September 2012