Integration of Depositional Facies and Sequence Stratigraphy in Characterizing Unconventional Reservoirs: Eagle Ford Shale, South Texas

Workman, Seth; Grammer, Michael

The Eagle Ford Shale is a mixed siliciclastic/carbonate unconventional resource play with considerable oil and natural gas. Reported variations in well producibility indicate small-scale geologic heterogeneities govern reservoir quality and potential. The limited availability of subsurface data constrains previous subsurface investigations. As a result, the internal variability of facies and reservoir attributes remain poorly understood. This warrants additional rock-based work constrained within a sequence stratigraphic framework to define the character and vertical stacking pattern of depositional facies and genetic units, as well as provide insight into probable lateral and vertical distributions and geometries of potential reservoir facies.

Four cores were incorporated to determine facies successions and establish a hierarchical classification of stacking patterns based on the analysis of depositional facies in order to delineate associations of intervals with reservoir-quality porosity/permeability values and specific facies and/or units. Thin sections (20 µm) were utilized to evaluate variability within higher frequency cycles, while enabling more detailed observation of diagenetic features and alterations. X-ray diffraction analyses guided estimates of mineralogy and interpretations. Gas shale core analyses were the fundamental measure of reservoir quality and basis for delineating associations of intervals with enhanced/marginal porosity and permeability and specific facies and/or units. Facies relationships and interpretations were further constrained by modern depositional analogs.

These cores show a regressive, shallowing-upward sequence with higher-frequency cycles (HFC) included within. This is indicated by an upward: change in lithology; transition from pelagic- to traction- modes of deposition; increase in grains/skeletal debris and bioturbation; and, decrease in TOC. These HFCs suggest a eustatic control and consist of transgressive/early highstand calcareous mudstones (primary reservoir) and well cemented, late highstand/early lowstand foraminiferal packstone/grainstones that compartmentalize the reservoir. Facies and HFCs correlate to wire-line log signatures. This work demonstrates how techniques of identifying and linking depositional facies to reservoir quality, and tying these to wire-line log data assist in the evaluation of unconventional reservoirs and enhance the predictability of reservoir potential away from core observations.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013