DEVELOPING FIRST AND SECOND ORDER MODELS FOR PREDICTING MUDROCK MINERALOGY AND FACIES DISTRIBUTIONS: UNDERSTANDING SEDIMENTARY PROCESSES AND REQUIRED CHARACTERIZATION METHODS

Although the goals of characterizing mudrock (shale) systems are the same as those for sandstones and carbonates, the methods needed to define mudrocks are significantly different. Conventional approaches (e.g., visual outcrop and core description, laboratory x-ray diffraction analysis, wireline logs) for defining variations in these rocks are at best of low resolution and at worst misleading or inaccurate.

Despite these challenges, integrated, multidisciplinary study of several North American mudrock systems (including the Woodford, Barnett, Haynesville, and Eagle Ford formations), has led to the development of first order models that define and predict the distribution of key mudrock attributes including facies, mineralogy, organic carbon content, and pore types. These models are tied to (1) sediment type, production, and transport, (2) distance from sediment source, and (3) degree of oxygenation. The models show that mudrock attributes commonly vary regularly from proximal (generally nearer to shore) to distal settings. However, studies also show that these changes can occur along both regional depositional strike and dip and that some systems do not conform to these trends.

Defining second order (smaller scale) variability in mudrock properties remains a major challenge to characterizing and predicting areal facies patterns. Key methods needed to define mudrock attribute variations at this scale are (1) high resolution x-ray fluorescence measurement of elemental abundances and mineralogy in cores or outcrops, (2) outcrop-based analysis of depositional processes and the controls of bed continuity, and (3) scanning electron microscope determination of grain assemblages. These disparate data sets and scales of observation must be integrated in order to effectively define patterns of facies development and the processes that control them.