oa Stratigraphic Heterogeneity in Shale-Gas Systems: Lessons from North America

North American data demonstrate that shale-gas productivity is a function of a suite of economically significant mudstone properties that vary laterally and vertically at a variety of length scales. Properties of interest include porosity type and amount, permeability, mechanical properties, organic matter type and concentration, stratification style, parasequence and bed thickness, natural fracturing, etc. Many of these variables are controlled, at least in part, by the processes and environments of mudstone deposition. The mudstones of North American shale-gas plays were deposited in tectonic settings that include passive margins (e.g., Haynesville, Barnett), foreland basins (e.g., Lewis, Marcellus) and intracratonic basins (e.g., Antrim). Knowledge of depositional processes and sequence stratigraphic concepts in these systems is used to help define which basins are more attractive as exploration targets, which portions of a basin are likely to produce more gas, which stratigraphic intervals are likely to be targets for horizontal drilling, and even how core sampling should be undertaken and interpreted. Fine-grained sedimentary deposits show heterogeneity at every level, and integration of microscopic- to sequence-scale observations (e.g., SEM imagery, thin sections, core/outcrop, wireline logs, seismic) and interpretations is essential.