Shale Gas
As natural gas demand increases in the United States, gas exploration across North America is increasingly focused on “unconventional” reservoirs, including shales (a broad term herein meant to include mudstone, claystone, and other fine-grained rocks). Although shales have been studied extensively in their role as source rocks (the rocks from which oil and gas generate), they have only recently become important reservoir rocks in North America, so research on them as it bears on their reservoir characteristics is extremely limited. Because North America is a mature petroleum province, shale gas (gas produced from shale reservoirs) is rapidly becoming an important domestic exploration target and an increasingly significant source of natural gas produced in the US and Canada. A systematic study of shale gas reservoirs is thus necessary in order to more fully appreciate the volume of contained petroleum resources and the mechanisms acting to create and preserve these reservoirs through geologic time. Such a study was recently launched by the USGS because a cursory look at some shale gas reservoirs in North America suggests that there are fundamental differences between them, making it difficult to generalize about these important reservoirs. Furthermore, there is little known about the role, if any, of clay minerals (Fig. 1A), the dominant mineral component of shale, in localizing organic material (the source of oil and/or gas) and their role in influencing the concentration of specific types of organic matter (Fig. 1B).
 
Figure 1A. Scanning electron photo showing clay minerals (example shown by arrow), the dominant mineral in shales.
Figure 1B. Scanning electron backscatter photo showing the complex mixture within shales of constituents including organic material, clays (I/S), fossil fragments, pyrite framboids, and detrital quartz (Q). Other inorganic constituents include carbonate (C) minerals.
Thus, scientists at the USGS are studying shales to address several fundamental questions concerning these important gas reservoirs such as, 1) what is the possible role of clay minerals in localizing and concentrating organic material in shale reservoirs? 2) what constitutes an effective shale gas source rock? and 3) can we define shale resources within a petroleum system framework? Addressing these questions will provide a greater understanding of the fundamental mechanisms responsible for shale gas accumulations and as such, construct a framework for future assessments of the undiscovered resources within these still enigmatic reservoirs.
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U.S. Department of the Interior |
U.S. Geological Survey
