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Organic-rich shales, which typically contain several weight-percent total organic carbon, are considered as key components of petroleum systems because these rocks were the probable source for petroleum in conventional reservoirs. Further, the characteristically low porosity and permeability of shales make them likely seals for reservoirs. Increasingly in North America, organic-rich shales are exploration targets because of their potential for containing exploitable gas in "unconventional" or continuous gas accumulations. Shales represent a complete petroleum system; gas produced from them is generated internally and the shale serves as reservoir, seal, and trap.
Although gas in shale gas systems was internally generated, there are key differences between the various shales that are currently in production, making it difficult to build a single genetic model for these unique systems. At one end of the spectrum is the prolific Mississippian Barnett Shale, Fort Worth Basin, Texas (estimated mean >26 tcf (7.4 bcm) of undiscovered, technically recoverable gas). This unit produces thermogenic gas generated both from kerogen (originally marine algal material), and by being cracked from oil (generated in the Barnett). In contrast, much of the gas produced from the Devonian Antrim Shale on the north margin of the Michigan Basin, Michigan (estimated mean >7 tcf (2 bcm) of undiscovered, technically recoverable gas), is bacterial in origin, and so differs markedly in origin from that in the Barnett. Bacterial gas in the Antrim was generated in the Holocene at shallow burial depths and at cool temperatures upon infiltration of meteoric waters and methane-generating microbes into the shale. A third shale, the Cretaceous Lewis Shale of the San Juan Basin, New Mexico (estimated mean ∼10 tcf (2.8 bcm) of undiscovered, technically recoverable gas), experienced both early bacterial and later thermogenic gas generation from a mixture of indigenous terrestrial (dominant) and marine (minor) organic matter; gas currently produced is largely thermogenic in origin.
Because artificial stimulation (induced fracturing) is required to successfully produce gas from shales due to their inherent low porosity and permeability, the physical character of the shale must be conducive to induced fracturing. The presence of a sufficient volume of non-clay minerals such as quartz (detrital, biogenic, and possibly authigenic) in the shales or in interbedded rocks, is critical because these minerals impart mechanical properties that allow for more successful artificial stimulation. Thus, depositional and diagenetic processes play an important role in gas production from shales.
As natural gas demand continues to increase in the United States, exploration will continue to focus on unconventional petroleum systems, including gas in shales. Expanded exploitation of shale gas reservoirs will require a closer view of shales as being more than just source rocks or reservoir seals.
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