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Abstract
DOI:10.1306/13321446M973489
Shale Resource Systems for Oil and Gas: Part 1—Shale-gas Resource Systems
Daniel M. Jarvie
Worldwide Geochemistry, LLC, Humble, Texas, U.S.A.
ACKNOWLEDGMENTS
Part 1 of this chapter is dedicated to Jack Donald Burgess, retired from Gulf/Chevron/Humble Geochemical, who helped launch the shale-gas revolution with his geochemical assessments of the Barnett Shale as well as many other shale resource systems. Although known for his work in kerogen assessments such as vitrinite reflectance and visual kerogen, he integrated these data with all available geochemical and geologic data, showing many of us how to prepare such reports for geologists and engineers who worked on shale resource systems.
I thank my former colleagues at Humble Geochemical Services; various individuals from Mitchell Energy, especially Dan Steward, Kent Bowker, Chris Veeder, and Chris Stamm; and Jeff Jones from Van Operating, who first got me involved in the Barnett Shale. All contributed to this work in some way during the course of three decades.
ABSTRACT
Shale resource systems have had a dramatic impact on the supply of oil and especially gas in North America, in fact, making the United States energy independent in natural gas reserves. These shale resource systems are typically organic-rich mudstones that serve as both source and reservoir rock or source petroleum found in juxtaposed organic-lean facies. Success in producing gas and oil from these typically ultra-low-permeability (nanodarcys) and low-porosity (15%) reservoirs has resulted in a worldwide exploration effort to locate and produce these resource systems. Successful development of shale-gas resource systems can potentially provide a long-term energy supply in the United States with the cleanest and lowest carbon dioxide-emitting carbon-based energy source.
Shale-gas resource systems vary considerably system to system, yet do share some commonalities with the best systems, which are, to date, marine shales with good to excellent total organic carbon (TOC) values, gas window thermal maturity, mixed organic-rich and organic-lean lithofacies, and brittle rock fabric. A general classification scheme for these systems includes gas type, organic richness, thermal maturity, and juxtaposition of organic-lean, nonclay lithofacies. Such a classification scheme is very basic, having four continuous shale-gas resource types: (1) biogenic systems, (2) organic-rich mudstone systems at low thermal maturity, (3) organic-rich mudstone systems at a high thermal maturity, and (4) hybrid systems that contain juxtaposed source and nonsource intervals.
Three types of porosity generally exist in these systems: matrix porosity, organic porosity derived from decomposition of organic matter, and fracture porosity. However, fracture porosity has not proven to be an important storage mechanism in thermogenic shale-gas resource systems.
To predict accurately the actual resource potential
, the determination of original hydrogen and organic carbon contents is necessary. This has been a cumbersome task that is simplified by the use of a graphic routine and frequency distribution (P50) hydrogen index in the absence of immature source rocks or data sets.
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