Online Journal for E&P Geoscientists

Jarvie, Daniel M.^*1; Jarvie, Brian M.²; Weldon, David ³; Maende, Albert ³
(1) Worldwide Geochemistry, Humble, TX. (2) Wildcat Technologies, LLC, Humble, TX. (3) Geomark Research Rock Lab, Humble, TX.

Unconventional shale resource systems can provide massive energy reserves for gas and oil if developed properly and soundly. Development of these systems has evolved rapidly over the past decade in North America, although it took 30 years to reach such a level.

The key component of an unconventional system is the source rock itself. Thus, basic factors such as shale thickness, porosity, permeability, mineralogy, rock mechanics, organic richness, kerogen type, and oil/water/gas saturation are important variables. An unconventional shale resource system may be described as a continuous organic-rich source rock with low porosity and permeability with or without juxtaposed (overlying, interbedded, or underlying) organic-lean tight rock units.

Unconventional thermogenic shale gas resource systems are divided into overlapping categories. There are highly productive organic and silica-rich mudstones that are most often gas window mature, but there are also some productive systems that are oil window mature producing high BTU gases. Systems dominated by a gas window mature (>1.00%Roe), organic-rich mudstone typically have 30-80% of their petroleum storage in organic porosity. Hybrid systems have both organic and matrix porosity and will usually be far more productive as a result of higher storage capacity.

Unconventional shale oil resource systems, while dominantly clay and organic-rich mudstones have produced significant amounts of oil, e.g., Barnett Shale in the oil window, systems with juxtaposed, organic-lean carbonates have proven more successful (e.g., Eagle Ford Shale, Bakken Formation, and Niobrara Shale). There are also fractured shale oil resource systems and even high porosity, high permeability shale systems including the Miocene Monterey Shale in California, and fractured Upper Bakken Shale fields in the Williston Basin, North Dakota. While silica content remains important for rock brittleness, the source of silica, whether biogenic or detrital, becomes very important as biogenic silica will result in adsorption of oil to organic matter associated with the biogenic silica. In this case there is a tight, albeit brittle rock, but there is also strong adsorption of oil components especially polar constituents such as the resin and asphaltene fractions that have not yet been cracked to lighter hydrocarbons in the oil maturity window.