Unconventional Shale-Gas Resource Systems and Processes Affecting Gas Generation, Retention, Storage, and Flow Rates

Daniel M. Jarvie, Tim E. Ruble, Richard Drozd, Hossein Alimi, Valentina Baum, and Brian J. Jarvie
Humble Geochemical Services, Humble, Texas

Geochemical and petrophysical characterization of various shale-gas systems in the U.S. indicates a variety of unconventional shale-gas system types (Fig. 1). The most basic distinction is gas type: biogenic and thermogenic, although there can also be mixtures of the two gas types. Thermogenic shale-gas systems are further segregated into various sub-types depending on geochemistry and geology. The shale-gas system categories are: (1) high thermal maturity shale; (2) low thermal maturity shales; (3) mixed lithology intra-formational systems containing shale, sands, and silts; (4) inter-formational systems where gas is generated in a mature shale and stored in a less mature shale, and (5) mixed systems. A key difference among these shale-gas systems are initial gas flow rates. High thermal maturity systems tend to have much higher gas flow rates than low maturity systems because of gas charge and storage mechanisms. Certainly other non-geochemical factors, such as shale mineralogy, are extremely important in being able to stimulate these shales to flow gas.

These systems show significant differences in gas type, organic richness, thermal maturity, and gas flow rates. First, the Antrim Shale is biogenic gas, whereas the Barnett Shale and much of the New Albany Shale gases are thermogenic in origin. Second, the Devonian Antrim and New Albany systems are more organic-rich than the Mississippian Barnett Shale and the difference is not just due to the high thermal maturity of the Barnett Shale. These Devonian systems typically have more mass of organic carbon per unit of rock at equivalent thermal maturities. Thus, original TOC values are higher in these Devonian systems and in many other Devonian systems for that matter (e.g., the Bakken Shale of the Williston Basin). The amount and thermal maturity of the shales impacts the amount of gas and how gas can be stored in these systems. Thermogenic gas can be derived from two processes: gas derived from the primary cracking of organic matter (kerogen) and gas derived from the secondary cracking of hydrocarbons generated from kerogen (i.e., oil cracking). Thus, high thermal maturity systems have the possibility of higher gas contents. For example, at low thermal maturity the Antrim and New Albany Shales derive their porosity from low compaction histories and retention of hydrocarbons is primarily by adsorption to organic matter. The Barnett Shale, on the other hand, is fully compacted and derives its limited porosity from the volumetric change in organic matter as a result of kerogen decomposition as well as any free volume in micro-factures. Gas flow rates are then dependent upon the amount of gas stored (or generated) and the ability to release gas from adsorption sites as well as connecting to micro-reservoir compartments (Barnett Shale only).

Figure 1. Shale gas resource plays in the eastern and south-central United States. Data from three systems are used to illustrate system types: (1)the biogenic Devonian Antrim Shale, (2) the low thermal maturity Devonian New Albany shale, and (3) the high thermal maturity Barnett Shale.

AAPG Search and Discover Article #90065©2007 AAPG Southwest Section Meeting, Wichita Falls, Texas