Hydraulic fracturing is in the news because more natural gas has meant substantially fewer carbon emissions - and it has also been implicated in a variety of environmental issues.

Man is doing what nature has always done, albeit on a different time scale. A new GSA BULLETIN study  examines how long it takes natural Earth processes to form hydraulic fractures and whether the formation is driven by sediment compaction, oil and gas generation or something else. Plus, in order to make environmental models about modern hydraulic fracturing production, it's important to know what role these natural fractures play.

The process of fracture formation by a natural increase in pore-fluid pressure has previously been referred to as natural hydraulic fracturing. Researchers work to understand these fractures through examination of fluid inclusions trapped in minerals within the fractures. In this study, András Fall and colleagues conclude that natural hydraulic fractures formed over time spans of 33 to 35 million years, driven by the slow generation of natural gas.

(A) Location map of the Piceance Basin, Colorado, with major gas-producing fields.
(B) Stratigraphic cross section across the northern Piceance Basin.
Credit: A. Fall et al. and GSA Bulletin

Natural fractures provide important pathways for the flow of water, natural gas, and oil in geologic formations, including unconventional tight-gas sandstone oil and gas reservoirs targeted for production by hydraulic fracturing. These fractures play an essential role during well completion and production by connecting pores in the reservoir rock storing oil and gas to the hydraulic fracture and wellbore that allow production. "Sweet spots," or zones of higher than average permeability, have been attributed to the presence of these open fractures.

Successful prediction of zones of increased fracture abundance provides an opportunity to minimize drilling and completion costs as well as the environmental footprint of production. Successful prediction of natural fracture occurrence and their hydraulic properties requires models of fracture formation that are based on realistic mechanical, hydraulic, and chemical principles that can be tested against core, well-log, and production data.

A. Fall et al.,'Natural hydraulic fracturing of tight-gas sandstone reservoirs, Piceance Basin, Colorado', GSA BULLETIN, 30 July 2014, http://dx.doi.org/10.1130/B31021.1