Abstract

The porosity of shales varies depending upon such attributes as the mineralogy, grain size, organic content, depth and duration of burial, and extent of water-rock reaction. Today, shales are being exploited when they contain significant natural gas, and the connectivity of pores are important toward controlling both recovery of gas after hydrofracking. In fact, the fine-scale nature of the pores controls aspects of release of natural gas and brines during hydrofracturing and gas exploitation.

Despite the importance of shale as a source rock for natural gas and petroleum, it remains difficult to quantify and image porosity in shales because of their fine-scale nature. We are using neutron scattering, FIB SEM, CT microtomography, and other techniques to understand pores in a black (Marcellus) and a grey shale (Rose Hill formation) sampled in Pennsylvania. Samples were recovered both from outcrop and from depth in wellbores. We also report a new approach for investigating pores in shales by using neutron scattering before and after removal of
organic matter.

Pores in the two shales are observed to be isotropic (i.e. in the plane of bedding) or anisotropic (i.e. perpendicular to bedding), as expected for sediments that have been compacted after burial. Some nanometer-sized pores are observed in the organic matter of the Marcellus to be spherical; other pores are observed to be present in pyrite framboids and among silicate grains in that rock. We have no evidence that significant porosity is present in the organic matter in the Rose Hill formation, a relatively organic-poor shale, but pores are observed between and in clay particles.

We also investigate how progressive water-rock reaction changes the primary porosity in the shales by investigating weathering samples. FIB SEM images document that organic matter is oxidized and removed significantly from the weathering Marcellus before the rock turns to soil, leaving behind porosity. Pyrite oxidation and dissolution also creates pores in the Marcellus bedrock before it turns to soil. Only the latter process is significant in the Rose Hill shale. Unlike other weathering rocks (granites, diabase, basalt), the pores in the shales comprise surface fractals both before and after weathering. Understanding how water enters and transforms pores in shales at depth and near surface will increase our ability to protect our water and soil resources, as well as inform our methods of gas recovery.

Citation

Brantley, S.L., Jin, L., Rother, G., Cole, D.R., Gu, X., Balashov, V.N. (2013): Water-Organic-Rock Reactions Recorded in Pores in Shales from the Marcellus and Rose Hill Formations (Invited). Abstract H33L-01 presented at 2013 Fall Meeting, AGU, San Francisco, CA, 9-13 Dec..

This Paper/Book acknowledges NSF CZO grant support.