How Common Are Naturally Occurring Microfractures in Organic-Rich Mudrocks? Observations from Samples Prepared with Ar-Ion Cross-Section Polishing

Reed, Robert M.

Numerous researchers have made the case for naturally occurring, open or previously open, nanometer- to micrometer-scale-aperture microfractures in organic-rich mudrocks. In particular, bedding-parallel microfractures have been proposed by several authors. But the question persists—which microfractures are naturally occurring and which are artifacts of drilling and sample preparation?

Ar-ion cross-section polishing produces excellent surfaces for examining microscale features such as porosity in mudrocks. Preparation of dozens of Ar-ion-milled samples from more than a dozen different mudrock formations has provided a useful dataset for examining microfracture development.

Gypsum-lined, bedding-parallel microfractures are common, but not naturally occurring because gypsum in mudrocks can form postcoring, following dissolution of carbonate by acids being released through oxidation of unstable iron sulfides. Proposed bitumen-lined horizontal fractures are difficult to distinguish from elongated or deformed pieces of kerogen. Differentiation of the two hinges largely on the ability to differentiate kerogen from bitumen by means of microanalysis.

Completely open microfractures cutting through the mudrock matrix are present in various abundances in many samples. Most of these have bedding-parallel traces, or they are localized at grain boundaries. These fractures typically lack mineralization and have smooth, parallel sides. The common interpretation of these microfractures is that they are stress relief features and not naturally occurring.

Open microfractures hosted entirely within organic matter are more difficult to interpret. Some of these fractures lack parallel sides. Rarely these are bridged by narrow bands of organic matter. Although they may represent shrinkage of organic matter at the surface, some of these microfractures may be naturally occurring.

Naturally occurring microfractures contribute little to the pore systems of organic-rich mudrocks. The paucity of open, naturally occurring microfractures is not surprising because previous studies in other rock types (sandstone, limestone, dolomite) have shown that the smaller their apertures become, the more likely microfractures are to be mineral filled.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013