Abstract

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 coring and/or sample preparation?

Broad-ion-beam (BIB) cross-section polishing produces low-artifact, relatively flat surfaces for examination of microscale features in mudrocks. Preparation of dozens of BIB-milled samples from more than 20 different mudrock formations has provided a useful dataset for scanning electron microscope examination of 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 acid being released through oxidation of chemically unstable iron sulfides. Proposed bitumen-lined, horizontal microfractures are difficult to distinguish from elongated or ductilely deformed pieces of kerogen. Distinguishing between these two organic-matter scenarios hinges largely on the ability of microanalysis to differentiate solid bitumen from kerogen.

Open microfractures cutting through the mud-rock matrix are present in various abundances in many samples. Most of these either have bedding-parallel traces or they are localized along grain boundaries. These fractures typically lack any mineralization and have smooth, parallel sides. They are particularly common in clay-mineral-rich lithologies. The typical 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. A few are bridged by narrow bands of organic matter. Although many may represent shrinkage of organic matter at surface conditions, 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, as previous studies in other rock types (sandstone, limestone, dolomite) have shown, the smaller their apertures become, the more likely microfractures are to be mineral filled.