THREE-DIMENSIONAL IMAGING OF MICROFRACTURES IN FINE-GRAINED ROCKS USING X-RAY MICROTOMOGRAPHY

Although there is general support for the presence of microfractures and their ability to conduct fluid through mudstones, the spatial and temporal distribution of permeability along microfractures in mudstones is poorly understood. The fine-grained and fragile nature of mudstones makes analyzing these rocks difficult using common techniques. In recent years advances in x-ray scanning instruments have provided higher resolution tomographic data. X-ray microtomography provides a non-destructive technique that is suited to imaging the three-dimensional distribution of microfractures in fine-grained rocks.

Using x-ray microtomography both vertical and horizontal microfractures have been imaged in small cores taken from Cenozoic mudstones in the North Sea Central Graben. These mudstones are situated in the transition zone between normal hydrostatic pressure and overpressure and may episodically rupture and expel fluids through the development of microfractures. Three general morphologies of microfractures were imaged. In one sample the microfractures imaged are subparallel and vertical while in other samples horizontal microfractures were imaged. Comparison of the samples containing the horizontal microfractures show a single, through-going microfracture in one sample and a blocky, disconnected fracture system in another sample. It is possible that some of the microfractures were induced during sampling, however, the presence of diagenetic anhydrite indicates that the microfractures formed in the subsurface. In addition, the microfractures have altered fracture margins. Higher Al/Si ratios have been determined for the North Sea samples based on qualitative EDS analyses suggesting that the fracture margins have undergone more complete alteration of smectite to illite. The widths of the fractures are, on average, 10 microns and if distributed over a larger distance could conduct a large volume of fluid over geologic time. In some samples the anhydrite has completely sealed part of the microfractures, thus destroying the ability of the microfractures to transmit fluids.