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The analysis of small-and ultra-small-angle neutron scattering data for sedimentary rocks shows that the pore-rock fabric interface is a surface fractal ͑D s 2.82͒ over 3 orders of magnitude of the length scale and 10 orders of magnitude in intensity. The fractal dimension and scatterer size obtained from scanning electron microscopy image processing are consistent with neutron scattering data.[S0031-9007(99)08945-0] PACS numbers: 61.43.Hv, 61.12.Ex, Owing to the limited size range over which the fractal properties are usually observed, the issue of the apparent fractal geometry of various natural objects is a contentious one. In their critique of 96 recent reports on the fractality of a wide range of physical systems, Avnir et al. pointed out the contradiction between the narrow range of the appropriate scaling properties for declared fractal objects (centered around 1.3 orders of magnitude) and the public image of the status of experimental fractals [1], which for rocks has previously been based on limited experimental evidence (about 1.5 decades in length scale). A notable exception is the x-ray study of Bale and Schmidt on coals (Ref. [2], 2 decades in length scale, 7.5 decades in intensity). In this study we extend the range of length scales studied for rocks to over 3 decades (10 decades in intensity) and show that sedimentary rocks are in fact one of the most extensive fractal systems found in nature.Sedimentary rocks are formed from a mixture of organic and inorganic debris deposited in an aqueous environment, buried and compacted at elevated temperatures over geological periods of time. Remarkably, there is no percolation threshold observed in sedimentary rocks, which indicates a microstructure more complex than one originating from just a collection of compacted grains. According to the antisintering hypothesis of Cohen, the rock/pore interface evolves by maximizing the internal surface area in response to the secular equilibrium between the rock matrix and the formation brine [3]. Various studies performed on rocks of different origin and lithology over length scales in the range 20 Å to 100 mm have shown that sedimentary rocks are often effective fractals [4]. Experimental tools used in these studies include molecular adsorption [5], microscopic techniques [6,7], and small-angle scattering (SAS) methods. SAS methods are particularly well suited for testing the porematrix interface: They are noninvasive, average over the entire sample volume, and include correlation information. Previous small-angle neutron and x-ray scattering (SANS and SAXS) studies on shales [8-10] and sand-stones [7,9,11] demonstrated the surface fractal geometry of the pore-matrix interface in the scale range 20 Å to about 2000 Å.Recent progress in neutron scattering instrumentation enables one to access the microstructure of rocks well beyond the conventional SANS Q limit of Q min 3 3 10 23 Å 21 . The 80-m SANS instrument D11 at ILL [12] has resolution Q min 8 3 10 24 Å 21 and the Bonse-Hart geometry USANS facility at ORNL [13] c...

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