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The evolution of microstructure of shaly source rocks occuring during the natural oil generation is studied by the small-angle neutron and x-ray scattering for length scales 50-2000 Å. The study is performed on a set of rocks with several weight percent organic matter content, forming a natural maturity sequence for hydrocarbon generation. These rocks have been previously analyzed using various geochemical methods. The applicability of small-angle scattering techniques for the quantitative source rock studies is also tested using the laboratoryprepared rocks containing hydrogenated or deuterated eicosane. Although the natural source rocks are fivephase systems, they are perceived by neutrons as quasi-two-phase, which enables straightforward interpretation of the SANS data. The surfaces of immature source rocks are fractal ͑Dϭ2.5ϩ/Ϫ0.1͒ within the entire length scale region, regardless of the organic content. Upon maturation, two distinct fractal regimes develop ͑Dϭ2.8 ϩ/Ϫ0.1 and Dϭ2.0ϩ/Ϫ0.05͒ for the length scales below and above 600 Å, respectively. The SAXS and SANS data are compared with the geochemical thermal maturity indicators and a model of source rock structural evolution is presented. Our data suggest an oil generation scenario according to which hydrocarbons are produced from macerals finely dispersed inside the bulk of the rock and also concentrated on the grain boundaries for grain sizes larger than 600 Å. Upon reaching the thermal conditions necessary for oil generation the small grains crack and release oil into these microfractures, whereas the intergranular macerals produce oil and also wet the interface, thus forming an oil-wet network of conduits for primary migration.