There were investigated the Osinsk horizon rocks of Early Cambrian Usolsk suite, revealed in the well № 1 on the Eastern Sugdinsk Area (Irkutsk region). In the crosssection, there were distinguished two subhorizons (Osinsk-1 and Osinsk-2) according to the texture and structure features, rock composition and origin. The detailed lithopetrographic characteristics and genetic rock features were described, as well as the sedimentation and post-sedimentation alteration conditions made an influence on the formation of reservoir properties.