The development of oil field has entered the middle or late stage, and the dependence of oil and gas resources on imports is gradually increasing; therefore, the search for new oil and gas areas is an urgent task. Sanjiang Basin, which is located in the east of Heilongjiang Province, is the largest basin in the eastern periphery of Songliao Basin, but there has been no breakthrough in oil and gas exploration there. Through field geological survey and drilling exploration in Sanjiang Basin, the authors summarized the lithological and distribution characteristics of siliceous rock series of Dajiashan Formation deposited in early Mesozoic in this area. The siliceous rocks and dark mudstone strata are widely developed in the eastern part of the basin and has a large thickness, with total organic carbon abundance up to 0.95%; Ⅱ1 is the main organic matter type and is developed at the high maturation stage, and hence it is expected to become a new layer of oil and gas exploration in the Sanjiang Basin.