There is a set of dense oil-bearing rock in Middle Permian Lucaogou Formation in the north of Xinjiang Province and the rock has been ever regarded as mudstone, dolomicrite, siltstone or oil shale. Attributing to the systematic analysis of X-ray, it is known that the rock called "mudstone" can be defined as the sedimentary volcanic dust tuff (SVD tuff), in which volcanic dust (<30μm in grain diameter) is dominant in composition and the minerals commonly are feldspar, quatz and authigenic silicon and a small mount of analcime and pyrite with a littl (<5,) or no clay. The X-ray analysis also shows that the SVD tuff presents a change gradually to dolostone with changing content of dolomite in Lucaogou Formation. The SVD tuff displays good fluorescent reflection in microscope. The TOC is ranged from 2, to 20, in and the organic materials are present in laminaroid and scattered occurrences. This tuff is of many types of significant reservoir spaces, such as water-drainage channels, gas-drainage pores, structural fractures, silicalite fissures, volcanic ash-analcitized solution pores, algae (?) solution pores, and other pores. Through petrologic correlation of the Lucaogou tuff with nonvolcanic mudstone and shale, it is shows that this tuff can be an important source rock. Not a great abundance of clay minerals are necessary during hydrocarbon generation in the SVD dust tuff, which does not accord with the clay-catalyzing theory. This SVD tuff may be a good owner-source and owner-reservoir rock.