Abstract：Oil sands, also known as asphaltic sands, containing a natural sandstone of asphalt or other rock. This article mainly discusses the occurrence of oil sands, petrology, porosity and oil content of the reservoir. To understand the fromation of the Fengcheng oilsands, this article analysis the structure, diagenesis and biodegradation, ie. three conditions for the formation of oil sands. The regional faults serve as trapping structures; the unconformities play a role of migration pathways；and the diagenesis and biological degradation of oil sands ultimately result in the formation of the oil-bearing sands. Study of the features and origin of oil-sand reservoir can help exploration and mining of the oil-sand resources in the northwest margin of the Junggar Basin.