Jurassic Yan'an Formation reservoir rocks in the Maling oil field, Ordos Basin

The Jurassic Yan'an Formation reservoir rocks occur as one of the main oil reservoirs of the Maling oil field, Ordos Basin. The characteristics and controlling factors are discussed on the basis of core, thin section and SEM analysis. Vertically, the Jurassic Yan'an Formation reservoir rocks in the Maling oil field are graded from the Yan 10 oil reservoirs upwards into the Yan 4 + 5 oil reservoirs, and the sedimentary facies has witnessed the evolution from the braided stream facies to the anastomosed river-delta facies. From the Yan 10 oil reservoirs to the Yan 4 + 5 oil reservoirs, the quartz contents in the clastic rocks gradually decrease and the feldspar contents gradually increase. Correspondingly, the grain sizes of the clastic rocks become finer, and the reservoir thickness becomes thinner upwards. The physical properties of the reservoir rocks are generally influenced by the source areas. For instance, the North area, South area and Zhenyuan area near the provenances have good physical properties, while the Shangliyuan area located in the lower reaches of the river has relatively poor physical properties. The reservoir rocks in the study area are now in the lower diagenetic degrees, i. e., from the early diagenetic stage B to the late diagenetic A. The compaction and cementation lead to the poor physical properties, and the dissolution leads to the good physical properties.