Sedimentary facies and evolution of the lower Urho Formation in the 8th area of Karamay oilfield of Xinjiang, NW China

Abstract On the basis of core descriptions, examinations of rock thin sections, and EMI logging, a fluvial-dominated fan delta depositional system consisting of the fan delta plain and front subfacies from the lower Urho Formation of the Upper Permian in the Karamay oilfield of Xinjiang, NW China, was recognized in this research. The typical characteristics of this kind of fan delta is the depositional processes that were dominated by the fluvial and tractive current structure that developed very well, while pro-fan delta and gravity deposition occurred very less. The key microfacies associations in the fan delta plain subfacies are braided channel, sheeted flow, mud flow, and sieve deposits, while the fan delta front subfacies commonly contains subaqueous channels, interdistributary channels, debris flow, grain flow, and subaqueous levee microfacies. A study of image logging, grain size, and compositions of rocks indicate that provenance directions are different from the early to the last depositional periods of the Urho Formation, i.e. it changed from the southwest initially to the northwest in the latest among the fifth period to the first period. In addition, with the corresponding provenance direction changing, the sedimentary facies also transform regularly. This paper illuminated the sedimentary facies, subfacies and microfacies and also discussed deeply the depositional distribution and evolution of the lower Urho Formation. A sedimentary model of the fluvial-dominated fan delta was presented in this paper.