Characteristics of unconformity between the Xishanyao Formation and Toutunhe Formation of Middle Jurassic in Junggar Basin and its significance in petroleum exploration
He DengfaZhou LuTang YongWu XiaozhiDu Shekuan
1 School of Energy Resources，China University of Geosciences(Beijing)，Beijing100083
2 Department of Resources and Environment，Southwest Petroleum University，Chengdu 610500，Sichuan 3 Institute of Exploration and Development，PetroChina Xinjiang Oilfield Company，Kelamay834000，Xinjiang
Unconformities are widespread at the base, top and within the Jurassic in Junggar Basin. Among these unconformities，the unconformity is quite distinct between the Xishanyao Formation in the lower part of Middle Jurassic and the Toutunhe Formation in the upper part of Middle Jurassic.With an eroded thickness of 100～340 meters， it is a regional unconformity，and has a lacuna （or hiatus） of 4～10 Ma. On the unconformity，sandstones were deposited and immediately below it were the weathered clay interval and the semiweathered rocks just below the unconformity. Generally，the unconformity shows a succession of sandstonemudstonesandstone with a thickness of 70～90 meters. Based on the weathered clay intervals，the sandstones of the Toutunhe Formation formed a series of the stratigraphic onlap traps sealed by the weathered clay intervals at the bottom，while the sandstones of the Xishanyao Formation formed the truncated traps sealed by the weathered clay intervals at the updip. The petroleum exploration has proved that the areas above or below this unconformity are favourable areas for oil and gas accumulation. The late stage tilting of the unconformity has played an important role in the oil and gas accumulation, and adjustment and reallocation.
. Characteristics of unconformity between the Xishanyao Formation and Toutunhe Formation of Middle Jurassic in Junggar Basin and its significance in petroleum exploration[J]. JOPC, 2007, 9(4): 387-396.
