Organic geochemical characteristics of source rocks in Wuyu Basin, Tibet

The Wuyu Basin is a Cenozoic intermontane basin in the Tibetan Plateau. Since Oligocene, it has been deposited in the basin from base to top the formations as followed: the Oligocene Rigongla Formation, the Miocene Mangxiang Formation, the Pliocene Gazhacun Formation and the Pliocene Zongdangcun Formation. Dark source rocks have been found during field explorations in these strata. Geochemical studies have indicated that source rocks from the Mangxiang Formation are featured by good organic type, high abundance and superior hydrocarbon generation potential. They are during the peak period of hydrocarbon generation. The nC_21-/nC₂₂₊ value is 1.2 in average, showing the lower aquatic organism sources for organic matters. The Pr/ Ph value is 1.2 in average, showing the deep or semi-deep lacustrine weak reduction environment for organic matter deposition. The average value of γ-acerance index has reached 3.32, indicating the stratification pattern and high salinity situation of water during deposition, which are favorable for organic matter preservation. The Mangxiang source rocks are the major source rocks in the Wuyu Basin. Coal samples from the Mangxiang Formation are good gas source rocks which have reached the mature stage and are abundant in organic materials, type III primarily.